Status review and conservation recommendations for the gull-billed tern (gelochelidon nilotica) in North America Biological Technical Publication BTP-R1013-2010

              Status Review and Conservation Recommendations
for the Gull-billed Tern (Gelochelidon nilotica) in North America
Biological Technical Publication
BTP-R1013-2010
U.S. Fish & Wildlife Service
Larry Ditto©i
Kathy C. Molina1
R. Michael Erwin2
Eduardo Palacios3
Eric Mellink4
Nanette W. H. Seto5, 6
1 Natural History Museum of Los Angeles County, Los Angeles, CA
2 U.S. Geological Survey, Patuxent Wildlife Research Center, University of Virginia, Charlottesville, VA
3 Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE Unidad La Paz), Pronatura Noroeste, A.C., La Paz, B.C.S. México
4 Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Ensenada, B.C., México
5 U.S. Fish and Wildlife Service, Migratory Birds and Habitat Programs, Portland, OR
6 Current Address: Migratory Bird Management, Arlington, VA
Status Review and Conservation Recommendations
for the Gull-billed Tern (Gelochelidon nilotica) in North America
Biological Technical Publication
BTP-R1013-2010
U.S. Fish & Wildlife Service
Cover image: Gull-billed Tern
Photo credit: Larry Ditto©ii Status Review and Conservation Recommendations for the Gull-billed Tern
For additional copies or information, contact:
U.S. Fish and Wildlife Service
Pacific Southwest Region, Migratory Birds Program
2800 Cottage Way
Sacramento, CA 95825
Recommended citation:
Molina, K. C., R. M. Erwin, E. Palacios, E. Mellink, and N. W. H. Seto. 2010. Status review and conservation recommendations for the Gull-billed Tern (Gelochelidon nilotica) in North America. U.S. Department of Interior, Fish and Wildlife Service, Biological Technical Publication, FWS/BTP-R1013-2010, Washington, D.C.
Series Senior Technical Editor:
Stephanie L. Jones
U.S. Fish and Wildlife Service, Region 6
Nongame Migratory Bird Coordinator
P.O. Box 25486
Denver Federal Center
Denver, Colorado 80225-0486
Author contact information:
Kathy C. Molina
Natural History Museum of Los Angeles County
Section of Ornithology
900 Exposition Blvd.
Los Angeles, CA 90007
Phone: (213) 763-3368
E-mail: kmolina@nhm.org
R. Michael Erwin
U.S. Geological Survey
Patuxent Wildlife Research Center
Department of Environmental Sciences
University of Virginia
291 McCormick Rd.
Charlottesville, VA 22904
Phone: (434) 924-3207
E-mail: rme5g@virginia.edu
Eduardo Palacios
Centro de Investigación Científica y de Educación Superior de Ensenada
(CICESE Unidad La Paz)
Pronatura Noroeste
A.C., Miraflores 334 e/ Mulegé y La Paz. Fracc.
Bella Vista
La Paz, B.C.S. 23050
México
Phone: (612) 121-3031, Ext. 111
E-mail: epalacio@cicese.mx
Eric Mellink
Centro de Investigación Científica y de Educación Superior de Ensenada
(CICESE)
Km 107 Carr. Tijuana-Ensenada
Ensenada, B.C.
