Endangered and Threatened Wildlife and Plants; 12-Month Finding on a Petition To List Van Rossem's Gull-billed Tern as Endangered or Threatened, 58650-58680 [2011-24048]
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Federal Register / Vol. 76, No. 183 / Wednesday, September 21, 2011 / Proposed Rules
DEPARTMENT OF THE INTERIOR
Fish and Wildlife Service
50 CFR Part 17
[FWS–R8–ES–2010–0035; MO 92210–0–
0008–B2]
Endangered and Threatened Wildlife
and Plants; 12-Month Finding on a
Petition To List Van Rossem’s Gullbilled Tern as Endangered or
Threatened
Fish and Wildlife Service,
Interior.
ACTION: Notice of 12-month petition
finding.
AGENCY:
We, the U.S. Fish and
Wildlife Service, announce a 12-month
finding on a petition to list van
Rossem’s gull-billed tern (Gelochelidon
nilotica vanrossemi) as endangered or
threatened and to designate critical
habitat under the Endangered Species
Act of 1973, as amended (Act). After
review of the best available scientific
and commercial information, we find
that listing van Rossem’s gull-billed tern
is not warranted at this time. However,
we ask the public to submit to us any
new information that becomes available
concerning the threats to van Rossem’s
gull-billed tern or its habitat at any time.
DATES: The finding announced in this
document was made on September 21,
2011.
ADDRESSES: This finding is available on
the Internet at https://
www.regulations.gov at Docket Number
FWS–R8–ES–2010–0035. Supporting
documentation we used in preparing
this finding is available for public
inspection, by appointment, during
normal business hours at the U.S. Fish
and Wildlife Service, Carlsbad Fish and
Wildlife Office, 6010 Hidden Valley
Road, Suite 101, Carlsbad, California
92011. Please submit any new
information, materials, comments, or
questions concerning this finding to the
above street address.
FOR FURTHER INFORMATION CONTACT: Jim
Bartel, Field Supervisor, Carlsbad Fish
and Wildlife Office, 6010 Hidden Valley
Road, Suite 101, Carlsbad, California
92011; by telephone at 760–431–9440;
or by facsimile to 760–431–9624. If you
use a telecommunications device for the
deaf (TDD), you may call the Federal
Information Relay Service (FIRS) at
800–877–8339.
SUPPLEMENTARY INFORMATION:
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SUMMARY:
Background
Section 4(b)(3)(B) of the Endangered
Species Act of 1973, as amended (Act)
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(16 U.S.C. 1531 et seq.) requires that, for
any petition to revise the Federal Lists
of Endangered and Threatened Species
that contains substantial scientific or
commercial information that listing the
species may be warranted, we make a
finding within 12 months of the date of
receipt of the petition. In this finding,
we will determine that the petitioned
action is: (1) Not warranted, (2)
warranted, or (3) warranted, but the
immediate proposal of a regulation
implementing the petitioned action is
precluded by other pending proposals to
determine whether species are
endangered or threatened, and
expeditious progress is being made to
add or remove qualified species from
the Federal Lists of Endangered and
Threatened Species. Section 4(b)(3)(C)
of the Act requires that we treat a
petition for which the requested action
is found to be warranted but precluded
as though resubmitted on the date of
such finding, that is, requiring a
subsequent finding to be made within
12 months. We must publish these 12month findings in the Federal Register.
Previous Federal Actions
In our November 15, 1994, Candidate
Notice of Review (59 FR 58982), we
included van Rossem’s gull-billed tern
as a Category 2 candidate. Category 2
taxa were defined as those taxa for
which information in the possession of
the Service, at that time, indicated that
proposing to list as endangered or
threatened was possibly appropriate but
for which persuasive data on biological
vulnerability and threats were not
available to support proposed rules. In
our February 28, 1996, Candidate Notice
of Review (61 FR 7596), we announced
our decision to discontinue recognition
of Category 2 candidates, including van
Rossem’s gull-billed tern. This decision
was finalized on December 5, 1996 (61
FR 64481). Since that time, van
Rossem’s gull-billed tern has not been
treated as a candidate for Federal listing
under the Act.
On June 8, 2009, we received a
petition from the Center for Biological
Diversity dated June 3, 2009, requesting
that we list the ‘‘western’’ or ‘‘van
Rossem’s’’ subspecies of gull-billed tern
(Gelochelidon nilotica vanrossemi) as
endangered or threatened under the Act,
and that we designate critical habitat
concurrent with listing (CBD 2009, pp.
1–40). Included in the petition was
supporting information regarding the
subspecies’ taxonomy, ecology,
distribution, status, and potential
threats. Although not expressly stated in
the petition, we assumed the petition
was a request to list van Rossem’s gull-
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billed tern as endangered or threatened
throughout the subspecies’ entire range.
In response to the Center for
Biological Diversity’s June 3, 2009,
petition to list van Rossem’s gull-billed
tern as endangered or threatened
throughout its range, we sent a letter to
Center for Biological Diversity, dated
August 18, 2009, acknowledging receipt
of the petition and informing the
petitioner that we concluded the
petition did not indicate that an
emergency situation existed for this
subspecies and that emergency listing
was not warranted. We also stated that
we were addressing a significant
number of listing and critical habitat
actions in Fiscal Year 2009 (October 1,
2008, through September 30, 2009)
pursuant to court orders, judicially
approved settlement agreements, or
other statutory deadlines; however, we
noted that we had secured funding to
begin reviewing the petition in that
fiscal year. Further, we said we
anticipated publishing our 90-day
finding in Fiscal Year 2010.
We published our 90-day finding on
the petition to list van Rossem’s gullbilled tern as endangered or threatened
in the Federal Register on June 9, 2010
(75 FR 32728). In that finding we
determined that the petition presented
substantial scientific or commercial
information, per section 4(b)(3)(A) of the
Act, indicating that listing the van
Rossem’s gull-billed tern throughout its
range may be warranted. The current
notice constitutes the 12-month finding
on the June 3, 2009, petition to list the
van Rossem’s gull-billed tern
throughout its range as endangered or
threatened under the Act.
Species Information
Species Description and Taxonomy
Van Rossem’s gull-billed tern
(Gelochelidon nilotica vanrossemi) is
medium-sized compared to other tern
species (Parnell et al. 1995, p. 2). Like
most tern species, its plumage is
generally pale gray above (dorsally),
white below (ventrally), with breeding
(alternate) plumage adults having black
on the top of the head (Parnell et al.
1995, p. 2). Gull-billed terns, including
van Rossem’s gull-billed tern, differ
from other species of terns by having a
proportionately stouter bill that is black
throughout the year (Bent 1921, p. 201;
Parnell et al. 1995, p. 2; Pyle 2008, p.
706). Gull-billed terns are powerful
flyers, and despite appearing heavier
bodied than most tern species, they
exhibit a buoyant agility, especially
while foraging (Audubon 1840, p. 1;
Bent 1921, p. 201; Molina and
Marschalek 2003, p. 3).
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Van Rossem’s gull-billed tern is a
seabird in the avian order
Charadriiformes (shorebirds, gulls and
terns, auks, and allies) and family
Laridae (skuas, gulls, terns, and
skimmers) (AOU 1998, pp. 141 and
181), although terns are sometimes
considered a separate family, Sternidae
(e.g., Ridgeway 1919, p. 458; Gochfeld
and Burger 1996, pp. 572 and 624;
Ericson et al. 2003, pp. 1–14).
Gelochelidon is a monotypic genus (a
genus with only one species,
Gelochelidon nilotica, the gull-billed
tern). Gelochelidon has historically been
placed in synonymy with Sterna (e.g.,
Saunders 1876, p. 644). However, a
more recent analysis using
mitochondrial DNA and morphological
features concluded that the gull-billed
tern is sufficiently differentiated from
other tern species to resurrect
Gelochelidon as a genus separate from
Sterna (Bridge et al. 2005, pp. 459–469;
see also Banks et al. 2006, p. 930).
The gull-billed tern (the species as a
whole) has a worldwide distribution,
albeit discontinuous, and may comprise
up to six subspecies (Parnell et al. 1995,
p. 3; Gochfeld and Burger 1996, p. 645).
Of those, two subspecies are described
in North America (Molina 2008, p. 188),
with Gelochelidon nilotica aranea
breeding along the Atlantic and Gulf of
Mexico coasts of the United States and
northeastern Mexico, and with G. n.
vanrossemi breeding along the Pacific
and Gulf of California coasts, primarily
in Mexico (see ‘‘Range and Distribution’’
section below) (Molina and Erwin 2006,
pp. 271–272).
Bancroft (1929, pp. 283–286)
described Gelochelidon nilotica
vanrossemi from specimens collected at
the Salton Sea, Imperial County,
California. According to Bancroft (1929,
p. 284), van Rossem’s gull-billed tern
differs from the nominate subspecies of
the Old World (G. n. nilotica) by its
shorter tail and bill shape (less angular
gonys), and from the subspecies of
eastern North America, G. n. aranea, by
its ‘‘decidedly larger size.’’ However, in
contrast to the petitioner’s assertion that
the validity of the subspecies (i.e., its
distinctiveness) has not been questioned
(CBD 2009, p. 4), information in the
scientific literature indicates that some
authors have questioned the
distinctiveness of van Rossem’s gullbilled tern. For example, Murphy (1936,
p. 1093) noted the paucity of specimens
from the New World and concluded
‘‘existing subspecific names have been
created far in advance of any adequate
study of the facts.’’ Murphy’s published
statements of dissatisfaction over the
available information, in turn, caused
Grinnell and Miller (1944, p. 172) to
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‘‘not recognize a western race’’ (i.e.,
subspecies) of gull-billed tern in their
authoritative review of the birds of
California. Although additional
specimens are now available, providing
larger sample sizes in mensural
(measurement) data, geographic
representation of specimens from
western North America, especially from
Mexico and Central America, are still
limited (Molina and Erwin 2006, pp.
273, 283, and 294–295).
Individual gull-billed terns are
typically not identifiable to subspecies
under field conditions, and because the
two North American subspecies are
distinguished on the basis of average
morphometric differences that show
substantial overlap, even individual
specimens are not necessarily
distinguishable in the hand (Molina and
Erwin 2006, p. 283). This suggested to
Unitt (2004, p. 249) that the
distinctiveness of the G. n. vanrossemi
as a subspecies remains not entirely
conclusive (see also Patten and Unitt
(2002, pp. 26–35) regarding the pitfalls
of differentiating subspecies based on
average differences). Moreover, Pyle
(2008, p. 706) stated that the
morphological differences of the
western North American gull-billed
terns are ‘‘too slight for subspecific
recognition.’’
In contrast, other authors have not
questioned the distinctiveness of
Gelochelidon nilotica vanrossemi as a
subspecies. For example, the American
Ornithologists’ Union (AOU) Committee
on Classification and Nomenclature
(AOU Committee), the long-standing
scientific body responsible for
standardizing North American avian
taxonomy, recognized G. n. vanrossemi
in its 1957 (fifth) edition of its checklist
of North American birds (AOU 1957, p.
233), which was the last time the AOU
Committee explicitly addressed
subspecies (AOU 1998, p. xii). More
recently, Patten et al. (2003, pp. 1–363),
who critically reviewed the taxonomy of
subspecies in their book on the birds of
the Salton Sea region (Patten et al. 2003,
p. 71), also recognized G. n. vanrossemi
as valid (distinctive) (Patten et al. 2003,
p. 188). Additionally, G. n. vanrossemi
is recognized by many other authors
(such as Parnell et al. 1995, p. 3;
Gochfeld and Burger 1996, p. 645;
Patten et al. 2001, p. 45; Dickinson
2003, p. 149; Molina and Erwin 2006, p.
273, but see p. 283; and Molina et al.
2010, p. 1). However, the authors of this
latter group of works may not have
conducted taxonomic assessments of
their own and may instead have relied
upon other publications. Thus, in total,
the available scientific literature is not
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consistent regarding the distinctiveness
of van Rossem’s gull-billed tern.
The Service is currently funding the
U.S. Geological Survey to conduct a
genetics-based study that may yield
additional information regarding the
distinctiveness between the eastern and
western North American subspecies of
the gull-billed tern, but only those two
subspecies. As of the preparation of this
status review and 12-month finding, the
results of this work are not yet available.
Although we anticipate the information
from this study will be helpful in
understanding the relationship between
the eastern and western subspecies of
gull-billed terns in North America, a
comprehensive, rangewide review is
needed to address fully the
distinctiveness of all of the subspecies,
including Gelochelidon nilotica
vanrossemi, that compose the gull-billed
tern species. We are not aware of any
modern, rangewide treatments that
evaluate the taxonomic distinctiveness
of gull-billed tern subspecies.
In summary, the available scientific
information presents differing opinions
regarding the distinctiveness of
Gelochelidon nilotica vanrossemi as a
subspecies. Although this contradicts
the petitioner’s assertion that the
subspecies’ distinctiveness has never
been questioned (CBD 2009, p. 4), the
available information does not
conclusively support the abandonment
of a long-standing, established taxon
that is accepted by the AOU Committee
and is widely used in the literature.
Therefore, for the purposes of evaluating
the petitioned action, we assume G. n.
vanrossemi, van Rossem’s gull-billed
tern, is a subspecies per section 3(16) of
the Act.
Range and Distribution
Van Rossem’s gull-billed terns are
migratory (Molina et al. 2010, p. 5),
which means they breed in one area
during the spring and summer and then
move (migrate) to a different area for the
winter. Like most birds in the Northern
Hemisphere, they nest in northerly
locations during the summer and
overwinter farther south, presumably
using the Pacific coast of North America
as a migratory route (Molina et al. 2010,
p. 5). In the U.S. portion of the
subspecies’ breeding range, where
monitoring is more intensive and data
sets are more complete, van Rossem’s
gull-billed terns generally arrive in midMarch and leave in late August,
although some birds stay until
September or October (Patten et al.
2003, p. 188; Patton 2009, Table 2). Less
is known about the migratory habits of
populations in Mexico.
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Nesting of what would later be
described as the van Rossem’s
subspecies of gull-billed tern was first
noted at the Salton Sea in 1927
(Pemberton 1927, pp. 253–258). Reports
of historical observations and museum
specimen data suggested van Rossem’s
gull-billed terns bred in Mexico (van
Rossem and Hachisuka 1937, p. 333;
Friedmann et al. 1950, p. 107; Binford
1989, p. 115; Molina and Erwin 2006,
pp. 273–274 and 294–295), but it was
not until the 1990s that nesting of the
subspecies was actually observed in that
country (Palacios and Mellink 2007, p.
214). The majority of nesting locations
were discovered in Mexico only after
2000 as a result of focused surveys
(Palacios and Mellink 2007, p. 217).
As detailed below, the current
breeding range for van Rossem’s gullbilled tern is western North America
from extreme southern California in the
United States to the State of Guerrero in
Mexico. Within this general range, the
subspecies occurs in discrete nesting
locations predominantly along the
Pacific coast of Mexico including the
Gulf of California (Molina and Erwin
2006, p. 273) (Table 1, Figure 1). An
additional coastal nesting colony is
located in San Diego Bay, San Diego
County, California (Molina 2008, p.
188). Nest colonies are also located at
inland localities in northeastern Baja
California, Mexico (Molina and Garrett
2001, p. 25; Palacios and Mellink 2007,
p. 215), and at the Salton Sea, Imperial
County, California (Pembarton 1927, p.
253; Molina 2004, p. 94; Molina 2009b,
p. 5). The Salton Sea and San Diego Bay
are currently the only locations where
the subspecies nests in the United States
(Molina and Erwin 2006, p. 273), and
together they define the northern extent
of the breeding range of van Rossem’s
gull-billed tern. However, as this
document was being finalized, a pair of
van Rossem’s gull-billed terns attempted
to nest at the San Joaquin Marsh and
Wildlife Sanctuary in Irvine, Orange
County, California (Daniels 2011, in
litt.), which is roughly 135 kilometers
(km) (85 miles (mi)) north of the San
Diego Bay nesting location. It is too
early to know whether this location will
be regularly used by the subspecies in
the future.
TABLE 1—LIST OF KNOWN NESTING LOCATIONS OF VAN ROSSEM’S GULL-BILLED TERN (GELOCHELIDON NILOTICA
VANROSSEMI) IN THE UNITED STATES AND MEXICO (ARRANGED NORTH TO SOUTH)
[Approximate population size over the past decade for coarse-scale comparisons (Large—typically greater than 100 pairs, Medium—typically
between 15 and 100 pairs, and Small—typically less than 15 pairs]
Country
State
Nesting location a
Population
size b
Citations c
U.S. ..........
California .............
Salton Sea (multiple nest sites) ........
Large .........
U.S. ..........
Mexico .....
California .............
Baja California .....
San Diego Bay ..................................
´
Campo Geotermico Cerro Prieto (including Las Arenitas).
Medium ......
Large .........
Mexico .....
Baja California .....
Isla Montague, Colorado River Delta
Large .........
Mexico .....
Mexico .....
Baja California
Sur.
Sinaloa .................
Laguna Ojo de Liebre (Salinas de Small .........
Guerrero Negro).
´
´
Bahıa Santa Marıa (including Isla El Small .........
Rancho and Isla Altamura).
Mexico .....
Sinaloa .................
´
Bahıa de Ceuta .................................. Small .........
Mexico .....
Sinaloa .................
Mexico .....
Sinaloa/Nayarit ....
Mexico .....
Colima .................
Laguna del Caimanero (Las Tres
Tumbas).
Marismas
Nacionales
(including
´
Estero Teacapan and Laguna
Pericos (Laguna las Garzas),
Nayarit).
´
Laguna Cuyutlan ...............................
Pembarton 1927, p. 253; Molina 2004, pp. 92–99;
Molina 2010b, in litt., p. 3.
McCaskie 1987, p. 1488; Patton 2009, Table 2.
Molina and Garrett 2001, p. 24; Palacios and Mellink
2007, p. 217; Erickson et al. 2009, p. 508; Molina
2010b, in litt., p. 3; Palacios 2010, p. 11.
Palacios and Mellink 1993, p. 259; Peresbarbosa and
Mellink 1994, p. 201; Peresbarbosa and Mellink
2001, p. 266; Molina et al. 2006, p. 5; Palacios and
Mellink 2007, p. 217; Molina 2010b, in litt., p. 3;
Palacios 2010, p. 11.
Danemann and Carmona 2000, pp. 195–199; Palacios
and Mellink 2007, p. 217; Palacios 2010, p. 11.
´
˜
Gonzalez-Bernal et al. 2003, p. 176; Munoz del Viejo et
al. 2004, pp. 191–202; Palacios and Mellink 2007, p.
217; Palacios 2010, p. 11.
´
Gonzalez-Medina and Guevara-Medina 2008, p. 6;
Palacios 2010, p. 11.
Palacios and Mellink 2007, p. 217; Palacios 2010, p.
11.
Palacios and Mellink 2007, p. 217; Palacios 2010, p.
11.
Mexico .....
Guerrero ..............
´
Laguna Potosı .................................... Small .........
Medium ......
Large .........
Medium ......
Palacios and Mellink 2007, p. 217; Palacios 2010, p.
11.
Mellink et al. 2009, p. 8.
a Nesting
locations are general areas that may comprise more than one nest site. Some locations may not be occupied every year.
population size is for general comparison only; the level of accuracy and precision varies between sources and nesting populations differ
from year to year.
c Citations include noteworthy sources for the nesting location as well as sources for population ranges.
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b The
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The southern limit to the breeding
range of van Rossem’s gull-billed tern is
not precisely known. The southernmost
location where van Rossem’s gull-billed
terns have been observed nesting is
´
Laguna Potosı in the Mexican State of
Guerrero (Table 1, Figure 1).
Information in the literature shows that
gull-billed terns occur during the
breeding season in small numbers in
´
Mexico south of Laguna Potosı (Binford
1989, p. 115; Mellink et al. 1998, p. 381;
Molina and Erwin 2006, pp. 294–295;
Palacios and Mellink 2007, p. 220).
Although actual nesting has never been
observed at any of these southern
locations, breeding is suspected at some
(for example, Binford 1989, p. 115;
Mellink et al. 1998, p. 381). These areas
are all within the winter range of the
subspecies (Figure 1) and nonbreeding
birds may remain in this region during
the breeding season (Howell and Webb
1995, p. 303), which is a confounding
factor in assessing observations that do
not include actual detections of nests.
Additionally, Table 1 only includes
locations where actual nesting has been
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observed, but breeding behavior (such
as courtship) has been noted at other
locations, suggesting nesting may be
more widespread. These other locations
with observed breeding behavior but
without observation of actual nests
include locations in the Mexican States
of Sonora (historically) (van Rossem and
Hachisuka 1937, p. 333) and Jalisco
(Mellink et al. 2009, p. 48), both of
which are within the range van
Rossem’s gull-billed terns are known to
nest. Additionally, nesting may occur in
Mexico near or along the Colorado
River, north of the known nesting
location of Isla Montague at the delta
(Erickson et al. 2005, p. 498). Moreover,
there are likely smaller ephemeral sites
that are not used every year that are
probably missed during inconsistent
survey efforts. Also, gull-billed terns
have been observed nesting at inland
´
locations in Mexico (Gomez de Silva
2005, p. 501; Molina and Erwin 2006,
p. 274), which may consist of colonies
containing either North American
subspecies.
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Although some gull-billed tern
specimens from south of Guerrero have
been identified as van Rossem’s gullbilled terns (Hellmayr and Conover
1948, p. 297; Binford 1989, p. 115), the
majority of the occurrences reported in
the available literature are field
observations; thus, these records have
not been identified to subspecies. Gullbilled terns also nest farther south along
the Pacific coast of South America;
however, specimen data suggest that at
least some of these birds are the
‘‘eastern’’ subspecies of gull-billed tern,
Gelochelidon nilotica aranea (Molina
and Erwin 2006, p. 283; but see
Hellmayr and Conover 1948, p. 297,
footnote 1). The northern extent of the
range of the Pacific-breeding birds
presumed to be G. n. aranea is not
known and could potentially include
Central America, where available data
are limited. Thus, the southern limit of
the breeding range of van Rossem’s gullbilled tern extends at least as far south
as Guerrero, and possibly farther south,
but survey information from these
southern areas is limited and any
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conclusions drawn from observational
data are confounded by the potential
occurrence of birds of the ‘‘eastern’’
subspecies, G. n. aranea.
The winter range of the subspecies
includes the Gulf of California and the
Pacific coast of mainland Mexico,
possibly Pacific coastal Central America
and coastal northwestern South
America (Molina and Erwin 2006,
p. 272; Molina et al. 2009a, pp. 2–20;
Molina et al. 2010, p. 1), with the largest
concentrations found in the extensive
coastal lagoon systems of southern
Sonora, Sinaloa, and northern Nayarit
(Molina et al. 2009a, p. 9). However,
similar to the breeding range, the
southern part of the winter range is
poorly defined (Molina et al. 2009a,
pp. 9–11). Although at least one
specimen collected from Guatemala in
winter (Molina and Erwin 2006, p. 294)
was thought to be Gelochelidon nilotica
vanrossemi (Hellmayr and Conover
1948, p. 297), the potential mingling of
the ‘‘eastern’’ subspecies of gull-billed
terns along the Pacific coast of southern
Mexico and Central America
complicates our ability to delineate the
winter range of van Rossem’s gull-billed
tern (Molina et al. 2009a, p. 15). Not
only are individuals of the G. n. aranea
subspecies that breed in western South
America possible in the region (the
available literature is not specific as to
the winter range of these South
American-nesting birds), individuals
that breed in eastern North America
(G. n. aranea) may also cross from the
Gulf of Mexico (such as at the Isthmus
of Tehuantepec or Isthmus of Panama)
to winter along the Pacific coast
(Gochfeld and Burger 1996, p. 645;
Molina and Erwin 2006, pp. 283–284).
Such behavior has been documented
for other species of terns and gulls
(Molina and Erwin 2006, p. 84). As
such, ‘‘eastern’’ gull-billed terns
potentially intermingle with van
Rossem’s gull-billed terns within the
southern portion of the latter’s range.
However, we do not know how
prevalent this is. Moreover, the
available literature has evolved through
time. Contrary to earlier accounts (for
example, AOU 1957, p. 233; Molina and
Erwin 2006, p. 282), Molina et al.
(2009a, p. 15) suggested that the winter
range may not extend south of the
Isthmus of Tehuantepec; thus, without
firm data the subspecies’ range remains
equivocal. In addition to coastal
locations, small numbers of gull-billed
terns, presumably van Rossem’s gullbilled terns, regularly occur at inland
sites in western Mexico during the
winter, away from Pacific coastal
lowlands (Molina et al. 2010, p. 12);
thus, the winter range likely includes
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inland areas of western Mexico and
possibly Central America.
The best available information
indicates the breeding range of the
subspecies has expanded in recent
years. The first record for coastal
California (and the first record for the
Pacific coast north of the southern tip of
the Baja California Peninsula) was of an
adult detected along San Diego Bay in
July 1985 (McCaskie 1985, p. 962).
Evidence of nesting was noted there two
years later (McCaskie 1987, p. 1488;
Unitt 2004, p. 248). Initially, the
population grew slowly and
sporadically, but after 1999 the
population increased much more
quickly and steadily, totaling
approximately 59 pairs in 2010
(R. Patton, in litt., 2010, spreadsheet
summary). Moreover, despite multiple
earlier explorations of the avifauna of
the Baja California Peninsula, Mexico
(Bryant 1889, pp. 237–320; Grinnell
1928, p. 61; Wilbur 1987, pp. 94–95;
Massey and Palacios 1994, pp. 45–57),
van Rossem’s gull-billed terns were only
first noted in 1995 as nonbreeders along
the Pacific coast of the Baja California
Peninsula (Erickson et al. 2001, p. 125)
and first found nesting in 1996 at
Laguna Ojo de Liebre near Guerrero
Negro, Baja California Sur (Danemann
and Carmona 2000, p. 197). Laguna Ojo
de Liebre is the only known coastal
nesting location on either coast of the
1,200-km-long (750-mi-long) peninsula
(Molina et al. 2010, p. 61). The
colonization of these two new coastal
nesting locations suggests the breeding
range of the subspecies has expanded in
recent years.
Such range expansions are not
unprecedented; other colonial waterbird
species have similarly expanded their
range along the Pacific coast and
established nesting colonies, such as the
elegant tern (Thalasseus elegans)
(Collins et al. 1991, pp. 393–395) and
the black skimmer (Rynchops niger)
(Palacios and Alfaro 1992, pp. 173–176;
Collins and Garrett 1996, pp. 127–135;
Danemann and Carmona 2000, p. 197).
Black skimmers have also moved
northward along the Gulf of California
coast and even inland at the north end
of the Gulf; for example, establishing
nesting colonies at the Salton Sea
(McCaskie et al. 1974, pp. 337–338;
Collins and Garrett 1996, pp. 127–135)
and Cerro Prieto (Molina and Garrett
2001, p. 25). Van Rossem’s gull-billed
terns use similar nesting habitat as black
skimmers, often nesting near one
another at locations where their ranges
overlap (Parnell et al. 1995, p. 9).
Although the timing of the range
expansion of van Rossem’s gull-billed
terns has lagged behind the black
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skimmer and other species with
expanding ranges, it is possible that van
Rossem’s gull-billed terns may be
following a similar pattern and could
start to colonize new nesting locations
along the Pacific Coast.
There is some indication that van
Rossem’s gull-billed terns may
potentially continue to expand their
range northward along the California
coast. Birds that migrate long distances,
such as van Rossem’s gull-billed terns,
have the potential to occur outside their
expected range (i.e., vagrancy). Other
subspecies of gull-billed terns are
capable of long-distance flights and we
assume van Rossem’s gull-billed terns
are similarly capable. For example, an
individual of the nominate (European)
subspecies was banded as a nestling in
Denmark and collected a few months
later in Barbados in the Lesser Antilles
in the western Atlantic Ocean (Lincoln
1936, p. 331; see also Cooke 1945, p.
128)—roughly 4,500 km (3,000 mi)
outside of its expected winter range in
western Africa (Gochfeld and Burger
1996, p. 645). Another gull-billed tern,
probably of the Asian subspecies
Gelochelidon nilotica affinis (G. c.
addenda), was observed on the
Hawaiian islands of O’ahu, Moloka’i,
and Maui over a span of several months
(Pyle and Pyle 2009, no page number),
more than 8,000 km (5,000 mi) away
from its expected winter range in
Southeast Asia (Gochfeld and Burger
1996, p. 645). Although we do not have
information on similar long-distance,
extralimital movements for van
Rossem’s gull-billed tern, birds
presumed to be of this subspecies have
been observed north of the San Diego
Bay region (the northernmost nesting
location within the subspecies’ expected
range), including multiple detections of
single birds along the California coast as
far north as the San Francisco Bay area
(Patton 2009, Appendix B) and at inland
locations along the Colorado River and
elsewhere in Arizona (Speich and
Witzeman 1973, p. 148; Monson and
Phillips 1981, p. 50; Rosenberg et al.
1990, p. 193).
Such movements of van Rossem’s
gull-billed terns, though not
unexpected, occur too infrequently to
consider these areas as part of the
subspecies’ range. However, the number
of detections of van Rossem’s gull-billed
terns farther north along the coast of
California has increased as the San
Diego Bay breeding population has
increased (see discussion below in the
‘‘Population Size’’ section). As such,
areas where other species of terns nest
along the coast north of San Diego
should be monitored for nesting gullbilled terns. Confirmation of van
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Rossem’s gull-billed terns nesting north
of San Diego Bay, like the recent nesting
attempt detected in Orange County
mentioned above, would indicate a
continuing northward expansion of the
subspecies’ breeding range.
In summary, the current breeding
range of van Rossem’s gull-billed tern
extends from San Diego and the Salton
Sea along the Pacific and Gulf of
California coasts to at least as far south
as the State of Guerrero in Mexico.
Actual nesting locations are
discontinuously distributed within that
range (Table 1). However, survey
information is limited for most of the
Pacific coast of Mexico; additional
efforts may yet detect other nesting
locations in this region, including south
of Guerrero. The current winter range of
the subspecies includes the west coast
of mainland Mexico, potentially as far
south as Central America and coastal
northwestern South America, plus a few
inland locations.
Population Size
Historical data on population sizes are
generally lacking for the subspecies,
especially in western Mexico and
farther south into Central and South
America. As noted above, historical
information shows that van Rossem’s
gull-billed terns occurred in Mexico, but
these data largely consist of anecdotal
observations or museum collections
(specimens); there are few data to
indicate the size of historical
populations of van Rossem’s gull-billed
terns. Available literature that include
information on the historical avifauna of
western Mexico, such as Bryant (1889
pp. 237–320), Brewster (1902, pp. 1–
241), Salvin and Godman (1904, pp. 1–
505), Ridgway (1919 pp. 1–852),
Mailliard (1923, pp. 443–456), Huey
(1927, pp. 239–243), Grinnell (1928, pp.
1–300), van Rossem and Hachisuka
(1937, p. 333), van Rossem (1945, p. 93),
Hellmayr and Conover (1948, p. 297),
Friedmann et al. (1950, pp. 1–204),
Schaldach (1963, pp. 1–510), Binford
(1989, p. 115), and Russell and Monson
(1998, pp. 115–116) (see also summary
in Palacios and Mellink 2007, pp. 214–
215), present limited or no information
on gull-billed terns from the region.