México
U.S. Mailing Address:
P. O. Box 434844
San Diego, CA 92143-4844
Phone: (646) 175-0500
E-mail: emellink@cicese.mx
Nanette W. H. Seto (Current Address):
U.S. Fish and Wildlife Service
Migratory Bird Management
4401 N Fairfax Drive, MBSP 4107
Arlington, VA 22203
Phone: (703) 358-1835
E-mail: nanette_seto@fws.govTable of Contents iii
Table of Contents
List of Figures v
List of Tables vi
Executive Summary viii
Acknowledgments ix
Taxonomy 1
Legal Status 2
United States 2
Mexico 2
Central America and West Indies 2
Description 3
Geographic Distribution 4
Breeding 4
Gelochelidon nilotica aranea 4
Gelochelidon nilotica vanrossemi 4
Subspecies undetermined 4
Wintering 4
Gelochelidon nilotica aranea 4
Gelochelidon nilotica vanrossemi 5
Subspecies undetermined 5
Migration and Vagrancy 5
Gelochelidon nilotica aranea 5
Gelochelidon nilotica vanrossemi 5
Summer Non-breeding 5
Natural History 6
Breeding 6
Colonies, nests, and nest spacing 6
Reproductive phenology 6
Breeding site fidelity 6
Demography and limiting factors 6
Predators 7
Diet 7
Foraging Behavior 7
Post-breeding Dispersal 7
Population Estimates and Trends 8
Estimates 8
Gelochelidon nilotica aranea 8
Gelochelidon nilotica vanrossemi 8
Trends 10
Gelochelidon nilotica aranea 10
Gelochelidon nilotica vanrossemi 10
Monitoring Activities 11
Breeding Bird Survey 11
Regional and State Surveys 11
iv Status Review and Conservation Recommendations for the Gull-billed Tern
Habitat Requirements 12
Breeding Season 12
Winter and Summer Non-breeding Seasons 12
Threats 13
Habitat Loss and Degradation 13
Overutilization 13
Disease and Predation 14
Inadequacy of Existing Regulatory Mechanisms 14
Other Natural or Manmade Factors 14
Storm events and other natural disturbances 14
Pesticides or other contaminants 14
Population size and colony distribution 15
Introduced species 15
Conflicts with other species 15
Other interspecific interactions 15
Other conflicts 16
Disturbance to nesting sites 16
Management and Conservation 17
Habitat Management 17
Predator Management 17
Artificial Nesting Habitat 18
Education 18
Conservation Recommendations 19
Monitoring 19
Habitat Management and Protection 19
Research 20
Conclusion 20
Literature Cited 21
Appendix A. State and Regional Summaries of Gelochelidon nilotica aranea Status within the Breeding Range in the U.S., Mexico, and Caribbean 26
United States 26
Alabama 26
Delaware and Maryland 27
Florida 28
Georgia 30
Louisiana 31
Mississippi 34
New Jersey and New York 35
North Carolina 38
South Carolina 42
Texas 45
Virginia 53
Caribbean 55
Mexico 56
Literature Cited, Appendix A 57
Appendix B. State Summaries of Gelochelidon nilotica vanrossemi Status within the Breeding Range in the U.S. and Mexico 61
Mexico
Baja California and Baja California Sur 61
Colima, Nayarit, Sinaloa, and Sonora 63
United States
California 65
Literature Cited, Appendix B 67
Appendix C. State and Regional Contacts and Contributors 69List of Figures v
List of Figures
Figure 1. North American distribution of the Gull-billed Tern 1
Figure A-1. Locations and sizes of Gull-billed Tern colonies in Alabama, 2004 27
Figure A-2. Locations and sizes of Gull-billed Tern colonies in Florida, 2000 29
Figure A-3. Locations and sizes of Gull-billed Tern colonies in Georgia, 2003 30
Figure A-4. Locations and sizes of Gull-billed Tern colonies in Louisiana, 2001 33
Figure A-5. Locations and sizes of Gull-billed Tern colonies in Mississippi, 2003 33
Figure A-6. Locations and sizes of Gull-billed Tern colonies in New Jersey, 2001 37
Figure A-7. Locations and sizes of Gull-billed Tern colonies in New York, 2003 37
Figure A-8. Locations and sizes of Gull-billed Tern colonies in North Carolina, 2001 38
Figure A-9. Locations and sizes of Gull-billed Tern colonies South Carolina, 2003 44
Figure A-10. Locations and sizes of Gull-billed Tern colonies in the upper coast of Texas, 2003 45
Figure A-11. Locations and sizes of Gull-billed Tern colonies in central coast of Texas, 2003 48
Figure A-12. Locations and sizes of Gull-billed Tern colonies in the lower coast of Texas, 2003 51
Figure A-13. Locations and sizes of Gull-billed Tern colonies in Virginia, 2003 55
Figure B-1. Locations and sizes of Gull-billed Tern colonies in coastal northwest Mexico, 2003 62
Figure B-2. Locations and sizes of Gull-billed Tern colonies in coastal southwest Mexico, 2003 63
Figure B-3. Locations and sizes of Gull-billed Tern colonies in California, 2003 65
vi Status Review and Conservation Recommendations for the Gull-billed Tern
List of Tables
Table 1. State agency status of Gull-billed Terns in North America (continental United States only),
and National Heritage Status (NatureServe Explorer 2006). “No status” indicates that the state
has not it given the species a specific conservation status. 2
Table 2. Number of breeding pairs and colony sites of G. n. aranea in the continental
United States for years in which comprehensive state-wide censuses were conducted. 9
Table 3. Number of breeding pairs and colony sites of G. n. vanrossemi in the Pacific Coast
of the United States and Mexico in years surveys were conducted. 10
Table A-1. Number of Gull-billed Tern breeding pairs at colonies in Mobile County, Alabama,
1988–2004 (R. B. Clay, pers. comm.). Dash (—) indicates data are insufficient to discern
site availability, occupancy, or extent of survey coverage for a particular site and year. 26
Table A-2. Number of breeding pairs of Gull-billed Terns at colonies in Florida 1973–1977,
1979, 1985, 1995, 1998–2000a. Dash (—) indicates data are insufficient to discern site availability,
occupancy, or extent of survey coverage for a particular site and year. Data from Clapp et al.