Many of the cited historical texts
predate the 1929 formal description of
Gelochelidon nilotica vanrossemi, the
van Rossem’s subspecies of gull-billed
tern. Regardless of the subspecies or the
timing of the historical observations,
early observers would have been able to
identify the species as a whole—G.
nilotica, the gull-billed tern. As
discussed in the Species Description
and Taxonomy section, the available
information indicates that the
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subspecies of the gull-billed tern that
breeds in western Mexico (at least north
of the Isthmus of Tehuantepec) is G. n.
vanrossemi. Thus, the historical
observations of gull-billed terns in
western Mexico most likely pertained to
G. n. vanrossemi. The information that
is available from these sources indicates
that gull-billed terns were rarely
encountered, and when encountered,
were in small numbers. By comparison,
the information on other species of
colonial waterbirds in western Mexico
is much more complete. Although this
list of references is not a fully
exhaustive list of historical resources, it
illustrates the contrast between
historical information available on gullbilled terns and other species of colonial
waterbirds that occurred in western
Mexico. This contrast indicates that the
historical scientific explorations of the
region were adequate to detect many
other species of colonial waterbirds, but
were inadequate to detect gull-billed
terns or their nest sites in western
Mexico. It is reasonable to conclude that
van Rossem’s gull-billed terns were
encountered rarely because there were
comparatively few van Rossem’s gullbilled terns to encounter. Therefore, we
conclude based on the available
information, the historical population
size of van Rossem’s gull-billed terns in
western Mexico was small—or at least
not markedly larger than the population
today.
In the United States, when Pemberton
first discovered the nesting colony of
gull-billed terns at the Salton Sea in
1927, he estimated that there were
approximately 500 active nests
(Pemberton 1927, p. 256), which would
translate into a similar number of pairs.
It is not clear when this population
became established, but the Salton Sea
was created in its present form between
1905 and 1907 when Colorado River
floodwaters filled the dry lakebed
known as the Salton Sink; however,
previous historical and prehistorical
floods also periodically filled the Salton
Sink from time to time (with intervening
dry periods), forming an intermittent
body of water within the Salton Sink
now referred to as Lake Cahuilla (see
Patten et al. 2003, pp. 1–6 for a history
of Lake Cahuilla and the Salton Sea).
Although the Salton Sea population of
van Rossem’s gull-billed terns was not
systematically monitored until the
1990s, anecdotal evidence shows that
the population decreased over time to a
low somewhere in the range of 15 to 25
pairs in the early 1970s (Grinnell and
Miller 1944, p. 172; Pyle and Small
1961, p. 31; McCaskie 1973, p. 919;
McCaskie 1974, p. 949; McCaskie 1976,
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58655
p. 1004; Garrett and Dunn 1981, p. 189;
McCaskie pers. comm. 2010). Over the
next few decades, the population at the
Salton Sea increased to about 100 to 150
pairs, with more consistent monitoring
showing that it has remained fairly
constant since the early 1990s (Molina
2004, p. 94; Molina 2009b, p. 5). In San
Diego Bay, the nesting population of van
Rossem’s gull-billed terns has increased
from its inception in 1987 to 59 pairs in
2010 (R. Patton, in litt., 2010,
spreadsheet summary).
Today in Mexico, in addition to the
new, small colony at Laguna Ojo de
Liebre, van Rossem’s gull-billed terns
have colonized the islands in the
impoundments associated with the
´
Campo Geotermico Cerro Prieto (Cerro
Prieto geothermal generation facility) in
northeast Baja California. The facility
´
started operation in 1973 (GutierrezGalindo et al. 1988, p. 201) and van
Rossem’s gull-billed terns have been
observed there since at least 1996
(Molina and Garrett 2001, p. 25). Since
1996, fairly consistent monitoring at this
site indicates that it has grown to be one
of the largest populations (Table 1).
Additionally, the nesting colony at Isla
Montague has been fairly well
monitored since 1992 (Palacios and
Mellink 1993, p. 259; Molina 2010b, in
litt.). Although nesting at Isla Montague
was only just confirmed in 1992
(Palacios and Mellink 1993, p. 259),
nesting on the island was suspected
decades earlier based on specimens
collected there in the spring of 1915
(Friedmann et al. 1950, p. 107; Molina
and Erwin 2006, p. 294; Molina et al.
2010, p. 61).
As mentioned in the ‘‘Range and
Distribution’’ section, gull-billed terns
have been known to occur in western
Mexico for more than a century (see
Molina and Erwin 2006, p. 294) and
breeding there was likely; however,
nesting has only been documented
recently. Surveys at nesting locations
throughout the remainder of the
breeding range of van Rossem’s gullbilled tern in Mexico have been
sporadic and essentially consist of
‘‘snapshots’’ of nesting efforts over time.
During the breeding seasons of 2003 and
2005, Palacios and Mellink (2003, pp.
1–66; 2006, pp. 1–84; 2007, pp. 214–
222) surveyed at least 367 potential
nesting areas along the Pacific and Gulf
of California coasts of Mexico.
Additionally, of the nine known nesting
locations in Mexico (Table 1), all but
´
Laguna Potosı were resurveyed in June
and early July 2010 (Palacios 2010, pp.
1–28). However, the level of survey
effort compared with the number of
potential nesting locations along the
coast of Mexico suggests additional
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undetected nesting locations likely
exist. For example, one of the largest
single colonies of this subspecies (105 to
160 pairs) was only discovered in 2003
at Laguna Las Garzas (Laguna Los
Pericos) in Marismas Nacionales,
Nayarit (Table 1) (Palacios and Mellink
2003, p. 11; Palacios and Mellink 2007,
p. 217). New (but small) populations
´
were also found nesting in 2006 at Bahıa
´
de Ceuta, Sinaloa (Gonzalez-Medina and
Guevara-Medina 2008, pp. 6–7) and in
´
2007 at Laguna Potosı, Guerrero
(Mellink et al. 2009, p. 8) (Table 1).
Thus, although we expect additional
nesting locations to be found and
population estimates to change, we do
not expect refinements in those values
to alter substantially our understanding
of the subspecies or our analysis.
As summarized by Molina et al.
(2010, p. 10), 737 to 808 pairs of van
Rossem’s gull-billed terns appear to
have nested in western North America
in 2003 and 2005, with approximately
550 of those nesting in Mexico. Because
these values generally represent pairs of
nesting adults counted at nesting sites,
there are additional nonbreeding
individuals that are not represented in
these totals, underestimating the total
population size. Additionally, there may
be a limited number of pairs nesting at
undetected locations. Thus, these rough
estimates represent the minimum
population size for van Rossem’s gullbilled terns in the United States and
Mexico.
Population data for most of the
subspecies’ range are incomplete over
time; thus, population trends are
difficult to assess. Data from the Salton
Sea, which are fairly complete, shows a
marked decline in population compared
to the historical high in 1927, but this
population has remained fairly stable
since the 1990s (Molina et al. 2010, p.
10). Although preliminary data suggest
the numbers of nesting van Rossem’s
gull-billed terns at the Salton Sea during
the 2010 nesting season was
substantially smaller (Molina, in litt.,
2010, p. 3), it is not clear whether this
is a temporary or longer-term change;
marked declines have been observed
there in the past, but they have been
temporary (Molina, in litt., 2010, p. 3).
The available information from the
nesting locations in Mexico with the
most-complete population data (Isla
Montague and Cerro Prieto) shows that
population sizes at these locations are
variable (Palacios and Mellink 2007, p.
217). The populations at these sites also
appear to be connected, with
individuals moving between these
nesting locations and the Salton Sea
nesting location and, to a lesser extent,
the San Diego Bay nesting location
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(Molina and Garrett 2001, p. 26; Molina
2004, p. 98; Palacios 2010, pp. 12 and
15). In combination, the populations of
van Rossem’s gull-billed terns at Isla
Montague, Cerro Prieto, and the Salton
Sea are annually variable but, when
taken together, appear to have been
fairly stable since the 1990s (see Molina
et al. 2006, p. 5; Molina and Erwin 2006,
p. 279; Palacios and Mellink 2007, p.
217; Molina et al. 2010, p. 10). Data
from central and southern Mexico—the
bulk of the subspecies’ range
geographically but not, as suggested by
the data, in numerical terms—are
inadequate to define precise trends, but
they do not show any precipitous
declines (see Molina and Erwin 2006, p.
279; Palacios and Mellink 2007, p. 217).
Moreover, as discussed above, the
historical size of the van Rossem’s gullbilled tern population in the rest of
Mexico was likely never large.
Biology
Van Rossem’s gull-billed tern is
predominantly a coastal nesting species,
but it also nests at, or near, certain
inland saline lakes (Parnell et al. 1995,
p. 5; Molina and Erwin 2006, p. 284;
Molina et al. 2010, p. vii). During the
nonbreeding season, van Rossem’s gullbilled terns may occur at either saline or
freshwater areas (Molina et al. 2010, p.
12), but they are often found foraging
over tidal mudflats within large lagoons
and estuaries (Molina et al. 2009a, p.
12). Like other terns, gull-billed terns
(including van Rossem’s gull-billed
tern) are predators, but they differ from
most other tern species in how they
forage and in the types of prey they
consume. Unlike many other tern
species that eat only fish caught by
shallow dives into water, gull-billed
terns forage on a variety of prey items,
which varies by area. For example:
(1) Gull-billed terns capture flying
insects during foraging flights (Parnell et
al. 1995, p. 5);
(2) They swoop down and snatch up
terrestrial prey (such as small crabs,
lizards, insects, or small chicks of other
bird species) and aquatic prey (such as
small fish) near the water’s surface
(Parnell et al. 1995, p. 5; Molina and
Marschalek 2003, p. i); and
(3) They land to capture small prey
items from the water’s surface (Parnell
et al. 1995, p. 5).
Moreover, gull-billed terns—the
species as a whole, including van
Rossem’s gull-billed terns—are
opportunistic foragers (Parnell et al.
1995, p. 5; Gochfeld and Burger 1996, p.
645; Erwin et al. 1998a, p. 323; Molina
2009a, p. 6). Not only do they eat a wide
variety of prey items and forage over
wide range of areas, they also may
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opportunistically focus on certain prey
items when those items are abundant or
otherwise readily accessible. For
instance, gull-billed terns in western
Africa were observed preferentially
foraging on fiddler crabs (Uca tangeri),
despite being an energy-poor food
source, because the crabs were abundant
and easier to capture than other, more
energy-rich prey items (Stienen et al.
2008, p. 243). The diet and general
foraging habits of van Rossem’s gullbilled tern is similar to that of other
subspecies of gull-billed tern (Molina
and Marschalek 2003, p. 9; Molina and
Erwin 2006, pp. 286–287; Molina 2009a,
pp. 6–8; Molina et al. 2009a, p. 12).
Thus, van Rossem’s gull-billed terns
are generalist predators whose food
appears to be determined more by size
and availability of prey items rather
than strictly by the type of prey. The
foraging habitat of van Rossem’s gullbilled terns consists of open mudflats in
tidal estuaries, river margins, beaches,
salt marshes, freshwater marshes,
aquacultural impoundments (such as
shrimp ponds), and a variety of upland
habitats including open scrub,
pasturelands and irrigated agricultural
fields and associated canals and drains,
and the airspace over such areas
(Molina and Erwin 2006, p. 284; Parnell
et al. 1995, pp. 4–5). A university-based
study is currently underway in San
Diego Bay to evaluate the foraging
patterns and relative use of areas within
San Diego Bay and the adjacent
coastline; the results of this study are
not yet available.
Gull-billed terns, including van
Rossem’s gull-billed terns, nest in
colonies of 20 to 50 pairs, although
numbers may vary (Parnell et al. 1995,
p. 9). They display low nest-site fidelity;
that is, they are not closely tied to any
one nest site from year to year, even
moving to new sites and renesting
within the same year (e.g., after
disturbance or predation events)
(Parnell et al. 1995, p. 13; Erwin et al.
1998b, p. 970). Groups of van Rossem’s
gull-billed terns have displayed such
renesting behavior at the Salton Sea
´
(Molina 2009b, pp. 6–7) and at Bahıa
´
Santa Marıa (Palacios and Mellink 2007,
p. 218) (Table 1). Van Rossem’s gullbilled terns also readily take advantage
of new nest sites or sites that are not
available every year (for example,
Molina 2005, p. 4; Molina 2009b, p. 2).
Thus, van Rossem’s gull-billed terns
appear to be opportunistic and
adaptable nesters.
The term ‘‘nest colony’’ may refer to
the group of birds or a geographic
location. A nesting location (as used in
Table 1) may contain more than one
colony. In general, a colony consists of
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the terns that occupy a nest site during
a particular nesting attempt. A nest site
is the specific location where a group of
terns is nesting. Individual terns within
a colony may move between nest sites
between nesting attempts within a given
breeding season (within-year
movements). For example, after nest
failure at one nest site, members of a
colony may move within the same
breeding season to one (or more) nest
sites at a different location (or locations)
within the general nesting location
(Molina et al. 2010, p, 17). We also refer
to the groups of individuals that
collectively use nesting locations as
‘‘populations.’’ Even though it appears
that van Rossem’s gull-billed tern
populations return to nesting locations
(the general area), groups of individuals
may establish colonies at different nest
sites within those general areas from
year to year (between-year movements).
Moreover, these populations are not
necessarily fixed over time. Because van
Rossem’s gull-billed terns can fly long
distances, individuals of a population
may move between and among other
populations, more likely occurring
between years but potentially even
within years. For example, between-year
movements among nest locations
(populations) have been observed in the
northern portion of the subspecies’
range where many individual van
Rossem’s gull-billed terns are banded,
which allows specific birds to be
resighted, and thus tracked, over time
(Molina and Garrett 2001, p. 26; Patton
2001, p. 8; Molina 2004, p. 98; Palacios
2010, pp. 12 and 15).
Nests of van Rossem’s gull-billed
terns consist of shallow scrapes with
simple adornments, such as rocks,
shells, or fish bones (Parnell et al. 1995,
p. 10). Although some individuals may
form pairs during migration, breeding
activity reaches its peak when birds
arrive at nesting areas (Sears 1981, p.
192; Parnell et al. 1995, p. 8). The
breeding season generally occurs from
mid-March through August, at least
within the northern portion of its
breeding range (Parnell et al. 1995, pp.
4 and 9). The timing of nest initiation
varies from place to place and year to
year, with some colonies reinitiating
nesting after predation or disturbance
events and moving to other nearby nest
sites (Molina 2009b, pp. 6–7). Such
renesting can occur repeatedly in one
nesting season or birds may simply
abandon nesting at that nesting location
for a given year (Molina 2009b, pp. 6–
7).
Nesting habitat for van Rossem’s gullbilled terns consists of low, open areas
on natural and artificial beaches,
islands, and levees, usually with no or
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sparse vegetation (Parnell et al. 1995,
pp. 5 and 10; Palacios and Mellink
2007, p. 215). Typically, these areas are
located on islands or other remote areas
where the risk of predation is low.
Barren areas suitable as nest sites are
often kept clear by natural or artificial
disturbance regimes, especially tidal
inundation, that prevent or limit plant
growth. Although gull-billed terns
typically nest in areas above most high
tides (Bent 1921, p. 198; Parnell et al.
1995, p 4), it is not uncommon for active
nests to be destroyed by the highest
tides (Erwin et al. 1998b, p. 976;
Peresbarbosa and Mellink 2001, p. 268;
Molina and Erwin 2006, p. 286; Patton
2009, p. 9).
At San Diego Bay and the Salton Sea,
van Rossem’s gull-billed terns typically
lay two to three eggs per clutch (Parnell
et al. 1995, p. 12). The egg incubation
period is 22 to 23 days, and the young
fledge after 28 to 35 days (Parnell et al.
1995, p. 11). Similar to other tern
species (see Dunn 1972, pp. 360–366;
Buckley and Buckley 1974, pp. 1053–
1063; Shealer and Burger 1995, pp. 93–
99), juvenile gull-billed terns remain
dependent upon their parents for at
least 4 weeks after fledging and
probably longer, during which time they
learn to forage and fend for themselves
(Parnell et al. 1995, p. 12). Thus, van
Rossem’s gull-billed terns only raise one
brood per year (Parnell et al. 1995, p. 9);
any subsequent renesting attempts
typically follow a disturbance or
predation event that occurs early within
the breeding season.
Terns that survive to become adults
are generally long-lived (Gochfeld and
Burger 1996, p. 640) with lifespans of 10
to 20 years or even more (such as
Thompson et al. 1997, p. 15; Cuthbert
and Wires 1999, p. 19; Shealer 1999, pp.
17–18; Buckley and Buckley 2002, p. 18;
Hatch 2002, p. 25). Lifespan information
on the entire gull-billed tern species is
limited, with even less known about van
Rossem’s gull-billed tern. Other
subspecies of gull-billed terns are
known to first breed at 5 years old, but
can establish territories at nest sites
without breeding at 4 years old (Parnell
et al. 1995, p. 12). A few van Rossem’s
gull-billed terns of known age have been
observed nesting as 3-year-olds (Molina
et al. 2010, p. 6). Banded gull-billed
terns have been recovered in Europe
almost 16 years post-banding, and 14
years post-banding in eastern North
America (Parnell et al. 1995, p. 12).
Patton (2009, p. 9) noted a banded van
Rossem’s gull billed tern that was at
least 9 years old at the San Diego Bay
colony (and presumably breeding), and
10-year-old birds have been observed at
the Salton Sea (Molina et al. 2010, p. 6).
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We believe the lifespan of van Rossem’s
gull-billed tern to be similar to other
tern species (i.e., 10 to 20 years, possibly
more).
Management Actions
Through our Division of Migratory
Birds Management, the Service is the
lead Federal agency for managing and
conserving migratory birds in the
United States under the Migratory Bird
Treaty Act (MBTA). We provide
national and international leadership in
the conservation and management of
migratory birds by promoting, among
the Service and its partners, sciencebased management of both populations
and habitat on and off Service lands in
support of national and international
bird plans and initiatives.
In 2002 and 2008, pursuant to the
Fish and Wildlife Conservation Act of
1980, as amended (16 U.S.C. 2901 et
seq.), the Service included the gullbilled tern (the species as a whole) in
the list of Birds of Conservation Concern
(USFWS 2002, pp. 1–99; USFWS 2008,
pp. 1–87). The species was included as
a Bird of Conservation Concern both
nationally and in certain specific Bird
Conservation Regions, including the
U.S. portions of Bird Conservation
Regions 32 (Coastal California) and 33
(Sonoran and Mojave Deserts) (USFWS
2008, pp. 48 and 49). The gull-billed
tern subspecies that occurs in Bird
Conservation Regions 32 and 33 is the
van Rossem’s gull-billed tern
(Gelochelidon nilotica vanrossemi).
Conservation and management of van
Rossem’s gull-billed tern is one of the
Service’s regional priorities and
includes the following activities:
(1) Fall 2008—We funded a U.S.
Geological Survey (USGS) project to
clarify taxonomic status of gull-billed
terns in North America and define
population structure and status of the
species throughout its North American
range. Results from this work are
expected in 2011.
(2) September 2009—We held a
structured decisionmaking workshop,
bringing together interested parties to
address potential conflicts between van
Rossem’s gull-billed terns and species
listed under the Endangered Species
Act. Results of this workshop are still in
development.
(3) Spring/summer 2010—We
coordinated van Rossem’s gull-billed
tern population monitoring at Sonny
Bono Salton Sea and San Diego Bay
National Wildlife Refuges. This work
included population monitoring to
determine annual productivity, and
implementing measures to improve
habitat and nesting conditions.
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(4) Spring/summer 2010—The U.S.
Navy along with the Service supported
and is continuing to support universitybased research on foraging behavior of
van Rossem’s gull-billed tern within and
around San Diego Bay, which will
provide insight into main foraging sites
and frequency of visits to foraging sites.
Data analysis is currently underway and
results are not yet available.
Additionally, this work will continue in
2011 and planning is underway to
expand this research to include
migration and winter ecology using
satellite telemetry technology.
(5) Summer 2010—We funded
surveys for nine breeding colonies in
Western Mexico to gain a better
understanding of van Rossem’s gullbilled tern population size and estimate
2010 productivity (Palacios 2010 draft
report).
(6) Summer 2010—We have been and
continue to work on population models
to assess population and metapopulation dynamics of the van
Rossem’s gull-billed tern in California
colonies. Modeling will also evaluate
interactions of van Rossem’s gull-billed
terns with other tern and plover
populations in San Diego Bay. Further
modeling efforts are evaluating effects of
management actions on gull-billed tern
populations with a goal of maintaining
or increasing van Rossem’s gull-billed
tern numbers in California colonies.
(7) Fall 2010—We initiated
coordination with Mexican biologists,
the Sonoran Joint Venture, and the
Cerro Prieto Geothermal Facility to
develop a management plan for the
facility with an emphasis on best
management practices for colonial
nesting seabirds, including van
Rossem’s gull-billed terns. These
discussions and actions will also look
for additional opportunities for
conservation management in Mexico
(e.g., Las Arenitas Sewage Treatment
ponds).
(8) Fall 2010 and 2011—We are
participating in several planning efforts
for habitat restoration projects at the
Salton Sea. Two habitat restoration
projects are in the planning stages (one
by California Department of Fish and
Game (CDFG) and one by Sonny Bono
Salton Sea National Wildlife Refuge).
These planning efforts will emphasize
the development of suitable nesting
habitat for van Rossem’s gull-billed
terns and other colonial nesting birds.
(9) Fall 2010 and 2011—We are
coordinating the development of longterm conservation strategies for the
management of colonial nesting seabirds
in San Diego Bay, including efforts to
balance management of potentially
conflicting species like van Rossem’s
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gull-billed tern, the California least tern
(Sternula antillarum browni), and the
western snowy plover (Charadrius
alexandrinus nivosus).
Summary of Information Pertaining to
the Five Factors
Section 4 of the Act and
implementing regulations (50 CFR part
424) set forth procedures for adding
species to, removing species from, or
reclassifying species on the Federal
Lists of Endangered and Threatened
Wildlife and Plants. Under section
4(a)(1) of the Act, a species may be
determined to be endangered or
threatened based on any of the
following five factors:
(A) The present or threatened
destruction, modification, or
curtailment of its habitat or range;
(B) Overutilization for commercial,
recreational, scientific, or educational
purposes;
(C) Disease or predation;
(D) The inadequacy of existing
regulatory mechanisms; or
(E) Other natural or manmade factors
affecting its continued existence.
In making this 12-month finding,
information pertaining to van Rossem’s
gull-billed tern in relation to the five
factors provided in section 4(a)(1) of the
Act is discussed below. In making our
12-month finding on the petitioned
action, we considered and evaluated the
best available scientific and commercial
information.
In considering what factors might
constitute threats to a species, we must
look beyond the exposure of the species
to a factor to evaluate whether the
species may respond to the factor in a
way that causes actual impacts to the
species. If there is exposure to a factor
and the species responds negatively, the
factor may be a threat and we attempt
to determine how significant a threat it
is. The threat is significant if it drives,
or contributes to, the risk of extinction
of the species such that the species
warrants listing as an endangered or
threatened species as those terms are
defined in the Act.
Factor A. The Present or Threatened
Destruction, Modification, or
Curtailment of Its Habitat or Range
For this factor, we evaluate the
present (current) or threatened
(anticipated) impacts that may affect the
habitat or range of van Rossem’s gullbilled tern. This factor does not address
historical or past actions that resulted in
destruction, modification, or
curtailment of the species’ habitat or
range. Past actions that destroyed,
modified, or curtailed the species’
habitat or range are not threats in and
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of themselves. Any persisting
ramifications of such past actions that
may be threats to the species would be
addressed under Factor E (other natural
or manmade threats), below. However,
under Factor A, we do look to past
actions to inform our evaluation of
potential future threats affecting the
species’ habitat or range in that the
history of past actions allows us to
predict the likelihood of such actions
continuing into the foreseeable future.
As used here, habitat (in its general
sense) is an area that contains the
physical or biological features that are
important to the species’ biological
needs, such as breeding, feeding, or
sheltering. As highly mobile, migratory
birds, van Rossem’s gull-billed terns are
not necessarily confined to one
particular area that contains those
physical or biological features; that is,
individuals may move between or
among areas of habitat. Moreover, as a
subspecies of bird that migrates
seasonally, it breeds in certain areas
during the Northern Hemisphere spring
and summer; it then moves to other
areas where it spends the winter
(although, in some areas, there may be
overlap). Generally, the habitat needs of
van Rossem’s gull-billed tern can be
addressed by grouping its habitat into
two habitat types, (1) Foraging habitat,
which it needs all year, whether during
the breeding season (and within its
breeding range) or during the times it is
not breeding (within its winter range or
while migrating); and (2) nesting
habitat, which it needs for laying eggs
and raising young during the breeding
season.
Van Rossem’s gull-billed tern foraging
habitat, as discussed in the ‘‘Biology’’
section, comprises upland and aquatic
areas, including open mudflats in tidal
estuaries, river margins, beaches, salt
marshes, freshwater marshes,
aquacultural impoundments (such as
shrimp ponds), and a variety of upland
habitats including open scrub,
pasturelands and irrigated agricultural
fields and associated canals and drains,
and the airspace over such areas.
Nesting habitat consists of low, open
areas on natural and artificial beaches,
islands, and levees, usually with no or
sparse vegetation and are typically
located on islands or other remote areas
where the risk of predation is low.
As highly mobile, migratory birds,
van Rossem’s gull-billed terns can
choose among potential nesting
locations and specific nest sites within
those locations. For a nest site to be
suitable, it must have suitable foraging
habitat nearby, among other
considerations. Although it is not
known how gull-billed terns, including
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van Rossem’s gull-billed terns, make
such assessments of foraging habitat
(Biber 1989, p. 89), the available
information suggests that nesting gullbilled terns are typically not food
limited (Erwin et al. 1999, p. 52). In
contrast, breeding black skimmers,
which often nest near gull-billed terns
but eat fish almost exclusively
(Gochfeld and Burger 1994, pp. 4, 12–
13), may often be food limited (Erwin
1977, p. 715). This suggests that the
opportunistic foraging by van Rossem’s
gull-billed tern over a wide range of
foraging habitats allows the subspecies
to have a low sensitivity to impacts to
foraging habitat, even when confined to
smaller geographical areas during the
breeding season. This, in turn, suggests
that the subspecies will have a low
sensitivity to impacts to foraging habitat
during migration and on the wintering
grounds, when van Rossem’s gull-billed
terns are even less geographically
restricted. Moreover, this low sensitivity
to impacts to foraging habitat, as a
natural trait of the subspecies, is
unlikely to change over the foreseeable
future. Because foraging habitat for the
subspecies includes a wide range of
areas and nesting habitat comprises
specific nest sites, nesting habitat for
van Rossem’s gull-billed tern is likely to
be more limited than foraging habitat
under most situations.
United States
Salton Sea—The Salton Sea is a large,
inland lake in the Imperial and
Coachella Valleys and is within the
Sonoran Desert. The Salton Sea, in its
present form, was created in the early
1900s by flooding on the Colorado River
that followed canals dug for irrigation
(see Patten et al. 2003, pp. 1–6 for a
more detailed summary). The Salton Sea
has been maintained since then by
waste irrigation water associated with
extensive agricultural development in
the region. Thus, most of the
development of the region occurred in
the past. Today, the existing agricultural
fields and associated canals serve as
foraging habitat for van Rossem’s gullbilled terns.
However, the amount of water being
used for agriculture has declined
because of an agreement to transfer
water out of the Imperial Valley and
some fields in agricultural production
are being intentionally fallowed to
reduce the amount of water used in the
Imperial Valley (IID 2006, p. 1; IID 2009,
p. 71). Which fields are fallowed is
determined randomly (IID 2006, p. 1), so
we expect fallowed fields to occur over
a wide area in the Imperial Valley and
not concentrated near areas of van
Rossem’s gull-billed tern foraging
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activity. Moreover, the practice of
fallowing as a water conservation
measure is temporary; fallowing will
end after 2018 (IID 2009, p. 72). Over
the time fallowing is to be phased out
other water conservation measures will
likely be enacted in the Imperial Valley,
some of which may affect some areas of
foraging habitat for van Rossem’s gullbilled terns. For example, to conserve
water, there may be increased use of
sprinklers or other irrigation techniques
rather than the predominant current
practice of flooding fields (which makes
crickets, an important food source
(Molina 2009a, p. 1), and other
terrestrial prey items more accessible as
they flee the rising water), even where
van Rossem’s gull-billed terns forage
(IID 2007, pp. 17–19; Schoneman 2010,
in litt. p. 2). However, as noted
previously, van Rossem’s gull-billed
terns are opportunistic foragers—they
concentrate their foraging activity on
easily available food sources (Stienen et
al. 2008, p. 243)—yet they forage on a
wide variety of prey items (Parnell et al.
1995, p. 5). As such, van Rossem’s gullbilled tern foraging habitat includes a
number of areas. Thus, even if some of
the available foraging habitat is
destroyed or modified, it will likely not
affect a substantial amount of van
Rossem’s gull-billed tern foraging
habitat because the subspecies uses a
wide range of areas as foraging habitat
and they are capable of flying to those
areas.
Van Rossem’s gull-billed terns nest at
several different sites (primarily islands)
in the Salton Sea or nearby water
bodies. The subspecies’ use of particular
nest sites varies between and within
years, depending on local conditions.
Nest site conditions within the Salton
Sea vary because the Salton Sea has no
outflow and the elevation of the lake’s
surface depends upon the amount of
water input and loss. Input of water into
the Salton Sea is primarily from
agricultural runoff from nearby Imperial
Valley and, to a lesser extent, Coachella
Valley, with some input also from
natural precipitation, which is variable
and typically scant. Water loss is
through evaporation, which is high in
the desert environment.
Through recent history, shoreline
elevations of the Salton Sea have
fluctuated. As water levels rose, which
was the case through much of the midtwentieth century (Cohen et al. 1999, p.
10), many existing islands became
submerged and were no longer available
for nesting, while other small, higher
points of land (such as former levees)
became new islands. Some of the new
islands then became nest sites for birds,
including van Rossem’s gull-billed terns
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(Molina 2004, p. 96). As water levels
dropped, which has been the case over
the past several years, many of the small
islands (islets), such as those at Johnson
Street, Elmore Desert Ranch, and
Obsidian Butte, have again become part
of the mainland and have become
vulnerable to terrestrial predators, such
as coyotes (Canis latrans), feral dogs (C.
familiaris), or raccoons (Procyon lotor)
(Molina 2003, p. 2; Molina 2004, p. 96;
Molina 2005, p. 5; Molina 2009b, p. 7;
Molina 2010b, in litt., p. 3). The larger
Mullet Island has remained an island
over this time; however, conditions for
nesting of van Rossem’s gull-billed terns
at this site have varied because of other
factors (for example, predation,
competition, or disturbance) (Molina
2004, p. 96).
We expect water levels of the Salton
Sea to continue to drop in the
foreseeable future because the amount
of water used for irrigation in the
Imperial Valley (California) has
declined and has been transferred (sold)
to urban areas outside the region, thus
limiting the amount of agricultural
runoff entering the Salton Sea (IID 2006,
p. 1). As such, even the large Mullet
Island is expected to become attached to
the mainland in the near future (Molina
2010a, p. 9). As the water level drops in
the foreseeable future, it is likely that
most of the historical areas of
topographical relief that were once
islands will not again reemerge because
most of those areas eroded while
inundated (see Molina 2001, p. 97).
However, the dropping water level of
the Salton Sea may allow for new
islands to become exposed, allowing for
novel nest sites for van Rossem’s gullbilled terns, such as one south of
Obsidian Butte used by van Rossem’s
gull-billed terns in 2010 (Molina 2010a,
p. 6).
In addition to those nesting islands
that are or were isolated because of the
waters of the Salton Sea, van Rossem’s
gull-billed terns opportunistically use
nesting habitat on intentionally or
accidentally created islands in artificial
impoundments along the edge or near
the Salton Sea (Molina 2004, p. 93). For
example, the creation of the ‘‘saline
habitat ponds’’ near Hazard Road at the
southeastern corner of the Salton Sea in
2006 (Miles et al. 2009, p. 1), provided
nesting habitat for the subspecies from
2008 to 2010 (Molina 2009b, p. 2;
Molina 2010a, p. 8); the ponds were
dewatered and decommissioned
following the 2010 bird nesting season
(M. Walker, Bureau of Reclamation,
pers. comm. 2010). Another example of
opportunistic use of nesting habitat is
the 2005 nesting of van Rossem’s gullbilled terns at a pond some 25 km (15
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mi) from the Salton Sea that typically is
only full during the winter but had
water during the breeding season that
year (Molina 2005, p. 4). Although the
water levels in such artificial
impoundments may be independent of
the water levels of the Salton Sea, they
are also variable between and within
years. These ponds are dependent upon
artificial water inputs, and the
management of the water levels in some
of these ponds may not necessarily take
into account the needs of waterbirds
that may be nesting, including van
Rossem’s gull-billed terns. Thus, the
literature shows that such ponds have
provided, albeit inconsistently, nesting
habitat for the subspecies.
Additionally, a few nest sites located
on or near the Salton Sea are managed
for nesting waterbirds, including van
Rossem’s gull-billed tern, especially
those on the Sonny Bono Salton Sea
National Wildlife Refuge (Salton Sea
Refuge) (Schoneman 2010, in litt., p. 1).
Even so, the status of the nest sites on
the Salton Sea Refuge is not assured
over the long term because the Refuge
must purchase the water to maintain the
ponds that allow for the existence of the
nesting islands and adequate funding is
not guaranteed (C. Schoneman, Sonny
Bono Salton Sea National Wildlife
Refuge, pers. comm. 2010). Moreover,
the availability of the water itself is not
guaranteed; for example, during a water
shortage emergency, water availability
may be limited. Nevertheless, the Salton
Sea Refuge has consistently managed its
wetlands to support nesting van
Rossem’s gull-billed terns since 1995
(Molina 2004, p. 97; Schoneman 2010,
in litt., p. 1). Additionally, artificial
nesting platforms have been used at the
Salton Sea Refuge to provide additional
nest sites for van Rossem’s gull-billed
terns and other waterbird species
(Molina 2006, p. 3; Molina et al. 2009b,
p. 267). This or other management
actions could potentially be used to
provide additional nest site options for
van Rossem’s gull-billed terns at the
Salton Sea, even without the availability
of water for artificial ponds.
In summary, at the Salton Sea, even
if some of the available foraging habitat
is destroyed or modified, it will likely
not affect a substantial amount of van
Rossem’s gull-billed tern foraging
habitat because the subspecies uses a
wide range of areas as foraging habitat
and the birds are capable of flying to
those areas. We anticipate some loss of
existing nesting habitat at the Salton Sea
because the Sea’s decreasing water level
will reduce the number of nesting
islands that the subspecies has
traditionally used over the past 10 to 20
years. However, the lowering water
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level may result in the exposure of new
islands that may serve as nesting
habitat, as was shown in 2010.
Additionally, van Rossem’s gull-billed
terns have opportunistically used
suitable nesting habitat in artificial
impounds near the Salton Sea, even
though such habitat may only occur
from time to time. Thus, we expect
some reduction in the amount of nesting
habitat (i.e., a reduction in the number
of nest site options), but we do not
expect complete elimination of nesting
habitat in the region. The anticipated
reduction in the amount of nesting
habitat may force van Rossem’s gullbilled terns to nest in areas where
predation, disturbance, or other threats
may be more likely, potentially resulting
in lowered productivity of the
subspecies at this nesting location.
These potential threats are addressed in
the other factors, below.
San Diego Bay—The region around
San Diego Bay is highly urbanized,
nearly built-out, as a result of past
development, most of which occurred
before the subspecies colonized the
region in 1987. Much of south San
Diego Bay itself was developed for salt
production. Such areas of salt
production, or ‘‘saltworks,’’ comprise a
network of dikes that creates a series of
ponds from which water evaporates,
which leaves an ever-concentrating
solution of sea salt that is eventually
dried and harvested. The San Diego Bay
saltworks area is now part of the greater
San Diego Bay National Wildlife Refuge.
Many of the areas of foraging habitat for
the subspecies, such as the areas around
San Diego Bay (including San Diego Bay
National Wildlife Refuge, Silver Strand
State Beach, and certain lands owned or
operated by the U.S. Navy) and the
Tijuana River estuary (including Tijuana
Slough National Wildlife Refuge and
Borderfield State Park) (Patton 2009, pp.
10–11 and Figure 2), are largely
protected from future development. As
such, substantial destruction or
modification of foraging habitat in the
San Diego Bay region is not occurring
currently nor is it likely to occur in the
foreseeable future.
Potential nesting habitat for van
Rossem’s gull-billed terns occurs in
undeveloped areas in and around San
Diego Bay; nearly all occupied nest sites
are located on the saltworks dikes on
San Diego Bay National Wildlife Refuge
lands (Patton 2009, p. 8). These nesting
sites are protected and managed to
benefit several species of colonial
waterbirds, including van Rossem’s
gull-billed terns (USFWS 2006, pp. 1–
36). Thus, destruction or modification of
nesting habitat by urban development is
not a significant threat to the San Diego
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Bay colony of van Rossem’s gull-billed
terns.
Mexico
The availability of information on
specific nesting locations in Mexico
(Table 1; Figure 1) is variable and
generally less detailed than what is
available for nesting locations in the
United States. Using the information
available, the following discussion
provides our assessment of the status of
van Rossem’s gull-billed tern foraging
and nesting habitat at the locations in
Mexico. We are not aware of any van
Rossem’s gull-billed tern nesting
locations south of Mexico in Central
America.
´
Campo Geotermico Cerro Prieto—The
setting at this location is very similar to
the Salton Sea and has a comparable
history of agricultural development
(Furnish and Ladman 1975, pp. 84–88;
Molina and Garrett 2001, p. 23). Given
the similarity to the Salton Sea, foraging
by van Rossem’s gull-billed terns likely
occurs in the agricultural fields, along
the canals and drains in the area, and
over the neighboring desert (Molina and
Garrett 2001, pp. 23, 25, and 27;
Erickson et al. 2009, p. 508). The area
is not subject to the same water
agreements as the Imperial Valley. The
available literature does not identify any
significant threats to van Rossem’s gullbilled tern foraging habitat in the region
now or in the foreseeable future.
Van Rossem’s gull-billed terns nest on
islands in artificial ponds created by the
dumping of wastewater (brine) from the
geothermal electrical generation facility.
Since 1996, Cerro Prieto has grown to be
one of the larger populations of van
Rossem’s gull-billed terns (Molina and
Garrett 2001, p. 25; Palacios and
Mellink 2007, pp. 215–216). Recent
information suggests the facility is
managing its brine differently, reducing
the amount of water in the ponds,
thereby reducing the available nesting
habitat for van Rossem’s gull-billed
terns (Molina 2010b, in litt., p. 4;
Palacios 2010, pp. 11–14). However, we
do not know if this situation is
permanent and, as of 2010, the nesting
location still had areas of nesting habitat
(Palacios 2010, pp. 11–14).
Additionally, about 100 van Rossem’s
gull-billed terns were seen at the ‘‘new
Las Arenitas sewage ponds, near Cerro
Prieto’’ (Erickson et al. 2009, p. 508),
but these were likely birds from Cerro
Prieto and there was no evidence of
nesting observed at this site (R.
Erickson, Regional Editor, North
American Birds, 2010, pers. comm.).
The conditions at Cerro Prieto
illustrate the difficulty in accurately
assessing long-term threats to van
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Rossem’s gull-billed tern related to
management of artificial water
impoundments because these areas are
managed for reasons other than
maintaining nesting habitat. Because of
the combination of the loss of suitable
nesting habitat at Cerro Prieto proper,
and the uncertainty over the subspecies’
use of the new Las Arenitas ponds, we
are unable to predict the future of this
population at this nesting location;
however, because van Rossem’s gullbilled terns can opportunistically use
nesting habitat even under changing
conditions (see above), it is unlikely that
all nesting at this nesting location will
cease in the foreseeable future.
Isla Montague—Isla Montague, a
large, low island in the Colorado River
delta at the north end of the Gulf of
California in Baja California, is part of
the breeding range of the subspecies,
although some birds may winter there,
too (Molina et al. 2009a, p. 9). This area
is within the protective core zone of the
´
Alto Golfo de California y Delta del Rıo
Colorado Biosphere Reserve
(Peresbarbosa and Mellink 2001, p. 265).
Foraging habitat includes the deltaic
and coastal areas around the island,
including nearby aquacultural shrimp
ponds (Palacios and Mellink 2006, p.
60). Conversion of areas to shrimp
aquaculture may destroy or modify
areas of natural foraging habitat, but it
also is likely to result in manmade
foraging habitat that can have
concentrated prey, especially during
periods of shrimp harvest (Molina et al.
2009a, p. 12). As such, the development
of shrimp aquaculture is likely not a
substantial impact to van Rossem’s gullbilled tern foraging habitat here or
elsewhere in the subspecies’ overall
range.
Since 1992, when nesting was first
confirmed at Isla Montague, incomplete
though somewhat consistent data show
that the nesting habitat on this island
has supported as few as 30 and up to as
many as 200 breeding pairs of nesting
van Rossem’s gull-billed terns (Palacios
and Mellink 2007, p. 217; Molina et al.
2010, p. 61). This population was larger
in 2010, potentially because birds from
Cerro Prieto, the Salton Sea, or both,
relocated to this nesting location
(Palacios 2010, pp. 14–15). Moreover,
the nesting habitat at this site is low in
elevation and subject to flooding during
extreme high tides (Peresbarbosa and
Mellink 2001, pp. 267–268). Although
such flooding is a potential threat to
eggs or young (see Factor E), it does
suggest that substantial manmade
developments here are unlikely.
Therefore, we do not anticipate
destruction or modification of nesting
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habitat to be a significant threat at this
location.
Laguna Ojo de Liebre—This site is a
large lagoon along the Pacific coast of
the Baja California Peninsula in the
northwest corner of Baja California Sur.
´
The area is within the El Vizcaıno
Biosphere Reserve (Palacios 2010, p. 6).
Associated with this lagoon is the
salinas de Guerrero Negro (Guerrero
Negro saltworks), an extensive system of
artificial ponds used in the salt-making
process. Foraging habitat in the region is
likely within the greater lagoon area,
including portions of the saltworks, and
the nearby coastal areas and uplands.
Small islands within the network of
ponds provide potential nesting habitat
for colonial waterbirds, including a
small number of van Rossem’s gullbilled terns (Danemann and Carmona
2000, p. 197; Palacios and Mellink 2006,
p. 49; Palacios 2010, p. 16). Although
this nesting location is noteworthy
because it is the only one on the Baja
California Peninsula, the small number
(4 to 14 breeding pairs) of van Rossem’s
gull-billed terns that nest here do not
represent a significant number of birds
relative to the overall population of the
subspecies. The available information
does not suggest that this area is used
by van Rossem’s gull-billed terns during
the winter.
Foraging habitat in the region is likely
within the greater lagoon area, including
portions of the saltworks, and the
nearby coastal areas and uplands.
Although some future development is
possible, especially near the community
of Guerrero Negro, we do not anticipate
substantial destruction or modification
of van Rossem’s gull-billed tern foraging
habitat in this area because much of the
area away from Guerrero Negro and the
saltworks is largely uninhabited and the
area is designated a biosphere reserve,
which may limit any potential future
development. Even if some
development occurs, it will likely not
affect a substantial amount of van
Rossem’s gull-billed tern foraging
habitat because the subspecies uses a
wide range of areas as foraging habitat.
As suggested by the ponds at Cerro
Prieto, we should not consider the
islands associated with the saltworks
permanent; however, the available
information suggests that significant
changes in management are unlikely
over the foreseeable future (Palacios and
Mellink 2006, p. 54; Palacios 2010, p.
16).
´
´
Bahıa Santa Marıa—This area is a
large and extensive coastal lagoon
system with long barrier beaches in
Sinaloa. Foraging habitat in this area
likely includes the greater lagoon,
including areas of shrimp aquaculture;
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the coastline; and nearby agricultural
areas. The nesting habitat for van
´
Rossem’s gull-billed terns at Bahıa Santa
´
Marıa comprises two low, sandy islands
(and associated small islets), Isla El
Rancho and Isla Altamura, which are
part of the lagoon’s barrier islands
(Palacios and Mellink 2007, p. 218;
Palacios 2010, p. 19). Shrimp
aquaculture occurs within the large bay,
and agriculture occurs in nearby
uplands (Engilis et al. 1998, p. 333;
DeWalt 2000, pp. 61–62), but the
operations appear to be located in areas
at some distance from the nesting
islands (Robadue and Villalba 2001, p.
2). The Bahia Santa Maria nesting area
is included in the Islas del Golfo de
California (Gulf of California Island)
Park System and the Santa Maria Bay
Ecosystem Management Program
(Molina et al. 2010, p. 17; Palacios 2010,
p. 7). Areas within this lagoon are being
conserved for shorebirds and other
wildlife species through efforts of
nongovernmental organizations
(Robadue and Villalba 2001, p. 2; ABC
2007, p. 1). Together, these protections
restrict the destruction of nesting and
foraging habitat for van Rossem’s gullbilled terns in the lagoon. Loss or
modification of van Rossem’s gull-billed
tern nesting or foraging habitat at Bahia
Santa Maria does not appear to be a
significant threat now or in the
foreseeable future.
´
Bahıa de Ceuta—This site is a large,
long, coastal lagoon with barrier beaches
in Sinaloa. Foraging habitat in this area
likely includes the greater lagoon,
including areas of shrimp aquaculture;
the coastline; and nearby agricultural
areas. The area of van Rossem’s gullbilled tern nesting habitat appears to be
at the south end of the lagoon near an
area of artificial impoundments
´
(Gonzalez-Medina and Guevara-Medina
˜
2008, p. 7). Munoz del Viejo et al. (2004,
p. 197), describing perhaps the same
location from a study of other species of
nesting terns, identifies the area as ‘‘a
long-abandoned saltflat’’ (salt
production area or saltworks). The
nesting habitat at this site is low in
elevation and subject to flooding during
extreme high tides, which makes
substantial manmade developments
´
here unlikely. Gonzalez-Medina and
Guevara-Medina (2008, p. 7) have stated
that there seem to be no direct
anthropogenic threats to the nesting
habitat at this site. However, the
population of van Rossem’s gull-billed
terns at this nesting location is very
small, consisting of less than 10
individuals and only 1 nest was
´
observed in 2006 (Gonzalez-Medina and
Guevara-Medina 2008, p. 6); the nesting
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site, although apparently still present,
was not occupied in 2010 (Palacios
2010, pp. 20–21). Therefore, the
available information suggests that the
nesting and foraging habitat for van
Rossem’s gull-billed tern at this location
is currently not likely to be destroyed or
modified now or in the foreseeable
future; however, this nesting location
appears to be only intermittently
occupied by a very small population of
van Rossem’s gull-billed terns.
Laguna del Caimanero—This site is a
moderate-size lagoon in Sinaloa.
Foraging habitat in this area likely
includes the greater lagoon, including
areas of shrimp aquaculture; the
coastline; and nearby agricultural areas.
In 2005, the nesting area for van
Rossem’s gull-billed terns was located
on the southeastern part of the lagoon
on a large, dry, mudflat-island
surrounded by tidal channels (Palacios
and Mellink 2006, p. 66). In 2010, the
terns used a different mudflat-island, as
well as a dredge-spoil island (Palacios
2010, pp. 21–22) for nesting, which
indicates that multiple areas of nesting
habitat are available in the vicinity. Past
agricultural development of the
surrounding areas has altered the
landscape, vegetation, and surface flows
of water around the lagoon, leading to
increased siltation within the lagoon
(Ruiz-Luna and Berlanga-Robles 1999,
p. 37). Additionally, shrimp aquaculture
is practiced within the lagoon (Galindo
et al. 1997, p. 1072), including near the
nest sites (Palacios and Mellink 2006, p.
66).
The lagoon is artificially channelized,
which has increased siltation in the
southeast portion of the lagoon
´
(Hernandez-Cornejo and Ruiz-Luna
2000, p. 604), which in turn may have
contributed to the formation of the
mudflat-island nest sites. Such islands
likely flood during high tides in winter
(Palacios and Mellink 2006, p. 66),
which may increase habitat quality
because vegetation growth is inhibited.
However, high tides may also inundate
the nest sites during the breeding season
(Palacios 2010, p. 22), washing away
eggs or young chicks. Additionally,
fishermen used the 2005 mudflat-island
nest site to beach small boats, and they
erected a small, palapa-like shade
structure in the vicinity (Palacios and
Mellink 2006, p. 66). Given the limited
information we have regarding the
current and future human activities
within this nesting location and
variability of use by the van Rossem’s
gull-billed tern, we determine that the
destruction or modification of nesting or
foraging habitat is not a significant
threat at this location now or in the
foreseeable future.
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Marismas Nacionales—A portion of
this large, extensive lagoon system in
northwestern Nayarit, called Marismas
Nacionales Nayarit, has recently been
designated a Natural Protected Area, in
the Biosphere Reserve category, while
the Sinaloa portion of the lagoon has
been proposed for protection (E.
Palacios, pers. comm. 2010). Foraging
habitat in this area likely includes the
greater lagoon (including areas of
shrimp aquaculture, the coastline, and
nearby agricultural areas), and we
determine that the destruction or
modification of foraging habitat is not a
significant threat at this location now or
in the foreseeable future. Nesting habitat
for van Rossem’s gull-billed terns at this
´
large site includes Estero Teacapan,
which consists of a barrier beach at the
mouth of the lagoon, and some low,
small islands in Laguna Pericos.
Because the nesting habitat at Estero
´
Teacapan is at the mouth of the lagoon
on the barrier beach where natural
forces are likely to cause changes in the
landscape on a regular basis, it is
unlikely to be lost due to large-scale
development. However, the nesting area
is subject to lesser impacts resulting
from smaller human activities that
might affect the nesting habitat of van
Rossem’s gull-billed terns. The nesting
colony in 2003 appeared to be in use
despite the presence of a palapa-style
shade structure used by fishermen
(Palacios and Mellink 2006, p. 71).
The Laguna Pericos nesting area is
within a portion of the lagoon that has
been altered to promote shrimp harvest,
including the creation of ponds for
´
shrimp aquaculture (Hernandez-Cornejo
and Ruiz-Luna 2000, p. 604). Further
alteration of the area is possible for
development of shrimp fisheries and
aquaculture. Although such potential
alterations may affect van Rossem’s gullbilled tern nesting habitat, individual
van Rossem’s gull-billed terns readily
move between and among specific nest
sites, including manmade areas that
provide habitat. Because the Marismas
Nacionales area is very large with
multiple small islands, sand bars, and
manmade levees and thus suitable
alternative nest sites, we expect this
nesting population has the option to
move to other available sites to nest, if
necessary. Therefore, we determine that
destruction or modification of nesting
habitat is not a significant threat to the
van Rossem’s gull-billed tern at this
location now or in the foreseeable
future.
´
Laguna Cuyutlan—Compared to the
extensive lagoons in Sinaloa and
´
Nayarit, Laguna Cuyutlan in the
Mexican State of Colima is relatively
small, but it is the largest lagoon in a
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roughly 1,150-km (700-mi) stretch of
´
coastline (Mellink and Riojas-Lopez
2009, p. 1). Foraging habitat in this area
likely includes the greater lagoon
(including areas of shrimp aquaculture,
the coastline, and nearby agricultural
areas), and we determine that the
destruction or modification of foraging
habitat is not a significant threat at this
location now or in the foreseeable
future. Nesting habitat for van Rossem’s
gull-billed terns consists of a number of
small natural and artificial islands in
the lagoon (Palacios and Mellink 2006,
pp. 77–84). The lagoon is divided into
several subareas. The northwesternmost
portion of the lagoon is dredged
regularly to provide shipping access for
the industrial port city of Manzanillo
and is subject to oil spills and
additional development (Mellink and
´
Riojas-Lopez 2009, pp. 5–7). One island
used by nesting van Rossem’s gull-billed
terns in 2005 is located in this portion
of the lagoon (Palacios and Mellink
2006, p. 83). This island was created as
a byproduct of past dredging (Palacios
and Mellink 2006, p. 83). The other
islands used for nesting by van
Rossem’s gull-billed terns in 2005 are
located in a shallower portion of the
lagoon to the southeast. The nest site
near Manzanillo is likely to be
destroyed by future dredging or other
port-improvement or development
projects. The other nesting area used by
van Rossem’s gull-billed terns is in a
portion of the lagoon at some distance
from Manzanillo, and we determine that
development is not likely to
significantly threaten nesting habitat for
van Rossem’s gull-billed terns in this
portion of the lagoon in the foreseeable
future.
´
Laguna Potosı—This site is a
relatively small lagoon system in
Guerrero. Foraging habitat in this area
likely includes the greater lagoon
(including areas of shrimp aquaculture,
the coastline, and nearby agricultural
areas), and we determine that the
destruction or modification of foraging
habitat is not a significant threat at this
location now or in the foreseeable
future. The nesting habitat for van
Rossem’s gull-billed terns at this
location consists of low areas of salt
flats (Mellink et al. 2009, p. 44). The
nest site is subject to flooding during
high rains (which typically occur during
the latter part of the nesting season), but
the best available information suggests
the nest site is located away from
human activities and is, therefore,
protected from loss or modification
(Mellink et al. 2009, p. 51); thus, this
area does not appear to be significantly
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threatened with development now or in
the foreseeable future.
Other Areas of West Mexico and Central
America
We are not aware of any current
(confirmed) nesting locations south of
´
Laguna Potosı, Mexico. Although areas
of far-southern Mexico and Central
America may potentially be within the
breeding range of the species, Molina et
al. (2009a, p. 15) suggest that it is
unlikely that ‘‘appreciable’’ breeding
populations occur south of the Isthmus
of Tehuantepec. Therefore, even if
habitat destruction and modification is
occurring in this region, it is not a
significant threat to van Rossem’s gullbilled tern now or in the foreseeable
future.
During the nonbreeding season, when
the subspecies is migrating or is within
its winter range, van Rossem’s gullbilled terns may use other sites along
the Pacific coasts of Mexico and
(possibly) Central America. Foraging
habitat may include a wide array of
areas. As noted in the ‘‘Biology’’ section,
above, van Rossem’s gull-billed terns are
opportunistic, often focusing on easy-tocatch prey items. For example, in
western Mexico, wintering van
Rossem’s gull-billed terns were
observed foraging at aquacultural
shrimp ponds where prey is
concentrated (Molina et al. 2009a, p.
12). Tidal flats and seasonally flooded
flats were also found to be widely used
as foraging areas during the winter
(Molina et al. 2009a, p. 8). Although
coastal development is occurring
(Molina et al. 2009a, p. 14), there are
other areas that have been designated or
are proposed to become designated as
Natural Protected Areas, including
biosphere reserves, where development
is less likely (see Factor D).
Additionally, as noted above, the
development of shrimp aquaculture
does not necessarily result in the
elimination of foraging habitat.
Moreover, the subspecies is not tied to
any one particular geographical area or
even to any one type of foraging area
within its winter range. Thus,
destruction or modification of van
Rossem’s gull-billed tern foraging
habitat in western Mexico is not likely
a significant threat now, nor is it likely
to be within the foreseeable future.
It is unclear whether or to what extent
van Rossem’s gull-billed terns actually
winter in Central America. Even if they
do occur there, Molina et al. (2009a, p.
15) suggest that it is unlikely that
‘‘appreciable’’ wintering populations
occur south of the Isthmus of
Tehuantepec, Mexico. Moreover, the
subspecies is not tied to any one
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particular geographical area or even to
any one type of foraging area within its
winter range. Thus, it is unlikely that
the subspecies would be significantly
affected by any destruction or
modification of its foraging habitat in
Central America now or within the
foreseeable future.
Summary of Factor A
Van Rossem’s gull-billed tern foraging
habitat includes a wide range of areas,
including wetlands and uplands, and
van Rossem’s gull-billed terns forage
opportunistically within these areas.
Moreover, van Rossem’s gull-billed
terns are highly mobile, capable of
locating and utilizing different foraging
areas. Loss or modification of foraging
habitat does not appear to be a
significant threat to van Rossem’s gullbilled tern for the south San Diego Bay
population, and a wide range of foraging
habitat at Salton Sea will be maintained
such that losses or modification of some
foraging habitat areas do not constitute
a significant threat to the Salton Sea
population. The assessment of loss or
modification of foraging habitat in
Mexico and Central America is more
difficult to determine because the
quantity and specificity of the available
information is variable across the
region. It is even questionable whether
the subspecies occurs south of the
Isthmus of Tehuantepec in southern
Mexico. However, because of the
subspecies’ ability to forage in a wide
range of areas, including areas
developed for aquacultural shrimp
ponds, the subspecies is less susceptible
to destruction and modification of its
foraging habitat. Additionally, it is not
likely that the foraging areas in Mexico
and Central America will be
substantially affected by development,
in part because many areas have some
level of legal protection. Therefore, we
conclude that destruction or
modification of foraging habitat is not a
significant threat to van Rossem’s gullbilled tern throughout its range now or
in the foreseeable future.
The amount of nesting habitat for van
Rossem’s gull-billed tern is more
limited. In the United States, nesting
habitat in San Diego Bay is protected
and managed by the San Diego Bay
National Wildlife Refuge. The
population of van Rossem’s gull-billed
terns in the San Diego Bay nesting
location has increased since the early
1990s and is now expanding to other
areas of protected nesting habitat
outside of the Refuge. At the Salton Sea,
the amount and distribution of nesting
habitat has varied through time with
nest sites being lost and added with
changing conditions (primarily the
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water level of the Salton Sea, but also
the availability of manmade
impoundments that intentionally or
accidentally have areas suitable for
nesting). Although the continued
existence of individual nest sites into
the foreseeable future is unknown, the
evidence suggests that, even under
changing conditions, it is unlikely that
all nesting habitat would be lost.
Moreover, the Sonny Bono Salton Sea
National Wildlife Refuge, which has
regularly harbored several colonies of
nesting van Rossem’s gull-billed terns,
including the consistently productive D
pond, has been actively managing for
the benefit of van Rossem’s gull-billed
tern by creating and maintaining areas
of nesting habitat, including artificial
nesting platforms. Although we
acknowledge that Salton Sea Refuge
may not always be able to provide the
same type or same level of management
every year, its record of accomplishment
for more than 15 years suggests that
continued beneficial management will
likely continue into the foreseeable
future. Therefore, we determine that
nesting habitat for the Salton Sea
population of the van Rossem’s gullbilled tern is not significantly
threatened by permanent loss or
destruction.
In Mexico, the available information
on nesting habitat is not as extensive
and is less detailed than U.S. data, but
it suggests that many nesting habitat
areas are located in protected areas and
are not likely to be destroyed or
substantially modified, while other
areas are subject to loss from habitat
destruction or modification. The nest
sites at Isla Montague, Marismas
´
´
Nacionales, and Bahıa Santa Marıa are
located within protected areas.
Moreover, the nest sites at these nesting
locations, along with the nest sites at
Isla Montague, Laguna Ojo de Liebre,
´
Bahıa de Ceuta, Laguna del Caimanero,
´
and Laguna Potosı, are situated on low
islands that are subject to flooding
during winter storms or high tides; as a
result, substantial manmade
developments on the islands are
unlikely. The nest sites at Cerro Prieto
are dependent on the management of
waste water at the geothermal
generation facility, which is uncertain at
this time; some van Rossem’s gull-billed
terns from this nesting location may
have moved and nested at Isla Montague
in 2010 in response to changes in the
amount of available habitat at Cerro
´
Prieto. Portions of Laguna Cuyutlan near
port operations may be subject to
dredging activities, which may destroy
existing areas of nesting habitat for van
Rossem’s gull-billed terns but may also
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result in the creation of dredge-spoil
islands that may serve as additional
nesting habitat. The portions of Laguna
´
Cuyutlan away from the port are less
likely to be destroyed. Thus, most of the
van Rossem’s gull-billed tern nest sites
in Mexico are not likely to be
substantially destroyed or modified.
Moreover, because van Rossem’s gullbilled terns are resilient and can move
from one area of nesting habitat to
another, the loss of a limited amount of
nesting habitat will not likely
significantly affect the species.
Based on our review of the best
available scientific and commercial
information, we conclude that van
Rossem’s gull-billed tern is not
threatened by the present or threatened
destruction, modification, or
curtailment of its habitat or range now
or in the foreseeable future.
Factor B. Overutilization for
Commercial, Recreational, Scientific, or
Educational Purposes
Within the context of this factor,
overutilization is the capture or
collection of individuals of a species,
including its eggs or young, to an extent
(at a high enough rate) that it affects the
conservation status of the species. We
are not aware of any information
suggesting that adult van Rossem’s gullbilled terns are utilized (collected,
harvested) or will likely be utilized in
the foreseeable future for commercial,
recreational, scientific, or educational
purposes anywhere in the subspecies’
range (but see the ‘‘Intentional Killing’’
section under Factor E). The
information available to us regarding
capture or collection of eggs or chicks of
van Rossem’s gull-billed terns in the
United States indicates that risks to the
species from overutilization for
commercial, recreational, scientific, or
educational purposes is not a significant
threat, and we determine that this factor
will not become a significant threat to
the occurrences of van Rossem’s gullbilled tern in the United States in the
foreseeable future.
In western Mexico, egging, the
collection of wild bird eggs by people
for subsistence or other uses, has
occurred historically (for example, see
Mailliard 1923, pp. 443–456). More
recently, egging activities at Guerrero
Negro in the 1970s, prior to the first
known nesting of van Rossem’s gullbilled terns at this location, was so
severe that nesting waterbirds were
extirpated from several islands
(Castellanos et al. 2001 p. 367).
However, the available information on
the current impacts of egging or other
utilization activities on van Rossem’s
gull-billed tern lacks specificity and is
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somewhat conflicting. Molina et al.
(2010, p. 13) stated that this activity is
not a threat to van Rossem’s gull-billed
tern and Palacios (2010, p. 14) states,
‘‘Other than tidal flooding, no evident
direct threats were documented for this
colony.’’ However, Palacios and Mellink
(2006, p. 60) noted in a general
statement that egging occurred at Isla
Montague at some unspecified time in
the past and postulated that it could
occur again, but they did not provide
specific information on whether egging
activities had affected or were affecting
van Rossem’s gull-billed tern nests.
Thus, the likelihood of this threat
affecting the subspecies at this nesting
location is not clear, but none of the
information available suggests that
utilization occurs or is likely to occur
with any appreciable frequency.
Mellink et al. (2009, p. 51) also
considered egging as a potential threat
´
in Laguna el Potosı, should the colony
there be discovered by the human
inhabitants of the area, but again, the
authors did not provide specifics on the
likelihood of it affecting the subspecies.
˜
Munoz del Viejo et al. (2004, p. 196)
documented egg collection of royal terns
´
´
(Sterna maxima) at Bahıa Santa Marıa,
and in the same area they noted that
blue-footed booby (Sula nebouxii)
chicks had been taken by fishermen and
˜
used for bait (Munoz del Viejo et al.
´
2004, p. 196). However, at Bahıa Santa
´
Marıa, we have no available information
indicating that van Rossem’s gull-billed
terns were targeted for either activity.