1983, Florida Fish and Wildlife Commission 2003, Loftin and Sutton 1979, Ogden 1974, 1975, 1979,
Portnoy et al. 1981, Smith and Alvear 1997. 29
Table A-3. Numbers of breeding pairs of Gull-billed Terns in McIntosh County, Georgia
1993, 1995, 1999 and 2003 (B. Winn, pers. comm.). 30
Table A-4. Number of breeding pairs of Gull-billed Terns at colonies along the coast of Louisiana,
1976, 1990–1999 and 2001. Dash (—) indicates data are insufficient to discern site availability,
occupancy, or extent of survey coverage for a particular site and year. Topographic quad names
follow the Louisiana Natural Heritage Program (LNHP) colony identification numbers.
Data from Martin and Lester 1990, Michot et al. 2004, Portnoy 1977, G. D. Lester, pers. comm. 32
Table A-5. Number of Gull-billed Tern breeding pairs at colonies in Jackson County, Mississippi,
1988–2004 (M. P. Stevens, pers. comm.). Dash (—) indicates data are insufficient to discern
site availability, occupancy, or extent of survey coverage for a particular site and year. 34
Table A-6. Number of breeding pairs of Gull-billed Terns at colonies in New Jersey, 1977, 1985,
1995, and 2001 (Erwin and Korschgen 1979, R. Andrews and C. D. Jenkins, pers. comm.). 35
Table A-7. Number of breeding pairs of Gull-billed Terns at colonies in New York, 1975, 1984–1999,
and 2001–2003. Dash (—) indicates data are insufficient to discern site availability, occupancy,
or extent of survey coverage for a particular site and year. Data from Buckley et al. 1975,
Sommers et al. 1994, 2001, M. R. Wasilco, pers. comm. 36
List of Tables vii
Table A-8. Number of breeding pairs of Gull-billed Terns at colonies in North Carolina, 1976–1977, 1983–1993, 1995, 1997, 1999, and 2001. Surveys conducted statewide in 1976, 1977, 1985, and 1993
through 2001 were comprehensive. Dash (—) indicates data are insufficient to discern site availability, occupancy, or extent of survey coverage for a particular site and year. Data from Parnell
and Soots 1979, Parnell et al. 1995, Portnoy et al. 1981, D. H. Allen, pers. comm. 39
Table A-9. Number of breeding pairs of Gull-billed Terns at colonies in South Carolina, 1975–1976, 1979, 1986–1990, and 1992–2003 (Portnoy et al. 1981, T. M. Murphy, pers. comm.). Statewide
census effort begins about 1987; earliest censuses may not have been comprehensive.
Dash (—) indicates data are insufficient to discern site availability, occupancy, or extent of
survey coverage for a particular site and year. 43
Table A-10. Number of breeding pairs of Gull-billed Terns at colonies along the upper coast of Texas, 1988–2003 (U.S. Fish and Wildlife Service 2004). Dash (—) indicates data are insufficient to discern site availability, occupancy, or extent of survey coverage for a particular site and year. 46
Table A-11. Number of breeding pairs of Gull-billed Terns at colonies along the central coast of Texas, 1988–2003 (U.S. Fish and Wildlife Service 2004). Dash (—) indicates data are insufficient
to discern site availability, occupancy, or extent of survey coverage for a particular site and year. 49
Table A-12. Number of breeding pairs of Gull-billed Terns at colonies along the lower coast of Texas, 1988–2003 (U.S. Fish and Wildlife Service 2004, D. S. Stolley, pers. comm.).