˜
Additionally at this location, Munoz del
Viejo et al. (2004, p. 199) reported that
they successfully worked with the local
inhabitants to stop this practice, but
there are no assurances that such
activities could not again occur.
Thus, in Mexico, egging and other
forms of utilization have not been
specifically documented to impact van
Rossem’s gull-billed tern; however,
egging has affected, to varying extents,
other species of birds that can and do
nest close to where van Rossem’s gullbilled terns nest. This suggests egging
and other forms of utilization, regardless
of purpose, are a potential threat to van
Rossem’s gull-billed terns. We expect
such utilization—should it occur at a
van Rossem’s gull-billed tern nest
colony—would result in complete
reproductive failure for the affected nest
colony. However, like a nest
depredation event, the adult terns
would likely survive to nest again in the
future nesting seasons or, potentially, to
renest that same season (see Factor C,
below, for more details). The available
information does not suggest that such
utilization activities are occurring to an
extent (at a high enough rate) for it to
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affect the conservation status of the
species. Thus, we conclude that
overutilization for any purpose is not
significantly affecting van Rossem’s
gull-billed tern in Mexico at the present
time, nor do we expect it to be a
significant threat in the foreseeable
future.
Therefore, based on our review of the
best scientific and commercial
information available, we conclude that
van Rossem’s gull-billed is not
threatened by overutilization for
commercial, recreational, scientific, or
educational purposes now or in the
foreseeable future.
Factor C. Disease or Predation
Disease
Diseases occur naturally in wildlife
populations. The occurrence of a
disease within the range of a species
does not necessarily mean that it is
deleterious to that species. However, if
one or more diseases are virulent
enough, the conservation status of a
species may be affected. The
susceptibility of van Rossem’s gullbilled tern to disease has not been well
studied, but multiple diseases impacting
avian populations are present in the
areas where van Rossem’s gull-billed
terns nest. Avian botulism, avian
cholera, and other diseases have
impacted thousands of fish-eating birds
at the Salton Sea (Friend 2002, pp. 295,
303), including an outbreak of avian
botulism that killed more than 14,000
birds in the mid-1990s (Roberts 1997, p.
2). Throughout those and other disease
outbreaks at the Salton Sea, the
population of van Rossem’s gull-billed
terns at this location appeared to be
unaffected (Molina 2004, p. 98; Molina
et al. 2010, pp. 14 and 66). This is
probably because van Rossem’s gullbilled terns do not depend solely upon
fish for food and, at the Salton Sea, they
primarily forage for crickets (Molina
2009a, p. 1). Because of their diverse
foraging habits, van Rossem’s gull-billed
terns appear less likely to be exposed to
diseases like avian botulism and avian
cholera.
A serious disease threat to avian
populations in North America is West
Nile Virus (WNV). WNV has caused
significant declines in bird populations
since its arrival in the United States in
1999 (LaDeau et al. 2007, p. 711).
Originally detected in New York, the
disease was first detected in California
in 2003 in the Imperial Valley, and was
present at the Salton Sea in the late
summer of 2003 and in the San Diego
region by autumn (Reisen et al. 2004, p.
1371). The impact of WNV on van
Rossem’s gull-billed tern, and
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charadriiform waterbirds in general, has
not been assessed. Charadriiform
waterbirds are susceptible to WNV
infection, with carcasses confirmed
positive for WNV in California (Eidson
et al. 2001 p. 617; Komar et al. 2003, p.
313), including a California least tern
(Sternula antillarum browni) (Foster in
litt. 2008). The closest related species to
van Rossem’s gull-billed tern that
researchers have examined for
susceptibility to WNV is the ring-billed
gull (Larus delawarensis). In a
laboratory study, ring-billed gulls
showed high mortality and viral loads
when exposed to WNV (Komar et al.
2003, p. 313). However, this may not be
a good predictor of how van Rossem’s
gull-billed tern might be affected by
WNV because variance between species
in disease response is high (LoGludice
et al. 2003, pp. 568–569), and lab tests
of WNV have proven to be
undependable predictors of conditions
in the field (Walker et al. 2007, p. 694).
Thus, if van Rossem’s gull-billed terns
were particularly susceptible to WNV or
other diseases in the wild, we would
expect to see a marked decline in
populations of van Rossem’s gull-billed
terns that have been exposed to the
disease, as have been observed in other
bird species (LaDeau et al. 2007, p. 710).
As noted above, WNV has been
present at the two U.S. van Rossem’s
gull-billed tern nesting locations (Salton
Sea and San Diego Bay) since 2003.
Although van Rossem’s gull-billed tern
numbers at the Salton Sea have
fluctuated over the past decade, their
overall population size has remained
fairly stable since the arrival of WNV to
the region (K. Molina, in litt. 2010, p. 3).
Meanwhile, the San Diego Bay
population increased over that time
(Patton 2009, Table 2). Had van
Rossem’s gull-billed tern been
substantially affected by WNV, these
two populations would have shown a
decline when the disease arrived in
their respective regions. The
information available shows that these
two well-monitored populations did not
decline. This indicates that the U.S.
population of van Rossem’s gull-billed
terns is not significantly threatened by
WNV now or in the foreseeable future.
Further, it suggests that the subspecies
as a whole is not likely to be
substantially affected by the disease.
The amount of information on the
prevalence of WNV in western Mexico
is limited, but there is some indication
that the disease has been recorded there
(Komar and Clark 2006, p. 114).
Although the population data for van
Rossem’s gull-billed terns in Mexico is
limited, there is no indication of marked
population decline. Nevertheless, as in
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the United States where evidence of
substantial effects of the disease on van
Rossem’s gull-billed tern is lacking, we
similarly expect no significant effects to
populations of the subspecies in
western Mexico from WNV.
Unlike other bird species that are
sensitive to WNV, such as American
crow (Corvus brachyrhynchos) and
greater sage-grouse (Centrocercus
urophasianus) that experienced
substantial population declines from
WNV (Reisen et al. 2004, p. 1371;
Naugle et al. 2004, p. 711), the available
information shows that populations of
van Rossem’s gull-billed tern have not
declined upon exposure to WNV
throughout the subspecies’ range.
Moreover, the best available information
gives no indication that other diseases
are substantially affecting the
subspecies in western Mexico or
elsewhere in the subspecies’ range.
Therefore, we conclude that disease,
including WNV, is not a significant
threat to van Rossem’s gull-billed tern
now, and we have no indication that it
will be in the foreseeable future.
Predation
Predation of eggs or flightless young
(nest predation) is frequently observed
at monitored van Rossem’s gull-billed
tern nest sites, but predation of adults
is rarely observed (Molina 2000, p. 7;
2001, p. 8; 2004, p. 96; 2006, p. 7; 2007,
p. 11; 2008, p. 189; 2009, p. 8; Patton
2002, p. 7; 2006, p. 7; 2008, p. 8; 2009,
p. 10; Molina et al. 2010, p. 14); thus,
we do not consider predation of adults
a significant threat to the subspecies.
The nests of ground-nesting birds are
particularly susceptible to terrestrial
predators, primarily mammals (Kruuk
1964, pp. 1–129), although predation
from aerial predators also occurs (Sears
1979, pp. 202–203). Once a mammalian
predator discovers or gains access to a
nest colony, it typically eats all or
nearly all eggs or young within the
colony, causing that nest attempt by the
colony to fail. In contrast, avian nest
predators typically eat only a few eggs
or young, causing individual nests to
fail, but rarely is the entire colony’s
nesting attempt affected (Molina 2007,
p. 11). Thus, some level of nest
predation is expected to occur naturally.
Behaviors such as nesting colonially
and selecting islands and other hard-toreach places for nesting are, in part,
anti-predator strategies that have
evolved as life-history traits in groundnesting species (Gochfeld and Burger
1996, p. 628), including van Rossem’s
gull-billed terns. A species’ behavior of
selecting nest sites that would be less
likely to be affected by terrestrial
predators blurs the lines between the
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Act’s five listing factors; that is, a
species’ behavioral strategy to avoid
nest predators (which would reduce
threat of predation under Factor C) is
also a consideration in what determines
the species’ nesting habitat (Factor A).
Another adaptation to nest predation
is for birds to renest; that is, to nest
again in the same breeding season,
which typically occurs at a different
nest site. Although renesting is
energetically demanding on the adults,
it increases the likelihood that a colony
will have some level of reproduction
(productivity) that year. However, the
number of birds that renest is typically
fewer than the number of birds that
initially nested, and the later in the
season a nest is lost, the lower the
likelihood that a pair will attempt to
renest (Thompson et al. 1997, p. 13),
and the later in the season a nest is
started, the lower the likelihood that
nest will successfully fledge young
(Massey and Atwood 1981, p. 604).
Thus, persistent nest predation, despite
renesting behavior, typically results in
reduced annual productivity of the
nesting colony or even reproductive
failure for that colony that year.
However, as long-lived birds, van
Rossem’s gull-billed terns do not
necessarily need to reproduce
successfully every year to maintain
population levels over time.
Although we have some information
on the level of nest predation at certain
van Rossem’s gull-billed tern nesting
locations, and we expect it to occur at
other locations, we do not know how
prevalent nest predation is rangewide.
Of the two nesting locations that are
monitored regularly (Salton Sea and San
Diego Bay), nest predation has been
noted at nest sites at the Salton Sea,
including some that are managed by the
Sonny Bono Salton Sea National
Wildlife Refuge in an effort to reduce
the likelihood of this threat (Molina
2009b, p. 8). The frequency of nest
predation by mammalian predators may
be increasing at certain nest sites at the
Salton Sea because the lowering water
level of the Sea is allowing onceisolated nesting islands to become
accessible (Molina 2009b, p. 8; Molina
et al. 2010, p. 13). Of all the van
Rossem’s gull-billed tern nest sites at
the Salton Sea, nest predation by
terrestrial predators remains infrequent
at only one site, the Sonny Bono Salton
Sea National Wildlife Refuge
headquarters (Rock Hill) ponds, but
there is much inter-specific competition
for nesting and loafing space at this site
(Molina 2010a, pp. 9–10) (see also the
‘‘Inter-specific Nest-site Disturbance’’
section in Factor E). Nevertheless, van
Rossem’s gull-billed terns are
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successfully fledging young at the
Salton Sea (Molina 2006, p. 2; Molina
2007, p. 4; Molina 2009b, p. 2) and even
in 2010, which had few nesting attempts
and high nest abandonment for a variety
of reasons, had some (albeit very few)
fledging (Molina 2010a, p. 2).
Additionally, dropping water levels has
allowed other nest sites to become
exposed, where van Rossem’s
successfully nested in 2010 (Molina
2010a, p. 2). It is unclear whether
apparent reduction in nest sites with
lower likelihoods of being depredated
will substantially affect the Salton Sea
colony of van Rossem’s gull-billed terns,
but it may translate into fewer birds
attempting to nest at this location; the
remaining may potentially move to
other nesting locations (e.g., Isla
Montague, Cerro Prieto, San Diego Bay)
instead.
In contrast, at San Diego Bay, the
population of van Rossem’s gull-billed
terns has steadily increased in part
because active anti-predator
management has limited the amount of
nest predation since 1999 (USFWS
2006, Appendix M, p. 2; Patton 2009,
Table 2). The primary nest site for van
Rossem’s gull-billed terns (and other
species of colonial, ground-nesting
waterbirds) in San Diego Bay is rarely
substantially affected by terrestrial
predators because (1) The nests are
located on an extensive network of
dikes where access by terrestrial
predators is limited by barriers and
fences that have been intentionally
erected; and (2) nonlethal and, if
necessary, lethal predator control
methods are used against those
predators that do venture to the nesting
areas (USFWS 2006, Appendix M).
Thus, nest predation is not a
significant threat at the San Diego Bay
nesting location because predators are
managed to benefit nesting colonial
waterbirds, including van Rossem’s
gull-billed terns. Nest predation at the
Salton Sea is less clear. The available
information suggests the Salton Sea
colony of van Rossem’s gull-billed terns
is being affected by nest predation at
some nest sites, but other nest sites are
productive, including a recently
emerged nest site. The apparent
reduction in the total number of nest
sites where nest predation is unlikely
may mean fewer van Rossem’s gullbilled terns nest at the Salton Sea in the
foreseeable future, but it is unlikely that
the nesting location will be completely
abandoned in the foreseeable future.
Additionally, even though the Salton
Sea is an important nesting location,
there are other nesting locations for the
subspecies.
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Because van Rossem’s gull-billed
terns are long-lived birds that are not
limited to any particular nesting
location, the individual adult van
Rossem’s gull-billed terns that have
traditionally nested at sites in the Salton
Sea area may move to other nesting
locations to nest. However, such shifts
in nesting locations would likely result
in increased intraspecific competition
for nest sites at existing nest colonies,
the establishment of new nesting
locations, or both. As a result, some
birds may be forced to nest in lower
quality habitat where they may be
subject to increased interspecific
competition (Factor E) or where the
level of nest predation may also be high.
It is not clear how much of an impact
this would have on the conservation
status of the subspecies because the
extent to which birds would have to
relocate is unclear and reproductive
success at existing nesting locations is
variable from year to year. Thus,
although we acknowledge some level of
impact to the subspecies, the portion of
the total population that would be
affected would be limited, and it would
not result in a significant threat to the
subspecies now or in the foreseeable
future.
In Mexico, nest predation has
occurred or was suspected at some nest
sites (for example, Peresbarbosa and
Mellink 2001, p. 267; Palacios and
Mellink 2007, p. 216). Although
information from nest sites over
multiple years is limited, we have no
information to suggest that there are
sustained, elevated levels of nest
predation occurring at any of the nesting
locations. Some nest sites have been
found to be inactive in some years
(Palacios and Mellink 2007, p. 217).
Although not atypical for this
subspecies, inactivity in some years may
indicate predation events or other
disturbances that have caused nest site
abandonment, although abandoned or
unused nest sites could potentially be
used again in other years. In some cases,
other nesting locations are found
nearby, suggesting the colony
successfully relocated. Thus, although
nest predation likely occurs in Mexico,
it does not appear to be at above-normal
levels.
Despite the behaviors that van
Rossem’s gull-billed terns use to reduce
the effects of nest predation (e.g.,
nesting at remote nest sites, predatory
defense behaviors), it is likely that they,
like nearly all bird species, suffer some
natural level of nest predation. We do
not know what the natural level of nest
predation is for van Rossem’s gull-billed
tern because it varies from nest site to
nest site and from year to year. Natural
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and manmade changes may alter the
levels of nest predation. The level of
nest predation appears to be increasing
at the Salton Sea, and possibly at some
sites in Mexico where nest sites have
shifted. While the shifting of nest sites
may indicate changes in levels of nest
predation, the fact that the colony has
moved shows that the subspecies can
adapt to such changes. Moreover, adult
van Rossem’s gull-billed terns are
generally long-lived, which means that
even if an adult fails to successfully
reproduce in a given year, it will likely
have additional chances to reproduce in
the future. Therefore, we determine that
nest predation at the Salton Sea is not
a significant threat to the subspecies
now or in the foreseeable future.
Moreover, we determine that this is not
a population-limiting factor that
presents a significant rangewide threat
now or in the foreseeable future.
Summary of Factor C
Disease, including avian botulism and
WNV, occurs within the range of van
Rossem’s gull-billed tern. In the wellmonitored nesting locations of San
Diego Bay and the Salton Sea, the
populations of the subspecies are
growing or are reasonably stable, despite
the presence of WNV. Moreover, the
Salton Sea population of van Rossem’s
gull-billed terns was not significantly
affected by substantial outbreaks of
avian botulism or avian cholera. Thus,
the available information suggests that
disease is not a significant threat to the
subspecies throughout its range now or
within the foreseeable future.
Predation of adults is not a significant
threat to the subspecies. Predation of
eggs or young at nest sites (nest
predation) is a concern for groundnesting birds such as van Rossem’s gullbilled tern. Many colonial waterbirds
have adapted to this threat by nesting on
islands and remote areas to reduce the
risk of predation or by responding to
predation events by renesting during the
same breeding season. Within the
United States, nest predation does not
appear to pose a significant threat to the
San Diego Bay van Rossem’s gull-billed
tern population; however, the Salton
Sea appears to be experiencing high
levels of nest predation, at least in some
years. While the Salton Sea is an
important nesting location, the adult
van Rossem’s gull-billed terns that have
traditionally nested there are not
confined to the Salton Sea and may
move to other locations to nest.
Although such shifts in nesting may
result in increased use of lower quality
habitat, which may result in lower
reproductive success at those locations,
we determine such potential impacts
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would not significantly threaten the
subspecies because the numbers
affected and the level of impact are
likely to be limited. The level of nest
predation at nesting locations in Mexico
is less clear, but the available
information suggests that it is not
occurring at above-normal levels.
Therefore, based on our review of the
best scientific and commercial
information available, we conclude that
van Rossem’s gull-billed tern is not
threatened by disease or predation now
or in the foreseeable future.
airport runways to protect human health
and safety. We have not issued any
other depredation permits for the van
Rossem’s gull-billed tern since the
1990s. The three individual birds
intentionally killed between 2003 and
2007 represent an insignificant number
when compared to the overall
population (average of 42 nesting pairs
for this time period, Molina et al. 2010,
p. 66) of van Rossem’s gull-billed terns
in San Diego Bay, which increased
during that time period and has
continued to grow since 2007.
Factor D. The Inadequacy of Existing
Regulatory Mechanisms
National Environmental Policy Act
All Federal agencies are required to
adhere to the National Environmental
Policy Act (NEPA) of 1970 (42 U.S.C.
4321 et seq.) for projects they fund,
authorize, or carry out. The Council on
Environmental Quality’s regulations for
implementing NEPA (40 CFR parts
1500–1518) state that agencies shall
include a discussion on the
environmental impacts of the various
project alternatives (including the
proposed action), any adverse
environmental effects that cannot be
avoided, and any irreversible or
irretrievable commitments of resources
involved (40 CFR part 1502). The NEPA
itself is a disclosure law, and does not
require subsequent minimization or
mitigation measures by the Federal
agency involved. Although Federal
agencies may include conservation
measures for gull-billed terns as a result
of the NEPA process, any such measures
are typically voluntary in nature and are
not required by the statute.
Additionally, activities on non-Federal
lands are subject to NEPA if there is a
Federal nexus. NEPA does not itself
regulate activities that might affect gullbilled terns, but it does require full
evaluation and disclosure of
information regarding the effects of
contemplated Federal actions on
sensitive species and their habitats.
The Act requires us to examine the
adequacy of existing regulatory
mechanisms with respect to threats that
may place van Rossem’s gull-billed tern
in danger of extinction or likely to
become so in the foreseeable future.
Existing regulatory mechanisms that
may have an effect on potential threats
to van Rossem’s gull-billed tern can be
placed into three general categories: (1)
U.S. Federal laws, (2) State laws, and (3)
Mexico Federal laws.
U.S. Federal Laws
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Migratory Bird Treaty Act
The Migratory Bird Treaty Act of 1918
(MBTA) (16 U.S.C. 703–712) states that
it is unlawful ‘‘to pursue, hunt, take,
capture, kill, or attempt to take, capture
or kill, possess, offer for sale, sell, offer
to barter, barter, offer to purchase,
purchase, deliver for shipment, ship,
export, import, cause to be shipped,
exported, or imported, deliver for
transportation, transport or cause to be
transported, carry or cause to be carried,
or receive for shipment, transportation,
carriage, or export, any migratory bird,
any part, nest, or eggs of any such bird,
or any product, whether or not
manufactured.’’ Mexico is also a
signatory of the MBTA. Van Rossem’s
gull-billed tern is included in the list of
migratory birds internationally
protected by the MBTA (50 CFR 10.13).
The MBTA makes it unlawful to kill or
take eggs or nests of van Rossem’s gullbilled terns, but it does not provide
protection for habitat.
As described in the ‘‘Intentional
Killing’’ section under Factor E, below,
approximately nine adult van Rossem’s
gull-billed terns have been killed
around San Diego Bay under
depredation permits issued by the
Service’s Migratory Bird Permit Office,
including six killed in the early 1990s
to protect the federally endangered
California least tern and threatened
western snowy plover, and three killed
between 2004 and 2007 near active
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Fish and Wildlife Conservation Act
The Fish and Wildlife Conservation
Act of 1980 (16 U.S.C. 2901–2911)
encourages States and Federal
departments and agencies to conserve
and promote conservation of nongame
fish and wildlife and their habitats. The
1988 amendment to the Fish and
Wildlife Conservation Act mandates the
Service to ‘‘identify species, subspecies,
and populations of all migratory
nongame birds that, without additional
conservation actions, are likely to
become candidates for listing under the
Endangered Species Act (ESA) of 1973.’’
Our Division of Migratory Bird
Management published the Birds of
Conservation Concern in 2008 (USFWS
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2008, pp. 1–87). We identified the gullbilled tern (the species as a whole) as a
Bird of Conservation Concern (see the
‘‘Management Actions’’ section above).
The species was included as a Bird of
Conservation Concern both nationally
and in certain specific Bird
Conservation Regions, including the
U.S. portions of Bird Conservation
Regions 32 (Coastal California) and 33
(Sonoran and Mojave Deserts) (USFWS
2008, pp. 48 and 49). Because we
identified the gull-billed tern as a Bird
of Conservation Concern, we have
denied depredation permit requests
under the MBTA (USFWS 2010, p. 1)
(see ‘‘Intentional Killing’’ section under
Factor E).
National Wildlife Refuge System
Improvement Act
The National Wildlife Refuge System
Improvement Act of 1997 (Pub. L. 105–
57) establishes the protection of
biodiversity as the primary purpose of
the national wildlife refuge system. This
has led to various management actions
that have directly benefited van
Rossem’s gull-billed tern. For example,
at the Sonny Bono Salton Sea National
Wildlife Refuge, nesting islands and
artificial nesting platforms have been
created and maintained (see Factor A).
At the San Diego Bay National Wildlife
Refuge, predator control has resulted in
reduced nest predation levels on van
Rossem’s gulled-billed terns (see Factor
C).
U.S. State Laws
Van Rossem’s gull-billed tern is not a
listed species under the California
Endangered Species Act (CESA), the
State’s primary regulatory mechanism to
protect species. However, the van
Rossem’s gull-billed tern is considered a
bird species of special concern in
California (Molina 2008, p. 188), an
administrative designation that carries
no formal legal status. According to
Comrack et al. (2008, pp. 1–4), the
intent of this designation is to focus
attention on animal species deemed to
be at conservation risk, stimulate
research, and improve the species’
conservation status before they meet
California Endangered Species Act
criteria for listing as a State threatened
or endangered species. However,
impacts to van Rossem’s gull-billed tern
from any projects would require
evaluation and disclosure under the
California Environmental Quality Act
(CEQA) (see below) due to its
consideration as a species of special
concern.
Van Rossem’s gull-billed tern also
receives protection through the State
migratory bird provisions of the
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California Fish and Game (CFG) Code.
The CFG Code prohibits any take or
possession of birds that are designated
by the MBTA as migratory nongame
birds, except as allowed by Federal
rules and regulations promulgated
pursuant to the MBTA (Division 4, Part
2, Chapter 1, section 3513).
Additionally, under the CFG Code, it is
unlawful to take, possess, or needlessly
destroy the nest or eggs of any bird,
including van Rossem’s gull-billed tern,
except as otherwise provided (Division
4, Part 2, Chapter 1, section 3503). This
provides protection to van Rossem’s
gull-billed terns, including their nests,
from any unlawful take.
California Environmental Quality Act
The California Environmental Quality
Act (CEQA) (Public Resources Code
21000–21177) and the CEQA Guidelines
(California Code of Regulations, Title
14, Division 6, Chapter 3, Sections
15000–15387) requires State and local
agencies to identify the significant
environmental impacts of their actions
and to avoid or mitigate those impacts,
if feasible. CEQA applies to projects in
California proposed to be undertaken or
requiring approval by State and local
government agencies. The lead agency
must complete the environmental
review process required by CEQA,
including conducting an Initial Study to
identify the environmental impacts of
the project and determine whether the
identified impacts are ‘‘significant.’’ If
significant impacts are determined, then
an Environmental Impact Report must
be prepared to provide State and local
agencies and the general public with
detailed information on the potentially
significant environmental effects
(California Environmental Resources
Evaluation System, 2010).
‘‘Thresholds of Significance’’ are
comprehensive criteria used to define
environmentally significant impacts
based on quantitative and qualitative
standards. They include impacts to
biological resources such as candidate,
sensitive, or special status species in
local or regional plans, policies, or
regulations, or by the CDFG or the
Service; or any riparian habitat or other
sensitive natural community identified
in local or regional plans, policies,
regulations or by the CDFG or Service
(CEQA Handbook, Appendix G, 2010).
Defining these significance thresholds
helps ensure a ‘‘rational basis for
significance determinations’’ and
provides support to the final
determination and appropriate revisions
or mitigation actions to a project in
order to develop a mitigated negative
declaration rather than an
Environmental Impact Report
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(Governor’s Office of Planning and
Research, 1994, p. 5).
Section 15380 of the CEQA
Guidelines indicates that species
designated as ‘‘Species of Special
Concern’’ should be included in an
analysis of project impacts (Comrack et
al. 2008, p. 2). In assigning ‘‘impact
significance’’ to populations of unlisted
species, factors such as population-level
effects, proportion of the taxon’s range
affected by a project, regional effects,
and impacts to habitat features are
analyzed. If significant effects are
identified, the lead agency has the
option of requiring mitigation through
changes in the project or to decide that
overriding considerations make
mitigation infeasible (CEQA section
21002). Protection of listed species
through CEQA is, therefore, dependent
upon the discretion of the lead agency
involved.
where land alteration is limited
(Figueroa and Sanchez-Cordero 2008, p.
3232). Two of the largest nesting
populations of van Rossem’s gull-billed
terns are within biosphere reserves,
including Isla Montague and Marismas
Nacionales. Additionally, the small
population of van Rossem’s gull-billed
terns at Laguna Ojo de Liebre, including
the Guerrero Negro saltworks, is within
´
the El Vizcaıno Biosphere Reserve
(Palacios 2010, pp. 6 and 16), but the
level of protection afforded by the
reserve is likely limited within the salt
production facility. Yet LGEEPA, as
implemented with the aid of the
CONANP, provides benefits to van
Rossem’s gull-billed tern and its habitat,
benefits the subspecies would not have
in the absence of such regulatory
mechanisms.
Mexico Federal Laws
In Mexico, van Rossem’s gull-billed
tern is protected by what is known as
the Ecology Law (Ley General del
´
´
Equilibrio Ecologico y la Proteccion al
Ambiente, or LGEEPA). This law, first
enacted in 1988 and amended in 1996,
is designed to preserve ecosystems and
allow for sustainable use of biodiversity
and development of working groups to
organize management and protection of
the environment in designated Natural
Protected Areas (Gonzales and Gastelum
2000, p. 50; Bezaury-Creel 2005, p.
1031). Although management of
protected areas has typically been
inadequate in Mexico, the situation has
been greatly improved through the
establishment of The National Protected
´
Area Commission (Comision Nacional
´
de Areas Naturales Protegidas, or
CONANP) (Bezaury-Creel 2005, p.
1034). Many management plans for
protected areas are under development,
´
´
including one for Bahıa Santa Marıa
(Bezaury-Creel 2005, pp. 1021, 1034), a
nesting location for van Rossem’s gullbilled terns. However, enforcement
continues to be problematic in Mexico
due to the lack of collaboration between
different Federal agencies, and between
Federal and local governments (Fraga
and Jesus 2008, p. 21). Furthermore,
local reserve managers often lack the
legal authority to enforce environmental
laws (Fraga and Jesus 2008, p. 21).
LGEEPA does not necessarily preserve
lands in protected areas; instead, areas
are considered more as ‘‘multiple use
zones’’ where thresholds are imposed
on sustainable use of natural resources
to limit activities (Bezaury-Creel 2005,
pp. 1030–1031). One form of Natural
Protected Areas, the ‘‘biosphere
reserve,’’ includes established core areas
In the United States, the National
Wildlife Refuge System Improvement
Act benefits breeding populations of van
Rossem’s gull-billed tern at San Diego
Bay National Wildlife Refuge and the
Sonny Bono Salton Sea National
Wildlife Refuge. Additional Federal and
State regulations provide benefits to the
subspecies, through its migratory bird
status (Federal and State), and to its
habitat, through its designation as a
species of special concern (Federal and
State).
In Mexico, two of the largest nesting
populations of van Rossem’s gull-billed
terns are located within biosphere
reserves and a third, smaller population
is in a biosphere reserve where other
uses (salt production) is occurring.
Development is somewhat limited by
the LGEEPA, especially in core areas of
biosphere reserves. The CONANP was
established to assist in preserving
ecosystems and organizing management
and protection of the environment in
these Natural Protected Areas. While
enforcement continues to be a concern
regarding regulatory mechanisms in
Mexico and active management is
lacking in many areas, these regulatory
mechanisms provide benefits to the van
Rossem’s gull-billed tern, benefits that
the subspecies would not have
otherwise.
Based on our review of the best
scientific and commercial information
available, we conclude that van
Rossem’s gull-billed tern is not
threatened by inadequate regulatory
mechanisms now, nor is it likely to
become so in the foreseeable future.
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Factor E. Other Natural or Manmade
Factors Affecting Its Continued
Existence
Inter-Specific Nest-Site Disturbance
Van Rossem’s gull-billed terns
generally nest on small, low islands
with little or no vegetation. Many other
species also use these islands for nesting
and loafing, where they compete with
van Rossem’s gull-billed terns for space;
van Rossem’s gull-billed terns,
especially eggs and young, may be
inadvertently crushed, injured, or
affected by agonistic behavior from
other species. These interactions,
discussed below, may affect the
productivity of nesting van Rossem’s
gull-billed terns, but such competition
is primarily natural, and many colonial,
ground-nesting species are able to adapt
to colonial nesting dynamics.
Van Rossem’s gull-billed terns are
known to compete for nesting sites with
other shorebirds and waterbirds (Molina
2004, p. 98). At San Diego Bay and the
Salton Sea, territorial behavior between
van Rossem’s gull-billed tern and
species such as black skimmer and
elegant tern result in the loss of van
Rossem’s gull-billed tern nests on a
near-yearly basis (e.g., see Patton 2009,
p. 9). Extent of the damage to the colony
varies, with approximately 5 to 15 nests
(7 to 25 percent of total nests) in a
colony destroyed (e.g., see Patton 2003,
p. 8; 2009, p. 9). Territorial disputes
between other species in close
proximity to van Rossem’s gull-billed
tern colonies can result in temporary
displacement of adult gull-billed terns
from nests. This disturbance could
result in predation of eggs by gulls and
mortality of eggs due to high
temperatures (Molina 2000, p. 8). Van
Rossem’s gull-billed terns also compete
for nesting space at colonies in Mexico,
where they share most of their breeding
sites with black skimmers, Caspian terns
(Hydroprogne caspia), and laughing
gulls (Larus atricilla) (Palacios and
Mellink 2006, pp. 49–84). At the San
Diego Bay nesting colony, van Rossem’s
gull-billed tern chicks have been killed
and injured by aggressive behavior of
black skimmers (Patton 2009, p. 9).
Competition for space from
nonbreeding waterbirds can also cause
damage to van Rossem’s gull-billed tern
nests. For example, loafing Caspian
terns, double-crested cormorants
(Phalacrocorax auritus), or white and
brown pelicans (Pelecanus
erythrorhynchos and P. occidentalis)
have displaced van Rossem’s gull-billed
terns and trampled their eggs, chicks, or
both at Salton Sea and San Diego Bay
(Molina 2001, p. 10; 2007, p. 11; 2009,
p. 8; Patton 2001, p. 9, 2009, p. 9;
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Molina et al. 2010, p. 15). These larger
birds often use the same loafing sites
repeatedly, returning after foraging or as
nighttime roosts. The severity of van
Rossem’s gull-billed tern displacement
and egg trampling is dependent on the
extent of the use by other birds at a
particular colony. The presence of larger
birds at a colony site for a week or less
may result in a reduction in van
Rossem’s gull-billed tern nesting
success through displacement, egg
trampling, or damage of individual
nests. If their presence continues over a
period of weeks, van Rossem’s gullbilled terns may abandon the colony
(Molina 2007, p. 11). Additionally,
nesting van Rossem’s gull-billed terns
occasionally have to compete for space
with other species of wildlife. For
´
example, at Laguna Cuyutlan, eggs of
colonial-nesting birds were crushed by
an American crocodile (Crocodylus
acutus) when it crawled onto a nesting
island (Palacios and Mellink 2007, p.
220).
Inter-specific interactions often occur
naturally at colonies of ground-nesting
birds. As discussed in the ‘‘Biology’’
section above, van Rossem’s gull-billed
terns often adapt to such interactions by
renesting at the same or other nearby
nest sites after disturbances. Although
the productivity of an affected nest
colony of van Rossem’s gull-billed terns
may be reduced or prevented in a given
year if such disturbances occur
repeatedly, it is unlikely that a
substantial proportion of nesting
locations will be significantly affected
repeatedly from year to year. Therefore,
we do not expect any deleterious effects
associated with these events to be a
significant threat to van Rossem’s gullbilled tern.
Anthropogenic Nest-Site Disturbance
Colonial nesting waterbirds are
sensitive to disturbance from the actions
of humans and domesticated animals
(Sears 1978, p. 9; Safina and Burger
1983, p. 168, Blanc et al. 2006, p. 122).
Disturbance of colonies can cause
mortality of eggs and chicks due to
increased predation and heat stress
(Safina and Burger 1983, p. 169). Gullbilled terns may be especially sensitive
to the presence of humans and animals
in their nesting colonies and prolonged
disturbance can result in decreased
breeding success (Clapp et al. 1983, p.
348, Molina 2008, p. 190). Excessive
human disturbance at a particular nest
site may cause van Rossem’s gull-billed
terns to abandon the nesting attempt at
a given site in a given year, though in
some cases such abandonment results in
renesting at a different nearby site.
Abandonment is not necessarily
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58669
permanent; van Rossem’s gull-billed
terns may again use those nest sites in
subsequent years, if the sites are
available. However, as noted in the
‘‘Predation’’ section under Factor C,
persistent renesting typically results in
reduced annual productivity for that
colony because fewer pairs are
subsequently likely to renest and those
that do are less likely to successfully
fledge young (Massey and Atwood 1981,
p. 604; Thompson et al. 1997, p. 13).
In the United States, most van
Rossem’s gull-billed tern nesting areas
occur in areas that are managed for the
benefit of wildlife species, including
van Rossem’s gull-billed terns and other
colonial nesting waterbirds, which
limits the level of human disturbance.
However, because nesting occurs at
different sites within and between years,
including nest sites located outside of
protected or managed areas, the
subspecies is subject to disturbance in
some areas. For example, regular visits
from boaters and fishermen on Mullet
Island in the Salton Sea may have
caused van Rossem’s gull-billed terns to
move from that nest site (Molina 2001,
p. 14). Also at the Salton Sea, lower
water levels have allowed some nesting
islands to become reconnected to the
mainland, and feral dogs have intruded
onto an area used by van Rossem’s gullbilled tern for nesting, causing the
colony to permanently abandon this
nest site (Molina 2000, p. 7). Similarly,
nest sites in San Diego Bay have been
disturbed in the past (Patton 2001, p. 9),
but predator management actions,
including fencing, at this site have
decreased the incidence of such
disturbances (USFWS 2006, pp. 1–36).
Researchers may cause disturbance of
nesting birds, though monitors and
researchers typically conduct their
activities in such a way as to disturb the
population as little as possible (Patton
2009, pp. 4–5). Nonetheless, Palacios
and Mellink (2007, p. 216) suspected
that researcher activity may have been
a disturbance at some nest sites in
Mexico, but this appears to have been
events associated with individual
studies and not from monitoring, which
involves repeated visits within and
between years. Therefore, we do not
anticipate this to be an ongoing,
significant threat.
In Mexico, many nest sites are
protected from human disturbance by
beneficial or benign land uses, or
because the nest sites are not easily
accessed by humans (Molina and Garrett
2001, p. 27; Palacios and Mellink 2006,
pp. 71, 78), such as at the Guerrero
Negro saltworks (Palacios and Mellink
2007, p. 217). However, human
disturbance has been noted near van
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Rossem’s gull-billed tern nest sites,
including two of Mexico’s largest
colonies, Laguna Pericos (in Marismas
Nacionales) and Isla Montague, plus
also Laguna del Caimanero and Laguna
´
Cuyatlan (Palacios and Mellink 2006,
pp. 60, 67, 74 and 78). Additionally,
´
Estero Teacapan (in Marismas
Nacionales), unlike most other nest sites
in Mexico, is often visited by tourists
(Palacios and Mellink 2006, p. 71).
Available information on disturbance at
nest sites in Mexico is limited to those
data that were generated by only one or
two visits, which limits our ability to
determine the frequency of past
disturbances or the likelihood that such
disturbances will continue into the
foreseeable future. However, frequent
disturbance (among others) would likely
result in van Rossem’s gull-billed terns
abandoning nest sites. At Isla Montague,
a site for which we have intermittent
data since 1992, nesting has continued
at roughly the same levels despite the
apparent disturbances over that time
(Palacios and Mellink 1992, p. 43).
Similarly, in a qualitative assessment of
the terns’ reaction to the presence of
fishermen, Palacios and Mellink (2006,
p. 67) note that van Rossem’s gull-billed
terns at Laguna del Caimanero appeared
to become ‘‘habituated’’ to human
disturbance and continued to nest
despite the presence of people. Thus,
the limited information available to us
does not indicate that there is a longterm population-level threat associated
with manmade nest disturbance to the
van Rossem’s gull-billed tern now or in
the foreseeable future.
Intentional Killing
Human-related actions that result in
the death of individual van Rossem’s
gull-billed terns have the potential to
affect the continuing existence of the
species if the number of individuals
killed substantially affects the mortality
rate of the subspecies. The mortality rate
in a population may substantially affect
a population if it continually exceeds
the rate of increase (or birth rate)
(Thomas 1994). Intentional killing
activities may include take authorized
under existing laws or unauthorized
depredation. Because either action, by
definition, results in the death of
individual van Rossem’s gull-billed
terns (or, in certain cases, destruction of
eggs) we assess these potential actions
in this section; however, we note that
the motives and level of oversight differ
between the two categories. Below we
assess the effects of intentional killing of
van Rossem’s gull-billed terns as a
potential threat to the subspecies.
In the San Diego Bay region, three van
Rossem’s gull-billed terns have been
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intentionally killed as part of the U.S.
Navy’s Bird/Animal Aircraft Strike
Hazard (BASH) program. The Navy
deemed it necessary to kill three adult
van Rossem’s gull-billed terns near
active runways for human safety
reasons, two in 2004 on Naval Base
Coronado and one in 2007 at Naval
Outlying Landing Field, Imperial Beach
(Molina et al. 2010, p. 16). The Service
authorized these removals under a
migratory bird depredation permit for
airport operations pursuant to the
Migratory Bird Treaty Act (50 CFR part
21). The three van Rossem’s gull-billed
terns killed under the Navy’s BASH
program have been the only individuals
intentionally killed under this program
since the subspecies established a
nesting colony in San Diego Bay in
1987.
Additionally, six (or possibly seven)
adult van Rossem’s gull-billed terns
were killed between 1993 and 1995 in
San Diego because they were considered
potential threats to federally endangered
California least terns and federally
threatened western snowy plovers
(Patton 2002, in litt., p. 1; Molina et al.
2010, p. 15). These two species nest in
highly managed areas in the San Diego
Bay region, and management measures
include limiting the effects of predators
on listed species. Depredation of
California least tern chicks and western
snowy plover chicks by van Rossem’s
gull-billed terns has increased as the
van Rossem’s gull-billed tern population
has increased in San Diego Bay (Patton
2009, Appendix C; Marschalek 2010,
pp. 12–13, 20). Since 1995, only
nonlethal methods have been used by
local managers in what have largely
been unsuccessful attempts to dissuade
van Rossem’s gull-billed terns from
depredating the chicks of California
least terns and western snowy plovers.
The Navy does not currently have
authorization from the Service to use
limited lethal control of van Rossem’s
gull-billed terns in areas the Navy
manages to benefit California least terns
and western snowy plovers.
As the level of depredation of
California least terns and western snowy
plovers by van Rossem’s gull-billed
terns has increased in the San Diego Bay
region, local land managers have
considered methods other than direct
lethal control of adults to reduce the
impact of van Rossem’s gull-billed terns
on the other listed species. For example,
as published in a draft Environmental
Assessment under the National
Environmental Policy Act, we proposed
in an experiment at the San Diego Bay
National Wildlife Refuge to gather data
that would help us answer the following
management questions: (1) Could we
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reduce the loss of California least tern
and western snowy plover chicks to
predation by van Rossem’s gull-billed
terns in the vicinity of San Diego Bay by
lowering the productivity within the
van Rossem’s gull-billed tern colony at
San Diego Bay; and (2) could
productivity within the van Rossem’s
gull-billed tern colony at San Diego Bay
be reduced without causing significant
direct impacts to San Diego Bay’s
breeding population of adult van
Rossem’s gull-billed terns (USFWS
2009, p. 4). In part, the experiment
proposed to addle eggs of van Rossem’s
gull-billed terns nesting at the San Diego
Bay National Wildlife Refuge to
determine if population size of van
Rossem’s gull-billed terns in San Diego
Bay could be controlled while avoiding
a decline of the overall population of
van Rossem’s gull-billed terns (USFWS
2009, pp. 8–9). Although initially
proposed for the 2009 nesting season,
no further action on the proposed
project was taken. No additional
compliance with the National
Environmental Policy Act was prepared
related to the proposed project, and we
are not planning to implement this
proposed project now or in the
foreseeable future.
The killing of van Rossem’s gullbilled terns as predator control has only
occurred in San Diego Bay, and no van
Rossem’s gull-billed terns have been
killed there for predator control since
1995. We are not aware of any killing of
van Rossem’s gull-billed terns as BASH
management anywhere except San
Diego Bay, and only three individuals
were killed there, two in 2004 and one
in 2007. The population of van
Rossem’s gull-billed terns remains in
the San Diego Bay area and has
consistently grown since 1999 (Patton
2009, Figure 1, no page number). Given
the continued level of growth of the San
Diego Bay population of van Rossem’s
gull-billed terns over the same time
period as the three individuals were
killed under the BASH program, the
level of take under this program has not
significantly affected the San Diego Bay
population of van Rossem’s gull-billed
terns, or the subspecies rangewide.
Thus, lethal control of van Rossem’s
gull-billed terns for predator control and
BASH prevention is currently not a
significant threat to the subspecies
throughout its range and, because we do
not anticipate an increase in the lethal
control measure associated with the
Navy’s BASH program, this is not a
significant threat to the subspecies in
the foreseeable future.
In Mexico, van Rossem’s gull-billed
terns forage at commercial shrimp
aquaculture farms. Although lethal
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control (e.g., shooting) of predators is
not legally authorized in Mexico, it has
been documented at some of these
aquacultural operations (e.g., Palacios
and Mellink 2006, p. 60). Information
on whether this activity is widespread
is limited. DeWalt (2000, p. 47) implied
that it occurs more often than it is
reported. Molina and Erwin (2006, p.
287) suggested that such activities are
widespread in Mexico during times
when shrimp are being harvested.
Evidence of lethal control of van
Rossem’s gull-billed terns in Mexico is
circumstantial (e.g., Molina and Erwin
2006, p. 287; Molina et al. 2010, p. 16),
and we are not aware of any direct
reports of van Rossem’s gull-billed terns
being shot or otherwise killed at shrimp
ponds within its range. Some van
Rossem’s gull-billed terns may be killed
in this manner; however, given the lack
of evidence of lethal control of van
Rossem’s gull-billed terns at
aquacultural ponds, we conclude that
the practice does not occur frequently
enough to negatively affect the status of
the subspecies. We have no information
to suggest this will change in the
foreseeable future. Therefore, the use of
lethal control at aquacultural ponds is
not a significant threat to van Rossem’s
gull-billed tern now nor is anticipated to
be a significant threat in the foreseeable
future.
Contaminants
High levels of pesticides and heavy
metals are known to cause reproductive
harm in breeding birds (Longcore et al.
1971, p. 486; King et al. 1978, p. 17).
The organochlorine pesticide known as
DDT breaks down in the environment to
form DDE, which may cause thinning of
eggshells and decreased reproductive
success in birds (Longcore et al. 1971,
pp. 486, 489). Although DDT was
banned in the United States in the
1970s, it was used for malarial control
´
in Mexico until the early 1990s (Garcıa´
Hernandez et al. 2006, p. 1640). Coastal
lagoons in Mexico have widely varying
´
levels of pesticides (Paez-Osuna et al.
2002, p. 1305), with DDE found in
elevated levels in some lagoons that
contain nesting sites for van Rossem’s
gull-billed terns (Galindo et al. 1997, p.
´
´
1076; Garcıa-Hernandez et al. 2001, p.
90; Carvalho et al. 2002, p. 1262).
Additionally, selenium is a naturally
occurring element that may also act as
a contaminant and affect birds under
certain conditions. At low levels,
selenium is an essential trace nutrient
that serves multiple metabolic functions
in animals (Arthur and Beckett 1994, p.
620), but at higher concentrations it can
cause embryo malformation and death
(Hoffman et al. 1988, p. 521). The
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available information indicates that
levels of selenium are elevated within
sediments at the Salton Sea (Miles et al.
2009, p. 2) and along the Colorado River
channel close to the Isla Montague
´
´
nesting location (Garcıa-Hernandez et
al. 2001, pp. 72 and 73), but at levels
below thresholds known to cause
reproductive harm at Cerro Prieto
´
´
(Garcıa-Hernandez et al. 2001, pp. 72
and 73).
Birds accrue contaminants mainly
through the food they eat, with fisheating birds commonly accumulating
higher levels of contaminants than birds
that feed on seeds or invertebrates
(Frank et al. 1975, p. 214, Focardi 1988,
p. 253, Ruelas-Inzunza et al. 2009, p.
418). For example, past studies have
linked reproductive failure with
heightened pesticide levels in the
common tern (Sterna hirundo) and the
roseate tern (Sterna dougallii), both fisheating species (Hays and Risebrough
1972, p. 21; Fox 1976, p. 470), but are
less pronounced in the black tern
(Chlidonias niger), which is primarily
insectivorous (Frank et al. 1975, pp.
211, 214). Although the diet of van
Rossem’s gull-billed terns may include
fish, they typically eat a variety of prey
items, with high percentages of
invertebrates (Erwin et al. 1998a, p.
325). For example, at both Salton Sea
and San Diego Bay, van Rossem’s gullbilled terns primarily forage on
invertebrates, with fish composing only
about a quarter of their diet (Molina and
Marschalek 2003, p. 23; Molina 2009a,
p. 10). While van Rossem’s gull-billed
terns are known to prey on small chicks
of other bird species, this prey item
makes up the smallest portion of their
diets (Molina et al. 2010, p. 7).
Although few studies have measured
effects of contaminants on van Rossem’s
gull-billed tern, the available
information from a small number of
samples, as summarized in Molina et al.
(2010, p. 15), found elevated levels of
total DDT from one van Rossem’s gullbilled tern egg from San Diego Bay, but
this concentration was still below the
thresholds found to be harmful in other
species. Other contaminants, such as
selenium (from eggs collected at Salton
Sea), arsenic, cadmium, copper,
mercury, nickel, and zinc (from one San
Diego egg), were all found to be at
concentrations below threshold levels
(Molina et al. 2010, p. 15). Based on this
best available information, we do not
consider contaminants to be a
significant threat to the van Rossem’s
gull-billed tern now or in the
foreseeable future.
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Food Availability
During periods when the subspecies
is not nesting, including migration and
while wintering, van Rossem’s gullbilled terns, as highly mobile birds, can
cover wide areas to search for food. In
contrast, food availability near nesting
sites is critical for successfully raising
young. However, the availability of food
(prey items) is naturally variable.
Moreover, unlike other tern species that
are dependent on fish as their sole food
source, van Rossem’s gull-billed terns
opportunistically eat a variety of prey
items found over a range of aquatic and
terrestrial areas (Parnell et al. 1995, p.
1; Gochfeld and Burger 1996, p. 645). It
is unlikely that all potential prey items
for van Rossem’s gull-billed tern will be
affected at the same time, and this
subspecies is able to refocus its foraging
behavior to locate alternate sources of
prey. If the overall availability of prey
items is low during a given year in
breeding areas, it will likely result in the
reduction or loss of productivity for that
year.
However, the adult van Rossem’s gullbilled terns would likely survive
because they are highly mobile and can
find food elsewhere, even if it means
abandoning the nesting attempt and
flying to other nesting or foraging
locations within the subspecies’ range.
Additionally, because van Rossem’s
gull-billed terns are long-lived, most
individual adults will survive to nest
the following year—at the original
nesting location, or perhaps even
moving to a different nesting location.
For example, evidence suggests van
Rossem’s gull-billed terns regularly
move between the Salton Sea, Cerro
Prieto, Isla Montague, and San Diego
Bay nesting locations within or between
years, although food availability is not
suspected as the motivation for such
relocations (Molina and Garrett 2001, p.
26; Patton 2001, p. 8; Molina 2004, p.
98; Palacios 2010, p. 12 and 15). Thus,
we do not consider a lack of food
availability to be a significant threat to
the subspecies now or in the foreseeable
future.
Small Population Size
Small populations are
disproportionately affected by
demographic, genetic, and
environmental stochastic (random)
events, and natural catastrophes
(Caughley 1994, pp. 217–227; Asquith
2001, pp. 345–352). Genetic stochastic
events can further influence population
demographics through inbreeding
depression and genetic drift (Lande
¨
1988, pp. 624–635; Whitlock and Burger
2004, pp. 155–170). The point at which
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a population becomes a ‘‘small
population’’ is not clear and varies by
species-specific or situational-specific
factors. Moreover, there is disagreement
among scientists and considerable
uncertainty as to the population size
adequate for long-term persistence of
wildlife populations. There is, however,
agreement that population viability for
species of vertebrates (including birds)
is more likely to be ensured if
population sizes (typically breeding
adults) are in the thousands of
individuals rather than hundreds (Traill
et al., 2010, p. 32; Reed et al. 2003, p.
30, Table 3). However, as stated by
Thomas (1990, p. 324), ‘‘there is no
‘magic’ population size that guarantees
the persistence of animal populations.’’
Moreover, the amount of time that most
authors consider to be ‘‘long term’’ is
many decades or even centuries (for
´
example, see Shaffer 1981, p. 132; Soule
and Simberloff 1986, p. 28; Traill et al.
2010, p. 31; see also Reed et al. 2003,
p. 30, Table 3 therein).
Thus, we do not consider rarity alone
to meet the information threshold
indicating that the species may warrant
listing. In the absence of information
identifying threats to the species and
linking those threats to the rarity of the
species, the Service does not consider
rarity or small populations alone to be
a threat. A species that has always had
small population sizes or been rare, yet
continues to survive, could be well
equipped to continue to exist into the
future. Many naturally rare species have
persisted for long periods within small
geographic areas, and many naturally
rare species exhibit traits that allow
them to persist despite their small
population sizes. Consequently, the fact
that a species is rare or has small
populations does not necessarily
indicate that it may be in danger of
extinction now or in the foreseeable
future.
Although surveys were conducted
through much of the subspecies’
breeding range in 2010, the surveys
were conducted fairly late in the nesting
season, and, thus, the most complete
(best available) estimated breeding
population size of van Rossem’s gullbilled tern is from the 2003 to 2005
period at approximately 800 pairs of
adults rangewide. That translates to
approximately 1,600 individual adults.
This rough estimate of population size
is largely based on counts of adults at
nesting locations; as such, this figure
approximates the number of breeding
adults but does not include nonbreeding
individuals. However, as discussed in
the ‘‘Population Size’’ section, the data
we have suggests the overall population
of this subspecies has never been
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particularly large. Although Pemberton
(1927, p. 256) estimated that there were
about 500 pairs (1,000 individuals) at
the Salton Sea in 1927, there are no
estimates of population sizes from any
other location in western North America
within that timeframe.
The Salton Sea now supports roughly
100 to 200 pairs (200 to 400
individuals); thus, the Salton Sea
population has decreased since the
1920s. However, the Salton Sea (or Lake
Cahuilla) has existed only intermittently
through recent history and prehistory,
which means that over time it has not
served as a persistent and consistent
nesting location. The available historical
information suggests that the population
of the subspecies in Mexico has been
small since at least the early 1900s.
Additionally, many of the places that
van Rossem’s gull-billed terns nest
currently were not occupied
historically, including San Diego Bay,
Laguna Ojo de Liebre (Guerrero Negro
saltworks), and Cerro Prieto geothermal
plant (which opened in 1973),
suggesting the breeding range of the
subspecies has expanded recently.
However, we lack the information to
determine if these additional nesting
sites are the result of an actual increase
in total population or just a
redistribution of the breeding
population.
Additionally, inbreeding depression
and genetic drift are less likely in a
subspecies in which individuals
regularly move between and among
other nesting locations, allowing
opportunities for genetic mixing. Also,
the wide geographic range over which
the subspecies breeds suggests that it
would be unlikely that all van Rossem’s
gull-billed tern nesting locations would
be simultaneously affected by a
catastrophic environmental event (such
as a drought, flood, or extreme weather).
Even if a large storm event, such as a
hurricane, during the breeding season
were to move through the northern end
of the Gulf of California to the Salton
Sea area, where several large nesting
populations occur (Table 1, Figure 1), it
may have an effect on the subspecies’
reproductive efforts for that year;
however, it is unlikely to result in the
death of a significant number of adult
van Rossem’s gull-billed terns because
they are capable flyers. Therefore,
although the small population size may
possibly be cause for concern, threats
associated with small population sizes
(i.e., demographic or genetic
bottlenecks, inbreeding depression,
genetic drift, and catastrophic events)
are not significantly affecting van
Rossem’s gull-billed tern and they are
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not likely to affect the subspecies in the
foreseeable future.
Climate Change
Direct observations of recent climate
change include increases in global
average air and ocean temperatures,
widespread melting of snow and ice,
and rising global average sea levels, and
provide unequivocal evidence for global
warming of the Earth’s climate system
(Intergovernmental Panel on Climate
Change or IPCC 2007, p. 5). These
changes in climate are expected to have
an effect on many ecosystems; however,
wetlands are likely to be particularly
affected given their sensitivity to
changes in precipitation and
evapotranspiration (MacIean et al. 2007,
p. 12). However, there is little specific
information available that directly
pertains to the likely effects of
anthropogenic global climate change on
van Rossem’s gull-billed tern. Below, we
summarize the applicable information.
Climate change-related impacts were
recently evaluated for the San Diego
region, which includes the San Diego
Bay van Rossem’s gull-billed tern
nesting location, in a paper prepared by
the California Energy Commission’s
Public Interest Energy Research
Program’s California Climate Change
Center (CCCC). This paper used three
climate models and two greenhouse gas
emissions scenarios (A2 and B1, from
the IPCC 2007, p. 18) to develop
downscaled global predictions for
climate change impacts to the San Diego
region by 2050. The report concluded
that temperatures for San Diego County
would increase 1.5 °F to 4.5 °F (0.8 °C
to 2.5 °C), but warming along the coast
was likely to be more moderate than
inland locations (approximately 50 km
(30 mi) inland) due to the influence of
the Pacific Ocean (CCCC 2009, p. 12).
However, it is not clear whether or how
much this will affect van Rossem’s gullbilled terns that nest at the San Diego
Bay nesting location. We did locate one
published study addressing climate
change and the phenology (the timing of
climate-related annual patterns in
wildlife) of migration for the ‘‘eastern’’
subspecies of gull-billed tern and other
summer- and winter-resident coastal
birds along the Texas coast (Foster et al.
2010). In this study, the authors found
that (warming) temperatures did not
have a direct effect on migration
phenology of ‘‘eastern’’ gull-billed terns
at this location, but they speculated that
it might be important at other places or
times along migration routes (Foster et
al. 2010, p. 122). Thus, at least for
‘‘eastern’’ gull-billed tern at this study
site, increasing average temperature
appeared to have little effect on
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migration phenology. Therefore, this
study does not provide evidence to
support a premise that climate change is
a significant threat to van Rossem’s gullbilled tern.
We are not aware of similar
downscaled regional climate models for
the inland van Rossem’s gull-billed tern
nesting locations, but as suggested
above, inland temperatures are expected
to rise. The region containing the Salton
Sea and Cerro Prieto nesting locations is
very hot during the nesting season. Eggs
left unattended during the heat of the
day in this environment can exceed 50
°C (122 °F), some 5 to 10 degrees hotter
than the temperature range for embryo
development (Grant 1982, pp. 56 and
60). Thus, even under current
temperature regimes, ground-nesting
birds in this region must attentively cool
their eggs during the day. Van Rossem’s
gull-billed terns soak their belly feathers
in water and use other techniques to
cool their eggs (and themselves) when
daytime temperatures peak (Grant 1982,
p. 39). We do not know the maximum
temperature the subspecies can endure
while nesting; however, it is clear that
the subspecies has natural behavioral
adaptations to keep its eggs within an
acceptable temperature range for
development in very hot environments.
Because the remaining nesting locations
are coastal—and thus the existing
temperatures are milder and the
potential temperature increases are
more likely to be moderate—increasing
temperatures associated with global
climate change is not likely to be a
significant threat to the subspecies.
Additionally, in the CCCC study,
future precipitation projections for this
region were mixed, with three
simulations indicating drier conditions
and three simulations indicating wetter
conditions; however, all agreed on a
high degree of variability of annual
precipitation, which the authors suggest
as indicating high likelihood of drought
(CCCC 2009, p. 13). Substantial changes
in the amount of precipitation could
potentially affect terrestrial prey
availability for van Rossem’s gull-billed
tern in the San Diego region, but
because the modeled forecasts were
inconclusive, there is little evidence to
suggest that van Rossem’s gull-billed
terns in the San Diego Bay region would
be significantly affected. Moreover, van
Rossem’s gull-billed terns in the San
Diego Bay region can and often do
forage on marine prey and prey items
that depend on marine systems, which
are less likely to be substantially
affected by changes in precipitation
(Molina and Marschalek 2003, p. 8 and
Figure 8). Similarly, changes in
precipitation (increase or decrease) are
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not likely to affect van Rossem’s gullbilled tern at the other coastal nesting
locations.
However, prolonged drought could
potentially affect the amount of water in
the Colorado River (Karl et al. 2009, p.
130), which is the source of irrigation
water for agricultural fields near the
Salton Sea and Cerro Prieto nesting
locations. If agriculture is severely
curtailed in this region, the amount of
food available to van Rossem’s gullbilled terns will likely be substantially
affected. A drought of that magnitude
would also likely impact the amount of
water available for maintaining nest
sites at the Salton Sea. Even if a severe
drought resulted in the loss of nesting
habitat at the Salton Sea and Cerro
Prieto, adult van Rossem’s gull-billed
terns would likely move to other nesting
locations.
Further, three simulation scenarios in
the CCCC study were used to model sea
level rise for the San Diego region and
results indicate an increase in sea level
of 12 to 18 inches (30 to 46 centimeters)
by 2050 (CCCC 2009, p. 14). The study
also looked at the effects of sea level rise
in combination with wave activity for
six already flood-prone areas in San
Diego County, estimating sea level with
both tide and wave run-up elevation
recurrences (CCCC 2009, pp. 14–18).
South San Diego Bay, the current
nesting location of the van Rossem’s
gull-billed tern population, was not
included in the results; however, coastal
areas from South Imperial Beach to
Oceanside Beach were evaluated (CCCC
2009, pp. 16–18). Tidal fluctuations
alone were found to inundate sandy
beaches in many areas, including the
Tijuana River mouth (CCCC 2009, p.
16), and incorporating a moderately
common frequency of wave events for
this location resulted in flooding of
most of the sandy beaches here and in
other coastal areas in San Diego County
(CCCC 2009, p. 16).
However, in south San Diego Bay, van
Rossem’s gull-billed terns
predominantly nest on certain artificial
dikes within a network of dikes that
form salt evaporation ponds (saltworks)
(USFWS 2006, p. 3–67; Patton 2009,
Summary [no page number]). The
nesting dikes are within the outer
perimeter of the saltworks, which means
they are not directly exposed to the tidal
waters of San Diego Bay, and the dikes
in the saltworks range from about 3 to
8 feet (1 to 2.5 meters) above the water
level (USFWS 2006, p. 3–64). Although
the San Diego Bay National Wildlife
Refuge is considering several potential
alternatives for managing south San
Diego Bay in the future, they all include
maintaining colonial waterbird nest
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sites, including for van Rossem’s gullbilled tern (USFWS 2006, pp. 2–47 to 2–
107). Therefore, we do not expect sealevel rise associated with anthropogenic
climate change to be a significant threat
to van Rossem’s gull-billed tern in San
Diego Bay.
While we lack information regarding
the specifics of the saltworks nest sites
in Mexico, it seems reasonable to
assume that the nest sites at these
locations will be similarly insulated
from sea-level rise by a system of dikes
that will be maintained for salt
production. Inland nesting locations in
Mexico (Cerro Prieto) and the United
States (the Salton Sea) are also not
threatened by sea-level rise resulting
from climate change. Additionally,
coastal areas of Mexico generally do not
face the same magnitude of ‘‘coastal
squeeze’’ scenarios that are predicted to
occur with sea-level rise in coastal
California because coastlines in Mexico
are not as developed and new nest sites
and foraging areas may be created as
coastline migrates inland and current
upland areas are converted to saltmarsh
or intertidal flats (Galbraith et al. 2002,
p. 177). Therefore, despite a high level
of uncertainty, we do not expect sealevel rise associated with anthropogenic
climate change to be a significant threat
to van Rossem’s gull-billed tern
throughout the subspecies’ range now or
in the foreseeable future.
Other available information on the
potential effects of anthropogenic global
climate change on van Rossem’s gullbilled tern includes a vulnerability
assessment for migratory waterbirds
within the African-Eurasian Flyway
(MacIean et al. 2007, pp. 1–100). This
assessment found a ‘‘minimal threat
from climate change’’ for the gull-billed
tern (MacIean et al. 2007, p. 84), which,
by range, would be referring to the
nominate subspecies (Gelochelidon n.
nilotica) (Gochfeld and Burger 1996, p.
645). However, the methodologies used
by MacIean et al. (2007, pp. 1–100) were
not appropriate to our status assessment
of van Rossem’s gull-billed tern because
the criteria and score levels they used
were largely subjectively determined
and did not translate well to our threatsbased assessment under the Act.
Therefore, this study does not provide
evidence to support a premise that
climate change is a significant threat to
van Rossem’s gull-billed tern.
While we recognize that climate
change is an important issue with
potential effects to listed species and
their habitats, we lack adequate
information to make precise
oceanographic and atmospheric
predictions regarding its effects to van
Rossem’s gull-billed tern, its prey, or its
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habitat. However, based on our review
and evaluation of the best currently
available data, we determine that the
potential direct effects of predicted
climate change on the subspecies is not
a significant threat to the van Rossem’s
gull-billed tern now or in the
foreseeable future.