Dash (—) indicates data are insufficient to discern site availability, occupancy, or extent of
survey coverage for a particular site and year. 52
Table A-13. Number of breeding pairs of Gull-billed Terns at colonies in Virginia, 1975–1977, 1984, and 1993–2003 (Williams et al. 1990, R. Andrews, B. R. Truitt and B. D. Watts, pers. comm.). Comprehensive surveys conducted statewide in 1977, 1984, 1998, and 2003. 54
Table B-1. Number of breeding pairs of Gull-billed Terns at colonies in Baja California and Baja California Sur, México, 1992–1994, 1996–2005. Dash (—) indicates no survey was
conducted; n/a = birds were present but not counted. Data from Danemann and Carmona 2000,
Molina and Garrett 2001, Palacios and Mellink 1993, 2007, EM, KCM, EP. 61
Table B-2. Number of breeding pairs of Gull-billed Terns at colonies in western Mexico (Sinaloa through Colima), 2000–2005. Dash (—) indicates no survey was conducted.
Data from Palacios and Mellink 2007, X. Vega and M. A. Gonzalez, pers. comm., EM, EP. 64
Table B-3. Number of breeding pairs of Gull-billed Terns at colonies in California, 1992–2006 (Molina 2004, R. T. Patton, pers. comm., KCM). 66viii Status Review and Conservation Recommendations for the Gull-billed Tern
Executive Summary
The Gull-billed Tern (Gelochelidon nilotica) is a medium-sized tern that breeds in small, scattered, often ephemeral colonies, typically in habitat devoid of vegetation near marine waters or saline lakes. In North America, the species breeds along the Atlantic coast south of New York, the Gulf of Mexico, and the Pacific coast of California and Mexico. Its distribution has contracted from known historic range along the Atlantic Coast, but has expanded along the Pacific Coast. Range changes in Mexico are unknown due to fragmentary knowledge of historical colony locations, but some range contraction may have occurred. Two subspecies
(G. n. aranea and G. n. vanrossemi) occur in North America. The current population of
G. n. aranea in the United States is estimated to be approximately 3610 pairs, over 60% of which occur in Texas. The number of birds in Texas appears stable, but the number of individuals has declined in Maryland, Delaware, Virginia, North Carolina, Florida, and possibly Georgia. G. n. vanrossemi has 737 to 808 pairs breeding in western Mexico and southern California.
Gull-billed Terns are designated as a Bird of Conservation Concern by the U.S. Fish and Wildlife Service. G. n. aranea is designated as endangered, threatened or of management concern in nine states and G. n. vanrossemi is designated as a Bird Species of Special Concern in California.
The main causes of population declines in North America are disturbance of nesting colonies, loss of natural nesting islands, and development or modification of upland foraging habitats. This species often nests on artificially deposited substrates, suggesting it could respond to management of breeding habitat.
Management priorities for Gull-billed Terns are: (1) protection of known nesting colony sites; (2) enhancement and conservation of potential nesting and foraging areas; (3) predator control; (4) development of population viability models; and (5) resolution of conflicts with other species and aquaculture. Research and monitoring needs are: (1) resolution of the subspecific identity of birds breeding in North America; (2) demographic studies addressing population viability; (3) the identification and linkage of breeding and non-breeding ranges; (4) studies of habitat use and ecology during the breeding and non-breeding seasons, especially in Mexico and Central America; (5) continued monitoring of breeding colonies, particularly in the Gulf Coast of Mexico; and (6) the establishment of monitoring efforts in the West Indies.Acknowledgements ix
W. McFarlane, David J. Newstead, Robert D.
Purrington, Mary P. Stevens, Dorie S. Stolley and
Jennifer K. Wilson for responding to numerous
requests and providing additional information.