Summary of Factor E
We identified that both inter-specific
and manmade nest site disturbance may
have an effect on the productivity of van
Rossem’s gull-billed terns. However,
their ability to relocate and renest
following disturbance combined with
the minimal amount of human
disturbance to nest sites in both Mexico
and the United States indicates that nest
site disturbance is not a significant
threat to the subspecies now or within
the foreseeable future.
Intentional killing of van Rossem’s
gull-billed terns has been very limited
in the past and currently only occurs for
human safety reasons in the United
States. There is no indication that it will
increase in the future. Illegal killing of
birds at aquaculture facilities in Mexico
has been observed but the extent to
which it occurs and what effect this may
have on the subspecies is not known.
Although it is likely to occur at some
level, the lack of documentation that
van Rossem’s gull-billed terns are
affected by this practice suggests that it
does not occur frequently. Thus,
intentional killing is not a significant
threat to the subspecies throughout its
range, nor is it likely to become a
significant threat within the foreseeable
future.
Contaminants, particularly DDT/DDE
and selenium, can negatively affect bird
species including van Rossem’s gullbilled tern and have been found at
elevated levels at certain nesting
locations, although very little data are
available with respect to van Rossem’s
gull-billed terns and their nest sites.
Based on the locations for which we
have information, contaminant levels
were below known thresholds for other
species. Moreover, van Rossem’s gullbilled terns are less likely to be exposed
to high levels of contaminants because
they eat a variety of foods, including
invertebrates, and contaminants levels
are less concentrated in invertebrates.
Therefore, contaminants are not likely a
significant threat to the subspecies now
or in the foreseeable future. Food
availability was also identified as a
potential threat. However, food
availability is naturally variable for most
species and van Rossem’s gull-billed
terns are highly opportunistic and
readily eat a wide variety of prey,
making them less vulnerable to changes
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in available prey items than species
with more specialized diets. As such,
food availability is not likely to be a
significant threat to van Rossem’s gullbilled tern now or within the
foreseeable future.
Small population size is a threat that
could leave van Rossem’s gull-billed
terns more vulnerable to stochastic
environmental events and natural
disasters, as well as genetic or
demographic problems. The best
available information suggests that the
population size of this subspecies was
likely always small, and it would appear
that the range has recently expanded,
suggesting that the overall population of
the subspecies is not limited. Therefore,
it is unlikely that small population size
is a significant threat now or within the
foreseeable future. Van Rossem’s gullbilled terns move readily between and
among populations between and
potentially within years, and their wide
range further ensures that small
population size is currently not a
significant threat, nor likely to become
one in the foreseeable future.
Sea-level rise resulting from climate
change is generally predicted to impact
coastal-nesting waterbirds like van
Rossem’s gull-billed tern; however,
impacts are likely to vary from species
to species and from nesting location to
nesting location. While climate change
could potentially affect van Rossem’s
gull-billed tern or its habitat,
information that is currently available
fails to provide evidence to support a
premise that climate change is a
significant threat to van Rossem’s gullbilled tern. Climate change-related sealevel rise is not expected to be a
significant threat on the U.S. nesting
locations in the foreseeable future, and
we have no evidence to suggest it will
significantly threaten the subspecies’
habitat in Mexico. Additionally,
potential temperature increases
associated with global climate change
are not likely to significantly affect the
subspecies throughout its range because
van Rossem’s gull-billed terns have
behavioral adaptations to keep eggs
within an acceptable temperature range
for development even under very high
environmental temperatures. Also,
severe drought would likely not
constitute a significant threat to the
subspecies because most of its breeding
range is coastal and marine food
resources would likely be unaffected.
Based on our review of the best
scientific and commercial information
available, we conclude that van
Rossem’s gull-billed tern is not
threatened by other natural or manmade
factors including nest site disturbance,
intentional killing, contaminants, food
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availability, small population size, or
climate change now or in the
foreseeable future.
Finding
As required by the Act, we considered
the five factors in assessing whether van
Rossem’s gull-billed tern is threatened
or endangered throughout all or a
significant portion of its range. We
examined the best scientific and
commercial information available
regarding the past, present, and future
threats faced by the van Rossem’s gullbilled tern. We reviewed the petition,
information available in our files, and
other available published and
unpublished information. In
considering what factors might
constitute threats, we must look beyond
the mere exposure of the species to the
factor to determine whether the species
responds to the factor in a way that
causes actual impacts to the species. If
there is exposure to a factor, but no
response, or only a positive response,
that factor is not a threat. If there is
exposure and the species responds
negatively, the factor may be a threat
and we then attempt to determine how
significant a threat it is. If the threat is
significant, it may drive or contribute to
the risk of extinction of the species such
that the species warrants listing as
threatened or endangered as those terms
are defined by the Act. This does not
necessarily require empirical proof of a
threat. The combination of exposure and
some corroborating evidence of how the
species is likely impacted could suffice.
The mere identification of factors that
could impact a species negatively is not
sufficient to compel a finding that
listing is appropriate; we require
evidence that these factors, alone or in
combination, are operative threats that
act on the species to the point that the
species meets the definition of
threatened or endangered under the Act.
Although foraging and nesting habitat
has been lost in the past within the
range of van Rossem’s gull-billed tern,
the subspecies’ flexibility in foraging
and nesting reduces the impact such
losses have on the subspecies. Unlike
most tern species, the foraging habitat
for the subspecies includes both upland
habitat and wetland areas. Additionally,
because the subspecies is a capable
flyer, it can quickly and effectively
move between areas in search of food.
Nest sites for van Rossem’s gull-billed
terns are more restrictive; they nest on
islands and other remote areas where
the risk of predation, especially from
terrestrial predators, is low. However,
nest site fidelity is low, meaning van
Rossem’s gull-billed terns can and may
move from one nest site to another, both
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between years or within a given year, to
renest after a predation or disturbance
event. Thus, provided nesting habitat is
available, they have no obvious
behavioral limitations that prevent them
from using it. As such, the subspecies is
not highly susceptible to loss of nesting
habitat and appears to be resilient to
changes in habitat.
Although there is the potential for
eggs and young of ground-nesting
colonial waterbirds to be harvested in
some areas in Mexico, the activity has
never been reported to affect van
Rossem’s gull-billed terns. If it occurs
now or in the foreseeable future, it is
unlikely to occur at levels (temporally,
geographically, or both) that pose a
significant threat to the subspecies
throughout its range or at any particular
nesting location. Therefore,
overutilization (Factor B) does not
appear to be a significant threat to van
Rossem’s gull-billed tern at this time.
Similarly, disease (including WNV)
(Factor C) does not appear to be a
significant threat at this time, and
neither do contaminants (DDT/DDE and
selenium) despite their presence in the
environment where the subspecies nests
and forages (Factor E).
Nest predation (Factor C) and
disturbance (Factor E) are a perennial
problem for ground-nesting bird species.
Van Rossem’s gull-billed terns nest on
islands and other remote areas where
the risk of predation and disturbance is
generally low. Disturbance may be from
naturally occurring species, humans,
pets, or livestock. Should a major
predation or disturbance event occur at
a nest site, van Rossem’s gull-billed
terns frequently relocate and renest.
Thus, van Rossem’s gull-billed terns
may still reproduce even when faced
with nest predation or severe
disturbance, thereby reducing the
magnitude of these threats should they
occur. Moreover, gull-billed terns are
long-lived. Should a colony fail to
reproduce in a given year, most of the
adult birds will likely have other
chances to reproduce. Thus, nest
predation and disturbance do not
significantly threaten the subspecies
throughout its range now or within the
foreseeable future.
Managers of other species have
targeted Van Rossem’s gull-billed terns
because they are predators. In the past,
a few gull-billed terns were killed to
protect California least tern and western
snowy plover nest colonies (Factor E).
However, no gull-billed terns have been
killed recently for this purpose, and no
lethal take permits have been granted
for such activities. As such, predator
control efforts (with van Rossem’s gullbilled terns as the targets) are not a
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current threat. Although three van
Rossem’s gull-billed terns were killed to
protect human health and safety (within
the vicinity of active airport runways),
these numbers of intentional loss are
small and all such actions occurred
within a population (the San Diego Bay
population) that has grown continually
since 1999. Additionally, unauthorized
lethal control (shooting) of van Rossem’s
gull-billed terns over commercial
shrimp aquaculture farms in Mexico has
been observed. Although information on
whether this activity is widespread is
not readily available, our review of the
available information does not indicate
a significant level of impact on van
Rossem’s gull-billed terns.
Van Rossem’s gull-billed terns are
generalist predators, opportunistically
consuming a variety of available prey
items. As a result, van Rossem’s gullbilled terns may shift to other types of
prey items should one become
unavailable because of natural or
human-influenced changes. This is in
contrast to most other tern species that
depend on fish as their primary prey. It
is unlikely that all potential prey items
for van Rossem’s gull-billed tern will be
affected at the same time. However,
should this occur, van Rossem’s gullbilled terns are capable of flying to
different locations to forage. If reduced
abundance of prey was to occur in
breeding areas, it would likely result in
the loss of productivity for that year, but
because van Rossem’s gull-billed terns
are long-lived, most individuals would
be expected to survive to nest the
following year. We have no information
to suggest that van Rossem’s gull-billed
terns are facing food shortages.
Therefore, food availability (Factor E) is
not a significant threat to the
subspecies.
With an estimated minimum breeding
population of approximately 1,600
adults, the population size of van
Rossem’s gull-billed tern is one of the
smallest of any tern taxon in North
America. Compared to larger
populations, small populations may be
more likely to be affected
disproportionately by demographic,
genetic, or environmental factors.
Although the population of van
Rossem’s gull-billed tern may be
relatively small, its range appears to
have recently expanded. This suggests
that the population is not markedly
affected by demographic or genetic
bottlenecks. Additionally, inbreeding
depression and genetic drift is less
likely in a subspecies comprised of
individuals that regularly move long
distances and occur at different nesting
locations from time to time, which van
Rossem’s gull-billed terns are known to
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do. Moreover, the wide range over
which the subspecies breeds suggests
that not all of the nesting areas would
be simultaneously affected by
catastrophic environmental events
(droughts, floods, hurricanes).
Therefore, although the small
population size is a potential cause for
concern, it does not appear that the
threats associated with small population
sizes (Factor E) are significantly
affecting van Rossem’s gull-billed tern
and are not likely to in the foreseeable
future.
Sea-level rise resulting from climate
change is generally predicted to impact
coastal-nesting waterbirds like van
Rossem’s gull-billed tern (Factor E);
however, the actual impacts are likely to
vary from species to species and from
nesting location to nesting location.
While climate change could potentially
affect van Rossem’s gull-billed tern or
its habitat, the limited amount of
available information fails to provide
evidence to support a premise that
climate change is a significant threat to
van Rossem’s gull-billed tern.
A species may be affected by more
than one threat in combination. Within
the preceding review of the five listing
factors, we have identified multiple
threats that may have interrelated
impacts on the subspecies. For example,
the productivity of van Rossem’s gullbilled terns may be reduced because of
the effects of predators (especially
terrestrial predators) (Factor C) or nestsite disturbance (Factor E). Likewise, a
physical change in nesting habitat
(Factor A), such as an island becoming
part of the mainland because of changes
in water level, may allow for increased
depredation or disturbance. Moreover,
the subspecies’ behavior of not nesting
in areas where depredation or
disturbance is likely may mean a nest
site is ‘‘abandoned’’ before nesting is
even attempted. Thus, the subspecies’
productivity may be reduced because of
these threats, either singularly or in
combination. However, it is not
necessarily easy to determine (nor is it
necessarily determinable) which
potential threat is the operational threat.
As we discuss above, regardless of its
source, we determine that such threats,
either individually or in combination,
are not likely to occur at a sufficient
geographical or temporal scale to
significantly affect the status of the
species.
Based on our review of the best
available scientific and commercial
information pertaining to the five
factors, we find that the threats, alone or
in combination, are not of sufficient
imminence, intensity, or magnitude to
indicate that van Rossem’s gull-billed
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tern is in danger of extinction
(endangered), or likely to become
endangered within the foreseeable
future (threatened) throughout its range.
Therefore, we find that listing van
Rossem’s gull-billed tern as an
endangered or threatened species
throughout its range is not warranted at
this time.
Distinct Vertebrate Population
Segments/Significant Portion of the
Range
After assessing whether the
subspecies is endangered or threatened
throughout its range, we next consider
whether a distinct vertebrate population
segment (DPS) exists and meets the
definition of endangered or is likely to
become endangered in the foreseeable
future (threatened). We also consider
whether the subspecies is endangered or
threatened within a significant portion
of its range. These assessments are
discussed below.
Distinct Vertebrate Population Segment
Under the joint DPS policy (61 FR
4722; February 7, 1996) of the Service
and National Marine Fisheries Service,
three elements are considered in the
decision concerning the establishment
and classification of a possible DPS.
These are applied similarly for
additions to or removal from the Federal
List of Endangered and Threatened
Wildlife. These elements include:
(1) The discreteness of a population in
relation to the remainder of the species
to which it belongs;
(2) The significance of the population
segment to the species to which it
belongs; and
(3) The population segment’s
conservation status in relation to the
Act’s standards for listing, delisting, or
reclassification (i.e., is the population
segment endangered or threatened).
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Discreteness
Under the DPS policy, a population
segment of a vertebrate taxon may be
considered discrete if it satisfies either
one of the following conditions:
(1) It is markedly separated from other
populations of the same taxon as a
consequence of physical, physiological,
ecological, or behavioral factors.
Quantitative measures of genetic or
morphological discontinuity may
provide evidence of this separation.
(2) It is delimited by international
governmental boundaries within which
differences in control of exploitation,
management of habitat, conservation
status, or regulatory mechanisms exist
that are significant in light of section
4(a)(1)(D) of the Act.
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We reviewed available information to
determine whether there are population
segments of van Rossem’s gull-billed
tern that meet the first discreteness
condition of our 1996 DPS policy. We
found no evidence that population
segments existed that were markedly
separated from each other as a
consequence of physical, physiological,
ecological, or behavioral factors. We are
not aware of measures of genetic or
morphological discontinuity that
provide evidence of marked separation.
As noted previously, van Rossem’s gullbilled terns are highly mobile. They are
migratory and regularly move between
breeding and wintering areas every year.
In the subspecies’ winter range,
individuals can mix and mingle with
other individuals. At the northern end
of the subspecies’ range, individuals
have been observed to move between
nesting locations between years (Molina
and Garrett 2001, p. 26; Patton 2001, p.
8; Molina 2004, p. 98), and the
information we have suggests that such
movements occur elsewhere within the
subspecies’ range. Even though a
superficial examination of nesting
locations (Figure 1) shows clusters of
nesting locations somewhat
geographically distant from other
clusters, the biology of the subspecies
suggests that interchange of individuals
occurs between and among these
clusters. In other words, an individual
van Rossem’s gull-billed tern that occurs
within a given cluster of nesting
locations during a given breeding season
may occur within a different cluster of
nesting locations the next year. As such,
these geographically separated clusters
are not biologically separate from each
other. Therefore, no population of van
Rossem’s gull-billed tern meets the first
discreteness condition of our 1996 DPS
policy.
We next evaluated whether any
population segments meet the second
discreteness condition of our 1996 DPS
policy. Nest locations at San Diego Bay
and Salton Sea can be delimited from all
other nest locations in Mexico by an
international governmental boundary
(Figure 1). However, after evaluating
available information, we have
concluded that breeding populations at
San Diego Bay and Salton Sea do not
meet the second discreteness condition
because differences in control of
exploitation, management of habitat,
conservation status, or regulatory
mechanisms between the U.S. and
Mexican populations are not significant
in light of section 4(a)(1)(D) of the Act.
Mexico and the United States are both
signatories to the Migratory Bird Treaty
Act, and two of the largest nesting
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populations of van Rossem’s gull-billed
terns in Mexico are located within
biosphere reserves where development
is limited by the LGEEPA (see Factor D).
We determined, based on a review of
the best available information, that there
are no populations of van Rossem’s gullbilled tern that meet the discreteness
conditions of the 1996 DPS policy. The
DPS policy is clear that significance is
analyzed only when a population
segment has been identified as discrete.
Because we found no population
segments that meet the discreteness
element under the Service’s DPS policy,
we will not conduct an evaluation of
significance under that policy. We
conclude that no population segment
qualifies as a listable DPS under the Act.
Significant Portion of the Range
Having determined that the van
Rossem’s gull-billed tern is not
endangered or threatened throughout its
range, we must next consider whether
there are any significant portions of the
range where the van Rossem’s gullbilled tern is in danger of extinction or
is likely to become endangered in the
foreseeable future.
Decisions by the Ninth Circuit Court
of Appeals in Defenders of Wildlife v.
Norton, 258 F.3d 1136 (2001) and
Tucson Herpetological Society v.
Salazar, 566 F.3d 870 (2009) found that
the Act requires the Service, in
determining whether a species is
endangered or threatened throughout a
significant portion of its range, to
consider whether lost historical range of
a species (as opposed to its current
range) constitutes a significant portion
of the range of that species. While this
is not our interpretation of the statute,
we first address the lost historical range
before addressing the current range.
Lost Historical Range
The available literature provides little
information on the historical breeding
range of van Rossem’s gull-billed tern.
The only historical nesting location
where nesting was confirmed was the
Salton Sea (Pemberton 1927, p. 253).
However, nesting was suspected at
various locations along the west coast of
mainland Mexico, possibly as far south
as the state of Oaxaca (see Molina and
Erwin 2006, pp. 273–274; see also the
‘‘Range and Distribution’’ section,
above). Although nesting has been
confirmed in modern times at certain
nesting locations in western mainland
Mexico—thereby validating the
suspicions of historical observers at
some, but not all, potential nesting
locations—the historical breeding range
of van Rossem’s gull-billed tern
everywhere except the Salton Sea is
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ambiguous and will remain so forever.
Thus, the historical breeding range of
van Rossem’s gull-billed tern may be
characterized as follows: The Salton Sea
and probably western mainland Mexico.
With the exception of the Salton Sea
nesting location (which was known
historically, but could not have existed
before the Salton Sea’s creation in its
modern form in 1907), the confirmation
of all other van Rossem’s gull-billed tern
nesting locations occurred in modern
times (1987 and later). Available
information on modern nesting
locations is summarized in Table 1,
with additional discussion in the
‘‘Range and Distribution’’ section,
above. As noted in that section, the
current southernmost confirmed nesting
´
location is Laguna Potosı, Guerrero, but
nesting farther south in Mexico
continues to be a possibility. As such,
despite increased certainty of the
subspecies’ current breeding range in
western Mexico compared to its
historical range, the southern limit of
that range remains ambiguous. Thus, the
current breeding range of the subspecies
may be characterized as follows: The
Salton Sea and south through the greater
Colorado River delta region, San Diego
Bay, Laguna Ojo de Liebre (Baja
California Sur), and western mainland
Mexico at least as far south as Laguna
´
Potosı (Guerrero) but possibly farther
south.
Although we acknowledge that there
is ambiguity in the historical and
modern breeding ranges, the ambiguities
are from essentially the same
geographical area, the southern Pacific
coast of Mexico (and possibly the
Pacific coast of Central America). The
ambiguity in the modern breeding range
is essentially a perpetuation of the
ambiguity in the historical breeding
range. Thus, the best available
information indicates that the current
breeding range of van Rossem’s gullbilled tern—with the modern
colonizations of San Diego Bay and
Laguna Ojo de Liebre—is larger than the
subspecies’ historical breeding range.
Thus, we conclude that no portions of
the subspecies’ breeding range have
been lost.
Little information is available on the
historical winter range of van Rossem’s
gull-billed tern. Even today, the current
winter range is not well defined. The
lack of historical and modern
information, especially for the southern
portion of the subspecies’ range, results
in historical and current winter ranges
that are ambiguous (see the ‘‘Range and
Distribution’’ section for details), much
in the way the breeding ranges are
ambiguous. After reviewing the
available information, the historical and
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current winter ranges of van Rossem’s
gull-billed tern can be characterized as
follows: Coastal western Mexico and
possibly western Central America. We
are not aware of any differences
between the subspecies’ current winter
range compared to its historical winter
range. Thus, we conclude that no
portions of the subspecies’ winter range
have been lost.
Information on the areas over which
van Rossem’s gull-billed terns migrate is
also limited. That area has likely had a
corresponding increase associated with
the modern colonization of nesting sites
along the Pacific coast of the Baja
California Peninsula, Mexico, and
extreme southwestern United States.
Thus, we conclude that no portions of
the subspecies’ range used for migration
have been lost. Therefore, there is no
lost historical range of van Rossem’s
gull-billed tern that could constitute a
significant portion of the range of the
subspecies.
Current Range
The Act defines ‘‘endangered species’’
as any species which is ‘‘in danger of
extinction throughout all or a significant
portion of its range,’’ and ‘‘threatened
species’’ as any species which is ‘‘likely
to become an endangered species within
the foreseeable future throughout all or
a significant portion of its range.’’ The
definition of ‘‘species’’ is also relevant
to this discussion. The Act defines
‘‘species’’ as follows: ‘‘The term
‘species’ includes any subspecies of fish
or wildlife or plants, and any distinct
population segment [DPS] of any
species of vertebrate fish or wildlife
which interbreeds when mature.’’ The
phrase ‘‘significant portion of its range’’
(SPR) is not defined by the statute, and
we have never addressed in our
regulations: (1) The consequences of a
determination that a species is either
endangered or likely to become so
throughout a significant portion of its
range, but not throughout all of its
range; or (2) what qualifies a portion of
a range as ‘‘significant.’’
Two recent district court decisions
have addressed whether the SPR
language allows the Service to list or
protect less than all members of a
defined ‘‘species’’: Defenders of Wildlife
v. Salazar, 729 F. Supp. 2d 1207 (D.
Mont. 2010), concerning the Service’s
delisting of the Northern Rocky
Mountain gray wolf (74 FR 15123, Apr.
12, 2009); and WildEarth Guardians v.
Salazar, 2010 U.S. Dist. LEXIS 105253
(D. Ariz. Sept. 30, 2010), concerning the
Service’s 2008 finding on a petition to
list the Gunnison’s prairie dog (73 FR
6660, Feb. 5, 2008). The Service had
asserted in both of these determinations
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that it had authority, in effect, to protect
only some members of a ‘‘species,’’ as
defined by the Act (i.e., species,
subspecies, or DPS), under the Act. Both
courts ruled that the determinations
were arbitrary and capricious on the
grounds that this approach violated the
plain and unambiguous language of the
Act. The courts concluded that reading
the SPR language to allow protecting
only a portion of a species’ range is
inconsistent with the Act’s definition of
‘‘species.’’ The courts concluded that
once a determination is made that a
species (i.e., species, subspecies, or
DPS) meets the definition of
‘‘endangered species’’ or ‘‘threatened
species,’’ it must be placed on the list
in its entirety and the Act’s protections
applied consistently to all members of
that species (subject to modification of
protections through special rules under
sections 4(d) and 10(j) of the Act).
Consistent with that interpretation,
and for the purposes of this finding, we
interpret the phrase ‘‘significant portion
of its range’’ in the Act’s definitions of
‘‘endangered species’’ and ‘‘threatened
species’’ to provide an independent
basis for listing; thus there are two
situations (or factual bases) under which
a species would qualify for listing: A
species may be endangered or
threatened throughout all of its range; or
a species may be endangered or
threatened in only a significant portion
of its range. If a species is in danger of
extinction throughout an SPR, it, the
species, is an ‘‘endangered species.’’
The same analysis applies to
‘‘threatened species.’’ Based on this
interpretation and supported by existing
case law, the consequence of finding
that a species is endangered or
threatened in only a significant portion
of its range is that the entire species
shall be listed as endangered or
threatened, respectively, and the Act’s
protections shall be applied across the
species’ entire range.
We conclude, for the purposes of this
finding, that interpreting the SPR phrase
as providing an independent basis for
listing is the best interpretation of the
Act because it is consistent with the
purposes and the plain meaning of the
key definitions of the Act; it does not
conflict with established past agency
practice (i.e., prior to the 2007
Solicitor’s Opinion), as no consistent,
long-term agency practice has been
established; and it is consistent with the
judicial opinions that have most closely
examined this issue. Having concluded
that the phrase ‘‘significant portion of
its range’’ provides an independent
basis for listing and protecting the entire
species, we next turn to the meaning of
‘‘significant’’ to determine the threshold
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for when such an independent basis for
listing exists.
Although there are potentially many
ways to determine whether a portion of
a species’ range is ‘‘significant,’’ we
conclude, for the purposes of this
finding, that the significance of the
portion of the range should be
determined based on its biological
contribution to the conservation of the
species. For this reason, we describe the
threshold for ‘‘significant’’ in terms of
an increase in the risk of extinction for
the species. We conclude that a
biologically based definition of
‘‘significant’’ best conforms to the
purposes of the Act, is consistent with
judicial interpretations, and best
ensures species’ conservation. Thus, for
the purposes of this finding, and as
explained further below, a portion of the
range of a species is ‘‘significant’’ if its
contribution to the viability of the
species is so important that without that
portion, the species would be in danger
of extinction.
We evaluate biological significance
based on the principles of conservation
biology using the concepts of
redundancy, resiliency, and
representation. Resiliency describes the
characteristics of a species and its
habitat that allow it to recover from
periodic disturbance. Redundancy
(having multiple populations
distributed across the landscape) may be
needed to provide a margin of safety for
the species to withstand catastrophic
events. Representation (the range of
variation found in a species) ensures
that the species’ adaptive capabilities
are conserved. Redundancy, resiliency,
and representation are not independent
of each other, and some characteristic of
a species or area may contribute to all
three. For example, distribution across a
wide variety of habitat types is an
indicator of representation, but it may
also indicate a broad geographic
distribution contributing to redundancy
(decreasing the chance that any one
event affects the entire species), and the
likelihood that some habitat types are
less susceptible to certain threats,
contributing to resiliency (the ability of
the species to recover from disturbance).
None of these concepts is intended to be
mutually exclusive, and a portion of a
species’ range may be determined to be
‘‘significant’’ due to its contributions
under any one or more of these
concepts.
For the purposes of this finding, we
determine if a portion’s biological
contribution is so important that the
portion qualifies as ‘‘significant’’ by
asking whether without that portion, the
representation, redundancy, or
resiliency of the species would be so
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impaired that the species would have an
increased vulnerability to threats to the
point that the overall species would be
in danger of extinction (i.e., would be
‘‘endangered’’). Conversely, we would
not consider the portion of the range at
issue to be ‘‘significant’’ if there is
sufficient resiliency, redundancy, and
representation elsewhere in the species’
range that the species would not be in
danger of extinction throughout its
range if the population in that portion
of the range in question became
extirpated (extinct locally).
We recognize that this definition of
‘‘significant’’ (a portion of the range of
a species is ‘‘significant’’ if its
contribution to the viability of the
species is so important that without that
portion, the species would be in danger
of extinction) establishes a threshold
that is relatively high. On the one hand,
given that the consequences of finding
a species to be endangered or threatened
in an SPR would be listing the species
throughout its entire range, it is
important to use a threshold for
‘‘significant’’ that is robust. It would not
be meaningful or appropriate to
establish a very low threshold whereby
a portion of the range can be considered
‘‘significant’’ even if only a negligible
increase in extinction risk would result
from its loss. Because nearly any portion
of a species’ range can be said to
contribute some increment to a species’
viability, use of such a low threshold
would require us to impose restrictions
and expend conservation resources
disproportionately to conservation
benefit: listing would be rangewide,
even if only a portion of the range of
minor conservation importance to the
species is imperiled. On the other hand,
it would be inappropriate to establish a
threshold for ‘‘significant’’ that is too
high. This would be the case if the
standard were, for example, that a
portion of the range can be considered
‘‘significant’’ only if threats in that
portion result in the entire species’
being currently endangered or
threatened. Such a high bar would not
give the SPR phrase independent
meaning, as the Ninth Circuit held in
Defenders of Wildlife v. Norton, 258
F.3d 1136 (9th Cir. 2001).
The definition of ‘‘significant’’ used in
this finding carefully balances these
concerns. By setting a relatively high
threshold, we minimize the degree to
which restrictions will be imposed or
resources expended that do not
contribute substantially to species
conservation. But we have not set the
threshold so high that the phrase ‘‘in a
significant portion of its range’’ loses
independent meaning. Specifically, we
have not set the threshold as high as it
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was under the interpretation presented
by the Service in the Defenders
litigation. Under that interpretation, the
portion of the range would have to be
so important that current imperilment
there would mean that the species
would be currently imperiled
everywhere. Under the definition of
‘‘significant’’ used in this finding, the
portion of the range need not rise to
such an exceptionally high level of
biological significance. (We recognize
that if the species is imperiled in a
portion that rises to that level of
biological significance, then we should
conclude that the species is in fact
imperiled throughout all of its range,
and that we would not need to rely on
the SPR language for such a listing.)
Rather, under this interpretation we ask
whether the species would be
endangered everywhere without that
portion, i.e., if that portion were
completely extirpated. In other words,
the portion of the range need not be so
important that even the species being in
danger of extinction in that portion
would be sufficient to cause the species
in the remainder of the range to be
endangered; rather, the complete
extirpation (in a hypothetical future) of
the species in that portion would be
required to cause the species in the
remainder of the range to be
endangered.
The range of a species can
theoretically be divided into portions in
an infinite number of ways. However,
there is no purpose to analyzing
portions of the range that have no
reasonable potential to be significant or
to analyzing portions of the range in
which there is no reasonable potential
for the species to be endangered or
threatened. To identify only those
portions that warrant further
consideration, we determine whether
there is substantial information
indicating that: (1) The portions may be
‘‘significant,’’ and (2) the species may be
in danger of extinction there or likely to
become so within the foreseeable future.
Depending on the biology of the species,
its range, and the threats it faces, it
might be more efficient for us to address
the significance question first or the
status question first. Thus, if we
determine that a portion of the range is
not ‘‘significant,’’ we do not need to
determine whether the species is
endangered or threatened there; if we
determine that the species is not
endangered or threatened in a portion of
its range, we do not need to determine
if that portion is ‘‘significant.’’ In
practice, a key part of the determination
that a species is in danger of extinction
in a significant portion of its range is
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whether the threats are geographically
concentrated in some way. If the threats
to the species are essentially uniform
throughout its range, no portion is likely
to warrant further consideration.
Moreover, if any concentration of
threats to the species occurs only in
portions of the species’ range that
clearly would not meet the biologically
based definition of ‘‘significant,’’ such
portions will not warrant further
consideration.
After reviewing the potential threats
throughout the range of van Rossem’s
gull-billed tern, we determine that there
may be two portions of the tern’s
breeding range that could be considered
to have concentrated threats for the
subspecies there. Below, we outline the
elevated threats found at two nesting
locations, the Salton Sea in California
and the islands in the impoundments
´
associated with Campo Geotermico
Cerro Prieto (Cerro Prieto geothermal
generation facility) in northeast Baja
California (Table 1, Figure 1). We then
assess whether these portions of the
subspecies’ breeding range may meet
the biologically based definition of
‘‘significant,’’ that is, whether the
contributions of these portions of the
gull-billed tern’s range to the viability of
the subspecies is so important that
without those portions, the species
would be in danger of extinction.
The decreasing water levels at the
nesting location at Salton Sea and
changing water storage practices at the
nesting location at Cerro Prieto have the
potential to be considered as
concentrations of threats at each of these
nesting locations (see Summary of
Information Pertaining to the Five
Factors). The observed and anticipated
reduction in water levels at these
locations may lead to an increase in nest
predation (Factor C) at either site.
Increased nest predation would likely
result in reduced reproductive output.
Moreover, the subspecies’ behavior of
selecting islands and other areas where
terrestrial nest predators are less likely
to occur makes the relative lack of
predators part of what constitutes
nesting habitat for this subspecies.
Thus, observed and anticipated changes
in water levels may also lead to a loss
of nesting habitat at the respective
locations (Factor A).
In general, for taxa that are sessile
(anchored) or of limited mobility, loss of
habitat would typically translate into
some concurrent loss of individuals,
which in turn would translate into some
concomitant effect on the overall
population. However, individual adult
van Rossem’s gull-billed terns are highly
mobile; they can and do move, both in
terms of their seasonal migratory
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movements and in terms of their ability
to move between nesting locations from
year to year and within years. For
example, if van Rossem’s gull-billed
terns returning from their wintering
areas found that a particular nesting
location no longer provided nesting
habitat, the available information
suggests that the birds can and would
move to a different nesting location.
Thus, habitat loss at either of these
nesting locations would not necessarily
result in a direct reduction in the
subspecies’ overall population.
However, we expect that moving to a
different nesting location would not be
without consequences. Instead, we
expect that the relocated birds would
concentrate in other existing nesting
locations (in potentially lower quality
nest sites within existing nesting
locations) or that they would occupy
new, potentially less-suitable (lower
quality) nesting locations.
Consequently, the effects of the loss of
nesting habitat would likely result in
reduced reproductive output by the
subspecies.
Because the van Rossem’s gull-billed
tern faces elevated threats at the Salton
Sea and Cerro Prieto nesting locations,
we next assess whether these portions of
the subspecies’ breeding range may
meet the biologically based definition of
‘‘significant.’’ For both areas, we
evaluate whether the portion’s
biological contribution is so important
that the portion qualifies as
‘‘significant’’ by asking whether without
that portion, the representation,
redundancy, or resiliency of the species’
would be so impaired that the species
would have an increased vulnerability
to threats to the point that the overall
species would be in danger of
extinction.
Although each nesting location has
features that make it unique, we have no
evidence, whether based on the
locations’ geography or the subspecies’
biology, that suggests these nesting
locations are markedly different from
any other nesting location. For example,
the nesting habitat is essentially the
same at all nesting locations. As with
nesting habitat, the subspecies’ foraging
habitat is similar throughout its range,
whether during the breeding season,
winter, or migration. Although coastal
nesting locations are more common than
the inland nesting locations that Salton
Sea and Cerro Prieto represent, van
Rossem’s gull-billed terns essentially
nest in the same types of areas inland
as they do in coastal nesting locations.
Gull-billed terns (subspecies unknown)
have also been observed nesting at other
´
inland locations in Mexico (Gomez de
Silva 2005, p. 501; Molina and Erwin
PO 00000
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Fmt 4701
Sfmt 4702
58679
2006, p. 274) (see the ‘‘Range and
Distribution’’ section, above).
As mobile birds, individual van
Rossem’s gull-billed terns are not tied to
any particular nesting location, and
often move between nesting locations.
Van Rossem’s gull-billed terns that nest
at either the Salton Sea or Cerro Prieto
are not permanent occupants of either
location. Van Rossem’s gull-billed terns
leave each of these areas to winter
farther south. As stated under ‘‘Biology’’
in the Species Information section, van
Rossem’s gull-billed terns appear to be
opportunistic and adaptable nesters,
displaying low nest-site fidelity, and
even moving to new sites and renesting
within the same year. Groups of van
Rossem’s gull-billed terns have
displayed such renesting behavior at the
Salton Sea (Molina 2009b, pp. 6–7) and
´
´
at Bahıa Santa Marıa (Palacios and
Mellink 2007, p. 218). Van Rossem’s
gull-billed terns will readily take
advantage of new nest sites as well as
sites that are not available every year
(for example, Molina 2005, p. 4; Molina
2009b, p. 2). If the Salton Sea and Cerro
Prieto could no longer support nesting,
other existing and potential nesting
locations are distributed along a 2,250km (1,400-mi) stretch of the subspecies’
breeding range from southern California
to Guerrero, Mexico (see Figure 1).
There are currently nine nesting
locations along the coast with multiple
nest sites where breeding colonies have
been documented. There is sufficient
representation and redundancy of
nesting habitat in the subspecies’
breeding range such that van Rossem’s
gull-billed tern would not be in danger
of extinction if either or both of the
Salton Sea and Cerro Prieto nesting
locations were completely lost.
Elimination of the Salton Sea and
Cerro Prieto nesting locations would not
result in the elimination of the
individual van Rossem’s gull-billed
terns that would have otherwise nested
at those locations. The loss of both or
either of the Salton Sea or Cerro Prieto
portions of the subspecies’ range would
not directly result in a reduction in the
subspecies’ overall population, but there
may be a temporary reduction in the
local populations’ reproductive output
compared to what it would have been.
This potential reduction of reproductive
output is not expected to reduce the
subspecies’ range of variation or
adaptive capabilities to such a level that
they would be in danger of extinction.
Without these two nesting locations, we
expect that the resiliency of van
Rossem’s gull-billed tern would not be
appreciably impacted; the subspecies
would continue to be able to recover
from periodic disturbance and
E:\FR\FM\21SEP3.SGM
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withstand catastrophic events in other
parts of its range.
In summary, although there are
elevated threats related to potential
changes in water level at Cerro Prieto
and Salton Sea, these portions of the
van Rossem’s gull-billed tern’s range are
not significant portions of its range.
Even if these nesting colonies were
abandoned at some time in the future,
it is likely that van Rossem’s gull-billed
terns would move and nest elsewhere,
as they are not tied to any particular
nesting location. As noted above, there
is little that biologically distinguishes
either Cerro Prieto or the Salton Sea
from other nesting locations for van
Rossem’s gull-billed tern. They each
happen to be inland, which
undoubtedly contributes to the shared
threat of changes in water levels, but the
nesting and foraging areas at each of
these sites do not differ notably from
those in the subspecies’ entire range.
Existing and potential nesting locations
are distributed along a 2,250-km (1,400-
VerDate Mar<15>2010
15:35 Sep 20, 2011
Jkt 223001
mi) stretch of the subspecies’ breeding
range from southern California to
Guerrero, Mexico. Neither Cerro Prieto
nor the Salton Sea, nor even the two
nesting locations combined, is a
‘‘significant’’ portion of the species’
range because their contribution to the
viability of the subspecies is not so
important that the subspecies would be
in danger of extinction without those
portions.
We find that van Rossem’s gull-billed
tern is not in danger of extinction now,
nor is it likely to become endangered
within the foreseeable future throughout
all or a significant portion of its range.
Therefore, listing van Rossem’s gullbilled tern as endangered or threatened
under the Act is not warranted at this
time.
We request that you submit any new
information concerning the status of, or
threats to, van Rossem’s gull-billed tern
to our Carlsbad Fish and Wildlife Office
(see ADDRESSES section) whenever it
becomes available. New information
will help us monitor van Rossem’s gull-
PO 00000
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billed tern and encourage its
conservation. If an emergency situation
develops for the van Rossem’s gullbilled tern or any other species, we will
act to provide immediate protection.
References Cited
A complete list of references cited is
available on the Internet at https://
www.regulations.gov and upon request
from the Carlsbad Fish and Wildlife
Office (see ADDRESSES section).
Authors
The primary authors of this notice are
staff members of the Carlsbad Fish and
Wildlife Office.
Authority: The authority for this section
is section 4 of the Endangered Species Act of
1973, as amended (16 U.S.C. 1531 et seq.).
Dated: September 9, 2011.
Daniel M. Ashe,
Director, Fish and Wildlife Service.
[FR Doc. 2011–24048 Filed 9–20–11; 8:45 am]
BILLING CODE 4310–55–P
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[Federal Register Volume 76, Number 183 (Wednesday, September 21, 2011)]
[Proposed Rules]
[Pages 58650-58680]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2011-24048]
[[Page 58649]]
Vol. 76
Wednesday,
No. 183
September 21, 2011
Part III
Department of the Interior
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Fish and Wildlife Service
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50 CFR Part 17
Endangered and Threatened Wildlife and Plants; 12-Month Finding on a
Petition To List Van Rossem's Gull-Billed Tern as Endangered or
Threatened; Proposed Rule
Federal Register / Vol. 76 , No. 183 / Wednesday, September 21, 2011
/ Proposed Rules
[[Page 58650]]
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DEPARTMENT OF THE INTERIOR
Fish and Wildlife Service
50 CFR Part 17
[FWS-R8-ES-2010-0035; MO 92210-0-0008-B2]
Endangered and Threatened Wildlife and Plants; 12-Month Finding
on a Petition To List Van Rossem's Gull-billed Tern as Endangered or
Threatened
AGENCY: Fish and Wildlife Service, Interior.
ACTION: Notice of 12-month petition finding.
-----------------------------------------------------------------------
SUMMARY: We, the U.S. Fish and Wildlife Service, announce a 12-month
finding on a petition to list van Rossem's gull-billed tern
(Gelochelidon nilotica vanrossemi) as endangered or threatened and to
designate critical habitat under the Endangered Species Act of 1973, as
amended (Act). After review of the best available scientific and
commercial information, we find that listing van Rossem's gull-billed
tern is not warranted at this time. However, we ask the public to
submit to us any new information that becomes available concerning the
threats to van Rossem's gull-billed tern or its habitat at any time.
DATES: The finding announced in this document was made on September 21,
2011.
ADDRESSES: This finding is available on the Internet at https://www.regulations.gov at Docket Number FWS-R8-ES-2010-0035. Supporting
documentation we used in preparing this finding is available for public
inspection, by appointment, during normal business hours at the U.S.
Fish and Wildlife Service, Carlsbad Fish and Wildlife Office, 6010
Hidden Valley Road, Suite 101, Carlsbad, California 92011. Please
submit any new information, materials, comments, or questions
concerning this finding to the above street address.
FOR FURTHER INFORMATION CONTACT: Jim Bartel, Field Supervisor, Carlsbad
Fish and Wildlife Office, 6010 Hidden Valley Road, Suite 101, Carlsbad,
California 92011; by telephone at 760-431-9440; or by facsimile to 760-
431-9624. If you use a telecommunications device for the deaf (TDD),
you may call the Federal Information Relay Service (FIRS) at 800-877-
8339.
SUPPLEMENTARY INFORMATION:
Background
Section 4(b)(3)(B) of the Endangered Species Act of 1973, as
amended (Act) (16 U.S.C. 1531 et seq.) requires that, for any petition
to revise the Federal Lists of Endangered and Threatened Species that
contains substantial scientific or commercial information that listing
the species may be warranted, we make a finding within 12 months of the
date of receipt of the petition. In this finding, we will determine
that the petitioned action is: (1) Not warranted, (2) warranted, or (3)
warranted, but the immediate proposal of a regulation implementing the
petitioned action is precluded by other pending proposals to determine
whether species are endangered or threatened, and expeditious progress
is being made to add or remove qualified species from the Federal Lists
of Endangered and Threatened Species. Section 4(b)(3)(C) of the Act
requires that we treat a petition for which the requested action is
found to be warranted but precluded as though resubmitted on the date
of such finding, that is, requiring a subsequent finding to be made
within 12 months. We must publish these 12-month findings in the
Federal Register.
Previous Federal Actions
In our November 15, 1994, Candidate Notice of Review (59 FR 58982),
we included van Rossem's gull-billed tern as a Category 2 candidate.
Category 2 taxa were defined as those taxa for which information in the
possession of the Service, at that time, indicated that proposing to
list as endangered or threatened was possibly appropriate but for which
persuasive data on biological vulnerability and threats were not
available to support proposed rules. In our February 28, 1996,
Candidate Notice of Review (61 FR 7596), we announced our decision to
discontinue recognition of Category 2 candidates, including van
Rossem's gull-billed tern. This decision was finalized on December 5,
1996 (61 FR 64481). Since that time, van Rossem's gull-billed tern has
not been treated as a candidate for Federal listing under the Act.
On June 8, 2009, we received a petition from the Center for
Biological Diversity dated June 3, 2009, requesting that we list the
``western'' or ``van Rossem's'' subspecies of gull-billed tern
(Gelochelidon nilotica vanrossemi) as endangered or threatened under
the Act, and that we designate critical habitat concurrent with listing
(CBD 2009, pp. 1-40). Included in the petition was supporting
information regarding the subspecies' taxonomy, ecology, distribution,
status, and potential threats. Although not expressly stated in the
petition, we assumed the petition was a request to list van Rossem's
gull-billed tern as endangered or threatened throughout the subspecies'
entire range.
In response to the Center for Biological Diversity's June 3, 2009,
petition to list van Rossem's gull-billed tern as endangered or
threatened throughout its range, we sent a letter to Center for
Biological Diversity, dated August 18, 2009, acknowledging receipt of
the petition and informing the petitioner that we concluded the
petition did not indicate that an emergency situation existed for this
subspecies and that emergency listing was not warranted. We also stated
that we were addressing a significant number of listing and critical
habitat actions in Fiscal Year 2009 (October 1, 2008, through September
30, 2009) pursuant to court orders, judicially approved settlement
agreements, or other statutory deadlines; however, we noted that we had
secured funding to begin reviewing the petition in that fiscal year.
Further, we said we anticipated publishing our 90-day finding in Fiscal
Year 2010.
We published our 90-day finding on the petition to list van
Rossem's gull-billed tern as endangered or threatened in the Federal
Register on June 9, 2010 (75 FR 32728). In that finding we determined
that the petition presented substantial scientific or commercial
information, per section 4(b)(3)(A) of the Act, indicating that listing
the van Rossem's gull-billed tern throughout its range may be
warranted. The current notice constitutes the 12-month finding on the
June 3, 2009, petition to list the van Rossem's gull-billed tern
throughout its range as endangered or threatened under the Act.
Species Information
Species Description and Taxonomy
Van Rossem's gull-billed tern (Gelochelidon nilotica vanrossemi) is
medium-sized compared to other tern species (Parnell et al. 1995, p.
2). Like most tern species, its plumage is generally pale gray above
(dorsally), white below (ventrally), with breeding (alternate) plumage
adults having black on the top of the head (Parnell et al. 1995, p. 2).
Gull-billed terns, including van Rossem's gull-billed tern, differ from
other species of terns by having a proportionately stouter bill that is
black throughout the year (Bent 1921, p. 201; Parnell et al. 1995, p.
2; Pyle 2008, p. 706). Gull-billed terns are powerful flyers, and
despite appearing heavier bodied than most tern species, they exhibit a
buoyant agility, especially while foraging (Audubon 1840, p. 1; Bent
1921, p. 201; Molina and Marschalek 2003, p. 3).
[[Page 58651]]
Van Rossem's gull-billed tern is a seabird in the avian order
Charadriiformes (shorebirds, gulls and terns, auks, and allies) and
family Laridae (skuas, gulls, terns, and skimmers) (AOU 1998, pp. 141
and 181), although terns are sometimes considered a separate family,
Sternidae (e.g., Ridgeway 1919, p. 458; Gochfeld and Burger 1996, pp.
572 and 624; Ericson et al. 2003, pp. 1-14).
Gelochelidon is a monotypic genus (a genus with only one species,
Gelochelidon nilotica, the gull-billed tern). Gelochelidon has
historically been placed in synonymy with Sterna (e.g., Saunders 1876,
p. 644). However, a more recent analysis using mitochondrial DNA and
morphological features concluded that the gull-billed tern is
sufficiently differentiated from other tern species to resurrect
Gelochelidon as a genus separate from Sterna (Bridge et al. 2005, pp.
459-469; see also Banks et al. 2006, p. 930).
The gull-billed tern (the species as a whole) has a worldwide
distribution, albeit discontinuous, and may comprise up to six
subspecies (Parnell et al. 1995, p. 3; Gochfeld and Burger 1996, p.
645). Of those, two subspecies are described in North America (Molina
2008, p. 188), with Gelochelidon nilotica aranea breeding along the
Atlantic and Gulf of Mexico coasts of the United States and
northeastern Mexico, and with G. n. vanrossemi breeding along the
Pacific and Gulf of California coasts, primarily in Mexico (see ``Range
and Distribution'' section below) (Molina and Erwin 2006, pp. 271-272).
Bancroft (1929, pp. 283-286) described Gelochelidon nilotica
vanrossemi from specimens collected at the Salton Sea, Imperial County,
California. According to Bancroft (1929, p. 284), van Rossem's gull-
billed tern differs from the nominate subspecies of the Old World (G.
n. nilotica) by its shorter tail and bill shape (less angular gonys),
and from the subspecies of eastern North America, G. n. aranea, by its
``decidedly larger size.'' However, in contrast to the petitioner's
assertion that the validity of the subspecies (i.e., its
distinctiveness) has not been questioned (CBD 2009, p. 4), information
in the scientific literature indicates that some authors have
questioned the distinctiveness of van Rossem's gull-billed tern. For
example, Murphy (1936, p. 1093) noted the paucity of specimens from the
New World and concluded ``existing subspecific names have been created
far in advance of any adequate study of the facts.'' Murphy's published
statements of dissatisfaction over the available information, in turn,
caused Grinnell and Miller (1944, p. 172) to ``not recognize a western
race'' (i.e., subspecies) of gull-billed tern in their authoritative
review of the birds of California. Although additional specimens are
now available, providing larger sample sizes in mensural (measurement)
data, geographic representation of specimens from western North
America, especially from Mexico and Central America, are still limited
(Molina and Erwin 2006, pp. 273, 283, and 294-295).
Individual gull-billed terns are typically not identifiable to
subspecies under field conditions, and because the two North American
subspecies are distinguished on the basis of average morphometric
differences that show substantial overlap, even individual specimens
are not necessarily distinguishable in the hand (Molina and Erwin 2006,
p. 283). This suggested to Unitt (2004, p. 249) that the
distinctiveness of the G. n. vanrossemi as a subspecies remains not
entirely conclusive (see also Patten and Unitt (2002, pp. 26-35)
regarding the pitfalls of differentiating subspecies based on average
differences). Moreover, Pyle (2008, p. 706) stated that the
morphological differences of the western North American gull-billed
terns are ``too slight for subspecific recognition.''
In contrast, other authors have not questioned the distinctiveness
of Gelochelidon nilotica vanrossemi as a subspecies. For example, the
American Ornithologists' Union (AOU) Committee on Classification and
Nomenclature (AOU Committee), the long-standing scientific body
responsible for standardizing North American avian taxonomy, recognized
G. n. vanrossemi in its 1957 (fifth) edition of its checklist of North
American birds (AOU 1957, p. 233), which was the last time the AOU
Committee explicitly addressed subspecies (AOU 1998, p. xii). More
recently, Patten et al. (2003, pp. 1-363), who critically reviewed the
taxonomy of subspecies in their book on the birds of the Salton Sea
region (Patten et al. 2003, p. 71), also recognized G. n. vanrossemi as
valid (distinctive) (Patten et al. 2003, p. 188). Additionally, G. n.
vanrossemi is recognized by many other authors (such as Parnell et al.
1995, p. 3; Gochfeld and Burger 1996, p. 645; Patten et al. 2001, p.
45; Dickinson 2003, p. 149; Molina and Erwin 2006, p. 273, but see p.
283; and Molina et al. 2010, p. 1). However, the authors of this latter
group of works may not have conducted taxonomic assessments of their
own and may instead have relied upon other publications. Thus, in
total, the available scientific literature is not consistent regarding
the distinctiveness of van Rossem's gull-billed tern.
The Service is currently funding the U.S. Geological Survey to
conduct a genetics-based study that may yield additional information
regarding the distinctiveness between the eastern and western North
American subspecies of the gull-billed tern, but only those two
subspecies. As of the preparation of this status review and 12-month
finding, the results of this work are not yet available. Although we
anticipate the information from this study will be helpful in
understanding the relationship between the eastern and western
subspecies of gull-billed terns in North America, a comprehensive,
rangewide review is needed to address fully the distinctiveness of all
of the subspecies, including Gelochelidon nilotica vanrossemi, that
compose the gull-billed tern species. We are not aware of any modern,
rangewide treatments that evaluate the taxonomic distinctiveness of
gull-billed tern subspecies.
In summary, the available scientific information presents differing
opinions regarding the distinctiveness of Gelochelidon nilotica
vanrossemi as a subspecies. Although this contradicts the petitioner's
assertion that the subspecies' distinctiveness has never been
questioned (CBD 2009, p. 4), the available information does not
conclusively support the abandonment of a long-standing, established
taxon that is accepted by the AOU Committee and is widely used in the
literature. Therefore, for the purposes of evaluating the petitioned
action, we assume G. n. vanrossemi, van Rossem's gull-billed tern, is a
subspecies per section 3(16) of the Act.
Range and Distribution
Van Rossem's gull-billed terns are migratory (Molina et al. 2010,
p. 5), which means they breed in one area during the spring and summer
and then move (migrate) to a different area for the winter. Like most
birds in the Northern Hemisphere, they nest in northerly locations
during the summer and overwinter farther south, presumably using the
Pacific coast of North America as a migratory route (Molina et al.
2010, p. 5). In the U.S. portion of the subspecies' breeding range,
where monitoring is more intensive and data sets are more complete, van
Rossem's gull-billed terns generally arrive in mid-March and leave in
late August, although some birds stay until September or October
(Patten et al. 2003, p. 188; Patton 2009, Table 2). Less is known about
the migratory habits of populations in Mexico.
[[Page 58652]]
Nesting of what would later be described as the van Rossem's
subspecies of gull-billed tern was first noted at the Salton Sea in
1927 (Pemberton 1927, pp. 253-258). Reports of historical observations
and museum specimen data suggested van Rossem's gull-billed terns bred
in Mexico (van Rossem and Hachisuka 1937, p. 333; Friedmann et al.
1950, p. 107; Binford 1989, p. 115; Molina and Erwin 2006, pp. 273-274
and 294-295), but it was not until the 1990s that nesting of the
subspecies was actually observed in that country (Palacios and Mellink
2007, p. 214). The majority of nesting locations were discovered in
Mexico only after 2000 as a result of focused surveys (Palacios and
Mellink 2007, p. 217).
As detailed below, the current breeding range for van Rossem's
gull-billed tern is western North America from extreme southern
California in the United States to the State of Guerrero in Mexico.
Within this general range, the subspecies occurs in discrete nesting
locations predominantly along the Pacific coast of Mexico including the
Gulf of California (Molina and Erwin 2006, p. 273) (Table 1, Figure 1).
An additional coastal nesting colony is located in San Diego Bay, San
Diego County, California (Molina 2008, p. 188). Nest colonies are also
located at inland localities in northeastern Baja California, Mexico
(Molina and Garrett 2001, p. 25; Palacios and Mellink 2007, p. 215),
and at the Salton Sea, Imperial County, California (Pembarton 1927, p.
253; Molina 2004, p. 94; Molina 2009b, p. 5). The Salton Sea and San
Diego Bay are currently the only locations where the subspecies nests
in the United States (Molina and Erwin 2006, p. 273), and together they
define the northern extent of the breeding range of van Rossem's gull-
billed tern. However, as this document was being finalized, a pair of
van Rossem's gull-billed terns attempted to nest at the San Joaquin
Marsh and Wildlife Sanctuary in Irvine, Orange County, California
(Daniels 2011, in litt.), which is roughly 135 kilometers (km) (85
miles (mi)) north of the San Diego Bay nesting location. It is too
early to know whether this location will be regularly used by the
subspecies in the future.
Table 1--List of Known Nesting Locations of Van Rossem's Gull-Billed Tern (Gelochelidon nilotica vanrossemi) in
the United States and Mexico (Arranged North to South)
[Approximate population size over the past decade for coarse-scale comparisons (Large--typically greater than
100 pairs, Medium--typically between 15 and 100 pairs, and Small--typically less than 15 pairs]
----------------------------------------------------------------------------------------------------------------
Population size
Country State Nesting location \a\ \b\ Citations \c\
----------------------------------------------------------------------------------------------------------------
U.S............ California........ Salton Sea (multiple Large........... Pembarton 1927, p. 253; Molina
nest sites). 2004, pp. 92-99; Molina 2010b,
in litt., p. 3.
U.S............ California........ San Diego Bay.......... Medium.......... McCaskie 1987, p. 1488; Patton
2009, Table 2.
Mexico......... Baja California... Campo Geot[eacute]rmico Large........... Molina and Garrett 2001, p. 24;
Cerro Prieto Palacios and Mellink 2007, p.
(including Las 217; Erickson et al. 2009, p.
Arenitas). 508; Molina 2010b, in litt., p.
3; Palacios 2010, p. 11.
Mexico......... Baja California... Isla Montague, Colorado Large........... Palacios and Mellink 1993, p.
River Delta. 259; Peresbarbosa and Mellink
1994, p. 201; Peresbarbosa and
Mellink 2001, p. 266; Molina et
al. 2006, p. 5; Palacios and
Mellink 2007, p. 217; Molina
2010b, in litt., p. 3; Palacios
2010, p. 11.
Mexico......... Baja California Laguna Ojo de Liebre Small........... Danemann and Carmona 2000, pp.
Sur. (Salinas de Guerrero 195-199; Palacios and Mellink
Negro). 2007, p. 217; Palacios 2010, p.
11.
Mexico......... Sinaloa........... Bah[iacute]a Santa Small........... Gonz[aacute]lez-Bernal et al.
Mar[iacute]a 2003, p. 176; Mu[ntilde]oz del
(including Isla El Viejo et al. 2004, pp. 191-202;
Rancho and Isla Palacios and Mellink 2007, p.
Altamura). 217; Palacios 2010, p. 11.
Mexico......... Sinaloa........... Bah[iacute]a de Ceuta.. Small........... Gonz[aacute]lez-Medina and
Guevara-Medina 2008, p. 6;
Palacios 2010, p. 11.
Mexico......... Sinaloa........... Laguna del Caimanero Medium.......... Palacios and Mellink 2007, p.
(Las Tres Tumbas). 217; Palacios 2010, p. 11.
Mexico......... Sinaloa/Nayarit... Marismas Nacionales Large........... Palacios and Mellink 2007, p.
(including Estero 217; Palacios 2010, p. 11.
Teacap[aacute]n and
Laguna Pericos (Laguna
las Garzas), Nayarit).
Mexico......... Colima............ Laguna Cuyutl[aacute]n. Medium.......... Palacios and Mellink 2007, p.
217; Palacios 2010, p. 11.
Mexico......... Guerrero.......... Laguna Potos[iacute]... Small........... Mellink et al. 2009, p. 8.
----------------------------------------------------------------------------------------------------------------
\a\ Nesting locations are general areas that may comprise more than one nest site. Some locations may not be
occupied every year.
\b\ The population size is for general comparison only; the level of accuracy and precision varies between
sources and nesting populations differ from year to year.
\c\ Citations include noteworthy sources for the nesting location as well as sources for population ranges.
[[Page 58653]]
[GRAPHIC] [TIFF OMITTED] TP21SE11.000
The southern limit to the breeding range of van Rossem's gull-
billed tern is not precisely known. The southernmost location where van
Rossem's gull-billed terns have been observed nesting is Laguna
Potos[iacute] in the Mexican State of Guerrero (Table 1, Figure 1).
Information in the literature shows that gull-billed terns occur during
the breeding season in small numbers in Mexico south of Laguna
Potos[iacute] (Binford 1989, p. 115; Mellink et al. 1998, p. 381;
Molina and Erwin 2006, pp. 294-295; Palacios and Mellink 2007, p. 220).
Although actual nesting has never been observed at any of these
southern locations, breeding is suspected at some (for example, Binford
1989, p. 115; Mellink et al. 1998, p. 381). These areas are all within
the winter range of the subspecies (Figure 1) and nonbreeding birds may
remain in this region during the breeding season (Howell and Webb 1995,
p. 303), which is a confounding factor in assessing observations that
do not include actual detections of nests.
Additionally, Table 1 only includes locations where actual nesting
has been observed, but breeding behavior (such as courtship) has been
noted at other locations, suggesting nesting may be more widespread.
These other locations with observed breeding behavior but without
observation of actual nests include locations in the Mexican States of
Sonora (historically) (van Rossem and Hachisuka 1937, p. 333) and
Jalisco (Mellink et al. 2009, p. 48), both of which are within the
range van Rossem's gull-billed terns are known to nest. Additionally,
nesting may occur in Mexico near or along the Colorado River, north of
the known nesting location of Isla Montague at the delta (Erickson et
al. 2005, p. 498). Moreover, there are likely smaller ephemeral sites
that are not used every year that are probably missed during
inconsistent survey efforts. Also, gull-billed terns have been observed
nesting at inland locations in Mexico (G[oacute]mez de Silva 2005, p.
501; Molina and Erwin 2006, p. 274), which may consist of colonies
containing either North American subspecies.
Although some gull-billed tern specimens from south of Guerrero
have been identified as van Rossem's gull-billed terns (Hellmayr and
Conover 1948, p. 297; Binford 1989, p. 115), the majority of the
occurrences reported in the available literature are field
observations; thus, these records have not been identified to
subspecies. Gull-billed terns also nest farther south along the Pacific
coast of South America; however, specimen data suggest that at least
some of these birds are the ``eastern'' subspecies of gull-billed tern,
Gelochelidon nilotica aranea (Molina and Erwin 2006, p. 283; but see
Hellmayr and Conover 1948, p. 297, footnote 1). The northern extent of
the range of the Pacific-breeding birds presumed to be G. n. aranea is
not known and could potentially include Central America, where
available data are limited. Thus, the southern limit of the breeding
range of van Rossem's gull-billed tern extends at least as far south as
Guerrero, and possibly farther south, but survey information from these
southern areas is limited and any
[[Page 58654]]
conclusions drawn from observational data are confounded by the
potential occurrence of birds of the ``eastern'' subspecies, G. n.
aranea.
The winter range of the subspecies includes the Gulf of California
and the Pacific coast of mainland Mexico, possibly Pacific coastal
Central America and coastal northwestern South America (Molina and
Erwin 2006, p. 272; Molina et al. 2009a, pp. 2-20; Molina et al. 2010,
p. 1), with the largest concentrations found in the extensive coastal
lagoon systems of southern Sonora, Sinaloa, and northern Nayarit
(Molina et al. 2009a, p. 9). However, similar to the breeding range,
the southern part of the winter range is poorly defined (Molina et al.
2009a, pp. 9-11). Although at least one specimen collected from
Guatemala in winter (Molina and Erwin 2006, p. 294) was thought to be
Gelochelidon nilotica vanrossemi (Hellmayr and Conover 1948, p. 297),
the potential mingling of the ``eastern'' subspecies of gull-billed
terns along the Pacific coast of southern Mexico and Central America
complicates our ability to delineate the winter range of van Rossem's
gull-billed tern (Molina et al. 2009a, p. 15). Not only are individuals
of the G. n. aranea subspecies that breed in western South America
possible in the region (the available literature is not specific as to
the winter range of these South American-nesting birds), individuals
that breed in eastern North America (G. n. aranea) may also cross from
the Gulf of Mexico (such as at the Isthmus of Tehuantepec or Isthmus of
Panama) to winter along the Pacific coast (Gochfeld and Burger 1996, p.
645; Molina and Erwin 2006, pp. 283-284).
Such behavior has been documented for other species of terns and
gulls (Molina and Erwin 2006, p. 84). As such, ``eastern'' gull-billed
terns potentially intermingle with van Rossem's gull-billed terns
within the southern portion of the latter's range. However, we do not
know how prevalent this is. Moreover, the available literature has
evolved through time. Contrary to earlier accounts (for example, AOU
1957, p. 233; Molina and Erwin 2006, p. 282), Molina et al. (2009a, p.
15) suggested that the winter range may not extend south of the Isthmus
of Tehuantepec; thus, without firm data the subspecies' range remains
equivocal. In addition to coastal locations, small numbers of gull-
billed terns, presumably van Rossem's gull-billed terns, regularly
occur at inland sites in western Mexico during the winter, away from
Pacific coastal lowlands (Molina et al. 2010, p. 12); thus, the winter
range likely includes inland areas of western Mexico and possibly
Central America.
The best available information indicates the breeding range of the
subspecies has expanded in recent years. The first record for coastal
California (and the first record for the Pacific coast north of the
southern tip of the Baja California Peninsula) was of an adult detected
along San Diego Bay in July 1985 (McCaskie 1985, p. 962). Evidence of
nesting was noted there two years later (McCaskie 1987, p. 1488; Unitt
2004, p. 248). Initially, the population grew slowly and sporadically,
but after 1999 the population increased much more quickly and steadily,
totaling approximately 59 pairs in 2010 (R. Patton, in litt., 2010,
spreadsheet summary). Moreover, despite multiple earlier explorations
of the avifauna of the Baja California Peninsula, Mexico (Bryant 1889,
pp. 237-320; Grinnell 1928, p. 61; Wilbur 1987, pp. 94-95; Massey and
Palacios 1994, pp. 45-57), van Rossem's gull-billed terns were only
first noted in 1995 as nonbreeders along the Pacific coast of the Baja
California Peninsula (Erickson et al. 2001, p. 125) and first found
nesting in 1996 at Laguna Ojo de Liebre near Guerrero Negro, Baja
California Sur (Danemann and Carmona 2000, p. 197). Laguna Ojo de
Liebre is the only known coastal nesting location on either coast of
the 1,200-km-long (750-mi-long) peninsula (Molina et al. 2010, p. 61).
The colonization of these two new coastal nesting locations suggests
the breeding range of the subspecies has expanded in recent years.
Such range expansions are not unprecedented; other colonial
waterbird species have similarly expanded their range along the Pacific
coast and established nesting colonies, such as the elegant tern
(Thalasseus elegans) (Collins et al. 1991, pp. 393-395) and the black
skimmer (Rynchops niger) (Palacios and Alfaro 1992, pp. 173-176;
Collins and Garrett 1996, pp. 127-135; Danemann and Carmona 2000, p.
197). Black skimmers have also moved northward along the Gulf of
California coast and even inland at the north end of the Gulf; for
example, establishing nesting colonies at the Salton Sea (McCaskie et
al. 1974, pp. 337-338; Collins and Garrett 1996, pp. 127-135) and Cerro
Prieto (Molina and Garrett 2001, p. 25). Van Rossem's gull-billed terns
use similar nesting habitat as black skimmers, often nesting near one
another at locations where their ranges overlap (Parnell et al. 1995,
p. 9). Although the timing of the range expansion of van Rossem's gull-
billed terns has lagged behind the black skimmer and other species with
expanding ranges, it is possible that van Rossem's gull-billed terns
may be following a similar pattern and could start to colonize new
nesting locations along the Pacific Coast.
There is some indication that van Rossem's gull-billed terns may
potentially continue to expand their range northward along the
California coast. Birds that migrate long distances, such as van
Rossem's gull-billed terns, have the potential to occur outside their
expected range (i.e., vagrancy). Other subspecies of gull-billed terns
are capable of long-distance flights and we assume van Rossem's gull-
billed terns are similarly capable. For example, an individual of the
nominate (European) subspecies was banded as a nestling in Denmark and
collected a few months later in Barbados in the Lesser Antilles in the
western Atlantic Ocean (Lincoln 1936, p. 331; see also Cooke 1945, p.
128)--roughly 4,500 km (3,000 mi) outside of its expected winter range
in western Africa (Gochfeld and Burger 1996, p. 645). Another gull-
billed tern, probably of the Asian subspecies Gelochelidon nilotica
affinis (G. c. addenda), was observed on the Hawaiian islands of O'ahu,
Moloka'i, and Maui over a span of several months (Pyle and Pyle 2009,
no page number), more than 8,000 km (5,000 mi) away from its expected
winter range in Southeast Asia (Gochfeld and Burger 1996, p. 645).
Although we do not have information on similar long-distance,
extralimital movements for van Rossem's gull-billed tern, birds
presumed to be of this subspecies have been observed north of the San
Diego Bay region (the northernmost nesting location within the
subspecies' expected range), including multiple detections of single
birds along the California coast as far north as the San Francisco Bay
area (Patton 2009, Appendix B) and at inland locations along the
Colorado River and elsewhere in Arizona (Speich and Witzeman 1973, p.
148; Monson and Phillips 1981, p. 50; Rosenberg et al. 1990, p. 193).
Such movements of van Rossem's gull-billed terns, though not
unexpected, occur too infrequently to consider these areas as part of
the subspecies' range. However, the number of detections of van
Rossem's gull-billed terns farther north along the coast of California
has increased as the San Diego Bay breeding population has increased
(see discussion below in the ``Population Size'' section). As such,
areas where other species of terns nest along the coast north of San
Diego should be monitored for nesting gull-billed terns. Confirmation
of van
[[Page 58655]]
Rossem's gull-billed terns nesting north of San Diego Bay, like the
recent nesting attempt detected in Orange County mentioned above, would
indicate a continuing northward expansion of the subspecies' breeding
range.
In summary, the current breeding range of van Rossem's gull-billed
tern extends from San Diego and the Salton Sea along the Pacific and
Gulf of California coasts to at least as far south as the State of
Guerrero in Mexico. Actual nesting locations are discontinuously
distributed within that range (Table 1). However, survey information is
limited for most of the Pacific coast of Mexico; additional efforts may
yet detect other nesting locations in this region, including south of
Guerrero. The current winter range of the subspecies includes the west
coast of mainland Mexico, potentially as far south as Central America
and coastal northwestern South America, plus a few inland locations.
Population Size
Historical data on population sizes are generally lacking for the
subspecies, especially in western Mexico and farther south into Central
and South America. As noted above, historical information shows that
van Rossem's gull-billed terns occurred in Mexico, but these data
largely consist of anecdotal observations or museum collections
(specimens); there are few data to indicate the size of historical
populations of van Rossem's gull-billed terns. Available literature
that include information on the historical avifauna of western Mexico,
such as Bryant (1889 pp. 237-320), Brewster (1902, pp. 1-241), Salvin
and Godman (1904, pp. 1-505), Ridgway (1919 pp. 1-852), Mailliard
(1923, pp. 443-456), Huey (1927, pp. 239-243), Grinnell (1928, pp. 1-
300), van Rossem and Hachisuka (1937, p. 333), van Rossem (1945, p.
93), Hellmayr and Conover (1948, p. 297), Friedmann et al. (1950, pp.
1-204), Schaldach (1963, pp. 1-510), Binford (1989, p. 115), and
Russell and Monson (1998, pp. 115-116) (see also summary in Palacios
and Mellink 2007, pp. 214-215), present limited or no information on
gull-billed terns from the region. Many of the cited historical texts
predate the 1929 formal description of Gelochelidon nilotica
vanrossemi, the van Rossem's subspecies of gull-billed tern. Regardless
of the subspecies or the timing of the historical observations, early
observers would have been able to identify the species as a whole--G.
nilotica, the gull-billed tern. As discussed in the Species Description
and Taxonomy section, the available information indicates that the
subspecies of the gull-billed tern that breeds in western Mexico (at
least north of the Isthmus of Tehuantepec) is G. n. vanrossemi. Thus,
the historical observations of gull-billed terns in western Mexico most
likely pertained to G. n. vanrossemi. The information that is available
from these sources indicates that gull-billed terns were rarely
encountered, and when encountered, were in small numbers. By
comparison, the information on other species of colonial waterbirds in
western Mexico is much more complete. Although this list of references
is not a fully exhaustive list of historical resources, it illustrates
the contrast between historical information available on gull-billed
terns and other species of colonial waterbirds that occurred in western
Mexico. This contrast indicates that the historical scientific
explorations of the region were adequate to detect many other species
of colonial waterbirds, but were inadequate to detect gull-billed terns
or their nest sites in western Mexico. It is reasonable to conclude
that van Rossem's gull-billed terns were encountered rarely because
there were comparatively few van Rossem's gull-billed terns to
encounter. Therefore, we conclude based on the available information,
the historical population size of van Rossem's gull-billed terns in
western Mexico was small--or at least not markedly larger than the
population today.
In the United States, when Pemberton first discovered the nesting
colony of gull-billed terns at the Salton Sea in 1927, he estimated
that there were approximately 500 active nests (Pemberton 1927, p.
256), which would translate into a similar number of pairs. It is not
clear when this population became established, but the Salton Sea was
created in its present form between 1905 and 1907 when Colorado River
floodwaters filled the dry lakebed known as the Salton Sink; however,
previous historical and prehistorical floods also periodically filled
the Salton Sink from time to time (with intervening dry periods),
forming an intermittent body of water within the Salton Sink now
referred to as Lake Cahuilla (see Patten et al. 2003, pp. 1-6 for a
history of Lake Cahuilla and the Salton Sea). Although the Salton Sea
population of van Rossem's gull-billed terns was not systematically
monitored until the 1990s, anecdotal evidence shows that the population
decreased over time to a low somewhere in the range of 15 to 25 pairs
in the early 1970s (Grinnell and Miller 1944, p. 172; Pyle and Small
1961, p. 31; McCaskie 1973, p. 919; McCaskie 1974, p. 949; McCaskie
1976, p. 1004; Garrett and Dunn 1981, p. 189; McCaskie pers. comm.
2010). Over the next few decades, the population at the Salton Sea
increased to about 100 to 150 pairs, with more consistent monitoring
showing that it has remained fairly constant since the early 1990s
(Molina 2004, p. 94; Molina 2009b, p. 5). In San Diego Bay, the nesting
population of van Rossem's gull-billed terns has increased from its
inception in 1987 to 59 pairs in 2010 (R. Patton, in litt., 2010,
spreadsheet summary).
Today in Mexico, in addition to the new, small colony at Laguna Ojo
de Liebre, van Rossem's gull-billed terns have colonized the islands in
the impoundments associated with the Campo Geot[eacute]rmico Cerro
Prieto (Cerro Prieto geothermal generation facility) in northeast Baja
California. The facility started operation in 1973 (Guti[eacute]rrez-
Galindo et al. 1988, p. 201) and van Rossem's gull-billed terns have
been observed there since at least 1996 (Molina and Garrett 2001, p.
25). Since 1996, fairly consistent monitoring at this site indicates
that it has grown to be one of the largest populations (Table 1).
Additionally, the nesting colony at Isla Montague has been fairly well
monitored since 1992 (Palacios and Mellink 1993, p. 259; Molina 2010b,
in litt.). Although nesting at Isla Montague was only just confirmed in
1992 (Palacios and Mellink 1993, p. 259), nesting on the island was
suspected decades earlier based on specimens collected there in the
spring of 1915 (Friedmann et al. 1950, p. 107; Molina and Erwin 2006,
p. 294; Molina et al. 2010, p. 61).
As mentioned in the ``Range and Distribution'' section, gull-billed
terns have been known to occur in western Mexico for more than a
century (see Molina and Erwin 2006, p. 294) and breeding there was
likely; however, nesting has only been documented recently. Surveys at
nesting locations throughout the remainder of the breeding range of van
Rossem's gull-billed tern in Mexico have been sporadic and essentially
consist of ``snapshots'' of nesting efforts over time. During the
breeding seasons of 2003 and 2005, Palacios and Mellink (2003, pp. 1-
66; 2006, pp. 1-84; 2007, pp. 214-222) surveyed at least 367 potential
nesting areas along the Pacific and Gulf of California coasts of
Mexico. Additionally, of the nine known nesting locations in Mexico
(Table 1), all but Laguna Potos[iacute] were resurveyed in June and
early July 2010 (Palacios 2010, pp. 1-28). However, the level of survey
effort compared with the number of potential nesting locations along
the coast of Mexico suggests additional
[[Page 58656]]
undetected nesting locations likely exist. For example, one of the
largest single colonies of this subspecies (105 to 160 pairs) was only
discovered in 2003 at Laguna Las Garzas (Laguna Los Pericos) in
Marismas Nacionales, Nayarit (Table 1) (Palacios and Mellink 2003, p.
11; Palacios and Mellink 2007, p. 217). New (but small) populations
were also found nesting in 2006 at Bah[iacute]a de Ceuta, Sinaloa
(Gonz[aacute]lez-Medina and Guevara-Medina 2008, pp. 6-7) and in 2007
at Laguna Potos[iacute], Guerrero (Mellink et al. 2009, p. 8) (Table
1). Thus, although we expect additional nesting locations to be found
and population estimates to change, we do not expect refinements in
those values to alter substantially our understanding of the subspecies
or our analysis.
As summarized by Molina et al. (2010, p. 10), 737 to 808 pairs of
van Rossem's gull-billed terns appear to have nested in western North
America in 2003 and 2005, with approximately 550 of those nesting in
Mexico. Because these values generally represent pairs of nesting
adults counted at nesting sites, there are additional nonbreeding
individuals that are not represented in these totals, underestimating
the total population size. Additionally, there may be a limited number
of pairs nesting at undetected locations. Thus, these rough estimates
represent the minimum population size for van Rossem's gull-billed
terns in the United States and Mexico.
Population data for most of the subspecies' range are incomplete
over time; thus, population trends are difficult to assess. Data from
the Salton Sea, which are fairly complete, shows a marked decline in
population compared to the historical high in 1927, but this population
has remained fairly stable since the 1990s (Molina et al. 2010, p. 10).
Although preliminary data suggest the numbers of nesting van Rossem's
gull-billed terns at the Salton Sea during the 2010 nesting season was
substantially smaller (Molina, in litt., 2010, p. 3), it is not clear
whether this is a temporary or longer-term change; marked declines have
been observed there in the past, but they have been temporary (Molina,
in litt., 2010, p. 3). The available information from the nesting
locations in Mexico with the most-complete population data (Isla
Montague and Cerro Prieto) shows that population sizes at these
locations are variable (Palacios and Mellink 2007, p. 217). The
populations at these sites also appear to be connected, with
individuals moving between these nesting locations and the Salton Sea
nesting location and, to a lesser extent, the San Diego Bay nesting
location (Molina and Garrett 2001, p. 26; Molina 2004, p. 98; Palacios
2010, pp. 12 and 15). In combination, the populations of van Rossem's
gull-billed terns at Isla Montague, Cerro Prieto, and the Salton Sea
are annually variable but, when taken together, appear to have been
fairly stable since the 1990s (see Molina et al. 2006, p. 5; Molina and
Erwin 2006, p. 279; Palacios and Mellink 2007, p. 217; Molina et al.
2010, p. 10). Data from central and southern Mexico--the bulk of the
subspecies' range geographically but not, as suggested by the data, in
numerical terms--are inadequate to define precise trends, but they do
not show any precipitous declines (see Molina and Erwin 2006, p. 279;
Palacios and Mellink 2007, p. 217). Moreover, as discussed above, the
historical size of the van Rossem's gull-billed tern population in the
rest of Mexico was likely never large.
Biology
Van Rossem's gull-billed tern is predominantly a coastal nesting
species, but it also nests at, or near, certain inland saline lakes
(Parnell et al. 1995, p. 5; Molina and Erwin 2006, p. 284; Molina et
al. 2010, p. vii). During the nonbreeding season, van Rossem's gull-
billed terns may occur at either saline or freshwater areas (Molina et
al. 2010, p. 12), but they are often found foraging over tidal mudflats
within large lagoons and estuaries (Molina et al. 2009a, p. 12). Like
other terns, gull-billed terns (including van Rossem's gull-billed
tern) are predators, but they differ from most other tern species in
how they forage and in the types of prey they consume. Unlike many
other tern species that eat only fish caught by shallow dives into
water, gull-billed terns forage on a variety of prey items, which
varies by area. For example:
(1) Gull-billed terns capture flying insects during foraging
flights (Parnell et al. 1995, p. 5);
(2) They swoop down and snatch up terrestrial prey (such as small
crabs, lizards, insects, or small chicks of other bird species) and
aquatic prey (such as small fish) near the water's surface (Parnell et
al. 1995, p. 5; Molina and Marschalek 2003, p. i); and
(3) They land to capture small prey items from the water's surface
(Parnell et al. 1995, p. 5).
Moreover, gull-billed terns--the species as a whole, including van
Rossem's gull-billed terns--are opportunistic foragers (Parnell et al.
1995, p. 5; Gochfeld and Burger 1996, p. 645; Erwin et al. 1998a, p.
323; Molina 2009a, p. 6). Not only do they eat a wide variety of prey
items and forage over wide range of areas, they also may
opportunistically focus on certain prey items when those items are
abundant or otherwise readily accessible. For instance, gull-billed
terns in western Africa were observed preferentially foraging on
fiddler crabs (Uca tangeri), despite being an energy-poor food source,
because the crabs were abundant and easier to capture than other, more
energy-rich prey items (Stienen et al. 2008, p. 243). The diet and
general foraging habits of van Rossem's gull-billed tern is similar to
that of other subspecies of gull-billed tern (Molina and Marschalek
2003, p. 9; Molina and Erwin 2006, pp. 286-287; Molina 2009a, pp. 6-8;
Molina et al. 2009a, p. 12).
Thus, van Rossem's gull-billed terns are generalist predators whose
food appears to be determined more by size and availability of prey
items rather than strictly by the type of prey. The foraging habitat of
van Rossem's gull-billed terns consists of open mudflats in tidal
estuaries, river margins, beaches, salt marshes, freshwater marshes,
aquacultural impoundments (such as shrimp ponds), and a variety of
upland habitats including open scrub, pasturelands and irrigated
agricultural fields and associated canals and drains, and the airspace
over such areas (Molina and Erwin 2006, p. 284; Parnell et al. 1995,
pp. 4-5). A university-based study is currently underway in San Diego
Bay to evaluate the foraging patterns and relative use of areas within
San Diego Bay and the adjacent coastline; the results of this study are
not yet available.
Gull-billed terns, including van Rossem's gull-billed terns, nest
in colonies of 20 to 50 pairs, although numbers may vary (Parnell et
al. 1995, p. 9). They display low nest-site fidelity; that is, they are
not closely tied to any one nest site from year to year, even moving to
new sites and renesting within the same year (e.g., after disturbance
or predation events) (Parnell et al. 1995, p. 13; Erwin et al. 1998b,
p. 970). Groups of van Rossem's gull-billed terns have displayed such
renesting behavior at the Salton Sea (Molina 2009b, pp. 6-7) and at
Bah[iacute]a Santa Mar[iacute]a (Palacios and Mellink 2007, p. 218)
(Table 1). Van Rossem's gull-billed terns also readily take advantage
of new nest sites or sites that are not available every year (for
example, Molina 2005, p. 4; Molina 2009b, p. 2). Thus, van Rossem's
gull-billed terns appear to be opportunistic and adaptable nesters.
The term ``nest colony'' may refer to the group of birds or a
geographic location. A nesting location (as used in Table 1) may
contain more than one colony. In general, a colony consists of
[[Page 58657]]
the terns that occupy a nest site during a particular nesting attempt.
A nest site is the specific location where a group of terns is nesting.
Individual terns within a colony may move between nest sites between
nesting attempts within a given breeding season (within-year
movements). For example, after nest failure at one nest site, members
of a colony may move within the same breeding season to one (or more)
nest sites at a different location (or locations) within the general
nesting location (Molina et al. 2010, p, 17). We also refer to the
groups of individuals that collectively use nesting locations as
``populations.'' Even though it appears that van Rossem's gull-billed
tern populations return to nesting locations (the general area), groups
of individuals may establish colonies at different nest sites within
those general areas from year to year (between-year movements).
Moreover, these populations are not necessarily fixed over time.
Because van Rossem's gull-billed terns can fly long distances,
individuals of a population may move between and among other
populations, more likely occurring between years but potentially even
within years. For example, between-year movements among nest locations
(populations) have been observed in the northern portion of the
subspecies' range where many individual van Rossem's gull-billed terns
are banded, which allows specific birds to be resighted, and thus
tracked, over time (Molina and Garrett 2001, p. 26; Patton 2001, p. 8;
Molina 2004, p. 98; Palacios 2010, pp. 12 and 15).
Nests of van Rossem's gull-billed terns consist of shallow scrapes
with simple adornments, such as rocks, shells, or fish bones (Parnell
et al. 1995, p. 10). Although some individuals may form pairs during
migration, breeding activity reaches its peak when birds arrive at
nesting areas (Sears 1981, p. 192; Parnell et al. 1995, p. 8). The
breeding season generally occurs from mid-March through August, at
least within the northern portion of its breeding range (Parnell et al.
1995, pp. 4 and 9). The timing of nest initiation varies from place to
place and year to year, with some colonies reinitiating nesting after
predation or disturbance events and moving to other nearby nest sites
(Molina 2009b, pp. 6-7). Such renesting can occur repeatedly in one
nesting season or birds may simply abandon nesting at that nesting
location for a given year (Molina 2009b, pp. 6-7).
Nesting habitat for van Rossem's gull-billed terns consists of low,
open areas on natural and artificial beaches, islands, and levees,
usually with no or sparse vegetation (Parnell et al. 1995, pp. 5 and
10; Palacios and Mellink 2007, p. 215). Typically, these areas are
located on islands or other remote areas where the risk of predation is
low. Barren areas suitable as nest sites are often kept clear by
natural or artificial disturbance regimes, especially tidal inundation,
that prevent or limit plant growth. Although gull-billed terns
typically nest in areas above most high tides (Bent 1921, p. 198;
Parnell et al. 1995, p 4), it is not uncommon for active nests to be
destroyed by the highest tides (Erwin et al. 1998b, p. 976;
Peresbarbosa and Mellink 2001, p. 268; Molina and Erwin 2006, p. 286;
Patton 2009, p. 9).
At San Diego Bay and the Salton Sea, van Rossem's gull-billed terns
typically lay two to three eggs per clutch (Parnell et al. 1995, p.
12). The egg incubation period is 22 to 23 days, and the young fledge
after 28 to 35 days (Parnell et al. 1995, p. 11). Similar to other tern
species (see Dunn 1972, pp. 360-366; Buckley and Buckley 1974, pp.
1053-1063; Shealer and Burger 1995, pp. 93-99), juvenile gull-billed
terns remain dependent upon their parents for at least 4 weeks after
fledging and probably longer, during which time they learn to forage
and fend for themselves (Parnell et al. 1995, p. 12). Thus, van
Rossem's gull-billed terns only raise one brood per year (Parnell et
al. 1995, p. 9); any subsequent renesting attempts typically follow a
disturbance or predation event that occurs early within the breeding
season.
Terns that survive to become adults are generally long-lived
(Gochfeld and Burger 1996, p. 640) with lifespans of 10 to 20 years or
even more (such as Thompson et al. 1997, p. 15; Cuthbert and Wires
1999, p. 19; Shealer 1999, pp. 17-18; Buckley and Buckley 2002, p. 18;
Hatch 2002, p. 25). Lifespan information on the entire gull-billed tern
species is limited, with even less known about van Rossem's gull-billed
tern. Other subspecies of gull-billed terns are known to first breed at
5 years old, but can establish territories at nest sites without
breeding at 4 years old (Parnell et al. 1995, p. 12). A few van
Rossem's gull-billed terns of known age have been observed nesting as
3-year-olds (Molina et al. 2010, p. 6). Banded gull-billed terns have
been recovered in Europe almost 16 years post-banding, and 14 years
post-banding in eastern North America (Parnell et al. 1995, p. 12).
Patton (2009, p. 9) noted a banded van Rossem's gull billed tern that
was at least 9 years old at the San Diego Bay colony (and presumably
breeding), and 10-year-old birds have been observed at the Salton Sea
(Molina et al. 2010, p. 6). We believe the lifespan of van Rossem's
gull-billed tern to be similar to other tern species (i.e., 10 to 20
years, possibly more).
Management Actions
Through our Division of Migratory Birds Management, the Service is
the lead Federal agency for managing and conserving migratory birds in
the United States under the Migratory Bird Treaty Act (MBTA). We
provide national and international leadership in the conservation and
management of migratory birds by promoting, among the Service and its
partners, science-based management of both populations and habitat on
and off Service lands in support of national and international bird
plans and initiatives.
In 2002 and 2008, pursuant to the Fish and Wildlife Conservation
Act of 1980, as amended (16 U.S.C. 2901 et seq.), the Service included
the gull-billed tern (the species as a whole) in the list of Birds of
Conservation Concern (USFWS 2002, pp. 1-99; USFWS 2008, pp. 1-87). The
species was included as a Bird of Conservation Concern both nationally
and in certain specific Bird Conservation Regions, including the U.S.
portions of Bird Conservation Regions 32 (Coastal California) and 33
(Sonoran and Mojave Deserts) (USFWS 2008, pp. 48 and 49). The gull-
billed tern subspecies that occurs in Bird Conservation Regions 32 and
33 is the van Rossem's gull-billed tern (Gelochelidon nilotica
vanrossemi).
Conservation and management of van Rossem's gull-billed tern is one
of the Service's regional priorities and includes the following
activities:
(1) Fall 2008--We funded a U.S. Geological Survey (USGS) project to
clarify taxonomic status of gull-billed terns in North America and
define population structure and status of the species throughout its
North American range. Results from this work are expected in 2011.
(2) September 2009--We held a structured decisionmaking workshop,
bringing together interested parties to address potential conflicts
between van Rossem's gull-billed terns and species listed under the
Endangered Species Act. Results of this workshop are still in
development.
(3) Spring/summer 2010--We coordinated van Rossem's gull-billed
tern population monitoring at Sonny Bono Salton Sea and San Diego Bay
National Wildlife Refuges. This work included population monitoring to
determine annual productivity, and implementing measures to improve
habitat and nesting conditions.
[[Page 58658]]
(4) Spring/summer 2010--The U.S. Navy along with the Service
supported and is continuing to support university-based research on
foraging behavior of van Rossem's gull-billed tern within and around
San Diego Bay, which will provide insight into main foraging sites and
frequency of visits to foraging sites. Data analysis is currently
underway and results are not yet available. Additionally, this work
will continue in 2011 and planning is underway to expand this research
to include migration and winter ecology using satellite telemetry
technology.
(5) Summer 2010--We funded surveys for nine breeding colonies in
Western Mexico to gain a better understanding of van Rossem's gull-
billed tern population size and estimate 2010 productivity (Palacios
2010 draft report).
(6) Summer 2010--We have been and continue to work on population
models to assess population and meta-population dynamics of the van
Rossem's gull-billed tern in California colonies. Modeling will also
evaluate interactions of van Rossem's gull-billed terns with other tern
and plover populations in San Diego Bay. Further modeling efforts are
evaluating effects of management actions on gull-billed tern
populations with a goal of maintaining or increasing van Rossem's gull-
billed tern numbers in California colonies.
(7) Fall 2010--We initiated coordination with Mexican biologists,
the Sonoran Joint Venture, and the Cerro Prieto Geothermal Facility to
develop a management plan for the facility with an emphasis on best
management practices for colonial nesting seabirds, including van
Rossem's gull-billed terns. These discussions and actions will also
look for additional opportunities for conservation management in Mexico
(e.g., Las Arenitas Sewage Treatment ponds).
(8) Fall 2010 and 2011--We are participating in several planning
efforts for habitat restoration projects at the Salton Sea. Two habitat
restoration projects are in the planning stages (one by California
Department of Fish and Game (CDFG) and one by Sonny Bono Salton Sea
National Wildlife Refuge). These planning efforts will emphasize the
development of suitable nesting habitat for van Rossem's gull-billed
terns and other colonial nesting birds.
(9) Fall 2010 and 2011--We are coordinating the development of
long-term conservation strategies for the management of colonial
nesting seabirds in San Diego Bay, including efforts to balance
management of potentially conflicting species like van Rossem's gull-
billed tern, the California least tern (Sternula antillarum browni),
and the western snowy plover (Charadrius alexandrinus nivosus).
Summary of Information Pertaining to the Five Factors
Section 4 of the Act and implementing regulations (50 CFR part 424)
set forth procedures for adding species to, removing species from, or
reclassifying species on the Federal Lists of Endangered and Threatened
Wildlife and Plants. Under section 4(a)(1) of the Act, a species may be
determined to be endangered or threatened based on any of the following
five factors:
(A) The present or threatened destruction, modification, or
curtailment of its habitat or range;
(B) Overutilization for commercial, recreational, scientific, or
educational purposes;
(C) Disease or predation;
(D) The inadequacy of existing regulatory mechanisms; or
(E) Other natural or manmade factors affecting its continued
existence.
In making this 12-month finding, information pertaining to van
Rossem's gull-billed tern in relation to the five factors provided in
section 4(a)(1) of the Act is discussed below. In making our 12-month
finding on the petitioned action, we considered and evaluated the best
available scientific and commercial information.
In considering what factors might constitute threats to a species,
we must look beyond the exposure of the species to a factor to evaluate
whether the species may respond to the factor in a way that causes
actual impacts to the species. If there is exposure to a factor and the
species responds negatively, the factor may be a threat and we attempt
to determine how significant a threat it is. The threat is significant
if it drives, or contributes to, the risk of extinction of the species
such that the species warrants listing as an endangered or threatened
species as those terms are defined in the Act.
Factor A. The Present or Threatened Destruction, Modification, or
Curtailment of Its Habitat or Range
For this factor, we evaluate the present (current) or threatened
(anticipated) impacts that may affect the habitat or range of van
Rossem's gull-billed tern. This factor does not address historical or
past actions that resulted in destruction, modification, or curtailment
of the species' habitat or range. Past actions that destroyed,
modified, or curtailed the species' habitat or range are not threats in
and of themselves. Any persisting ramifications of such past actions
that may be threats to the species would be addressed under Factor E
(other natural or manmade threats), below. However, under Factor A, we
do look to past actions to inform our evaluation of potential future
threats affecting the species' habitat or range in that the history of
past actions allows us to predict the likelihood of such actions
continuing into the foreseeable future.
As used here, habitat (in its general sense) is an area that
contains the physical or biological features that are important to the
species' biological needs, such as breeding, feeding, or sheltering. As
highly mobile, migratory birds, van Rossem's gull-billed terns are not
necessarily confined to one particular area that contains those
physical or biological features; that is, individuals may move between
or among areas of habitat. Moreover, as a subspecies of bird that
migrates seasonally, it breeds in certain areas during the Northern
Hemisphere spring and summer; it then moves to other areas where it
spends the winter (although, in some areas, there may be overlap).
Generally, the habitat needs of van Rossem's gull-billed tern can be
addressed by grouping its habitat into two habitat types, (1) Foraging
habitat, which it needs all year, whether during the breeding season
(and within its breeding range) or during the times it is not breeding
(within its winter range or while migrating); and (2) nesting habitat,
which it needs for laying eggs and raising young during the breeding
season.
Van Rossem's gull-billed tern foraging habitat, as discussed in the
``Biology'' section, comprises upland and aquatic areas, including open
mudflats in tidal estuaries, river margins, beaches, salt marshes,
freshwater marshes, aquacultural impoundments (such as shrimp ponds),
and a variety of upland habitats including open scrub, pasturelands and
irrigated agricultural fields and associated canals and drains, and the
airspace over such areas. Nesting habitat consists of low, open areas
on natural and artificial beaches, islands, and levees, usually with no
or sparse vegetation and are typically located on islands or other
remote areas where the risk of predation is low.
As highly mobile, migratory birds, van Rossem's gull-billed terns
can choose among potential nesting locations and specific nest sites
within those locations. For a nest site to be suitable, it must have
suitable foraging habitat nearby, among other considerations. Although