We are particularly grateful for the help provided
by Roger B. Clay, Philip Glass, Paul L. Leberg,
Thomas C. Michot, and William J. Vermillion. We
thank Michael T. Green and Tara S. Zimmerman
for their support, guidance and editorial advice.
We thank Geoff Sanders for assistance in data
compilation. We thank Elizabeth Cruz, National
Wildlife Refuge System, Pacific Region, Portland,
Oregon for assistance in development of figures. We
thank David Blankinship, Jenny Hoskins, William
H. Howe, William J. Vermillion, and a number of
anonymous reviewers for their helpful suggestions
on earlier drafts and particularly Kimball L.
Garrett for many insightful discussions, help with
literature references and suggestions on an earlier
draft of the manuscript. We thank Patricia Worthing
for her editorial assistance. Funding for this status
assessment was provided by the U.S. Fish and
Wildlife Service, Regions 1, 2, 4, 6, and 8 nongame
migratory bird programs.
We thank the following colonial waterbird survey
coordinators, database managers, breeding bird
atlas managers and researchers for graciously
providing unpublished information: David H. Allen
(North Carolina), David F. Brinker (Maryland),
J. Steve Calver (Georgia), Susan E. Cameron
(North Carolina), Roger B. Clay (Alabama), Brian
E. Collins (California), Michelle L. Gibbons (New
York), Philip Glass (Texas), Joe A. Halbrook
(Texas), Howard Horne (Alabama), C. David
Jenkins (New Jersey), Paul L. Leberg and Gary
D. Lester (Louisiana), Tim Manolis (California),
Robert W. McFarlane (Texas), Thomas C. Michot
(Louisiana), Thomas M. Murphy (South Carolina),
James F. Parnell (North Carolina), Robert T. Patton
(California), Elisa Peresbarbosa (Mexico), Robert
D. Purrington (Louisiana), James A. Rodgers
(Florida), Martha Roman (Mexico), Henry “Hank”
T. Smith (Florida), Todd Stefanic (California),
Mary P. Stevens (Mississippi), Jenny Thompson
(Mississippi), Barry R. Truitt (Virginia), Vincent
V. Turner (New Jersey), Xicotencatl Vega Picos
(Mexico), Michael R. Wasilco (New York), Bryan
D. Watts and Bill Williams (Virginia), Jennifer K.
Wilson (Texas), and Brad Winn (Georgia). We are
grateful to Steven W. Cardiff, Mark A. Goodman,
William H. Howe, Gregory D. Jackson, Robert
Acknowledgments
Taxonomy 1
Class: Aves
Order: Charadriiformes
Family: Laridae
Scientific name: Gelochelidon nilotica Gmelin 1789
Common name: Gull-billed Tern
The Gull-billed Tern has a cosmopolitan but
discontinuous distribution (Fig. 1) with six
subspecies described based on variation in size
and coloration of dorsal plumage. Two subspecies,
Gelochelidon nilotica aranea and G. n. vanrossemi,
occur in North America. The nominate subspecies
G. n. nilotica breeds in small numbers in northern
Germany and Denmark, and in scattered colonies
across southern Europe south to northwestern
Africa and east through Turkey and the Middle East
Taxonomy
to Asia Minor, India and southern Mongolia (Cramp
1985, Urban et al. 1986, Hagemeijer and Blair
1997); it winters mainly in Africa and India. Poorly
differentiated from nominate birds, the subspecies
G. n. addenda (“G. n. affinis” of many authors is a
synonym; Dickinson 2003) breeds in coastal China
and perhaps elsewhere in eastern Asia, wintering
south to southeast Asia and possibly northern
Australia (Higgins and Davies 1996, Wells 1999).
Australian breeding birds are the largest and palest
subspecies, G. n. macrotarsa (Higgins and Davies
1996). G. n. groenvoldi breeds locally in eastern
South America from Brazil to northern Argentina
(Blake 1977). The subspecific identity of breeding
birds in southwestern Ecuador is unknown, though
measurements are consistent with the subspecies
G. n. aranea (Ridgely and Greenfield 2001).
Figure 1. North American distribution of the Gull-billed Tern.
Breeding only
Year-round (Breeding and wintering)
2 Status Review and Conservation Recommendations for the Gull-billed Tern
Legal StatusSouth Carolina, but has no specific conservation status in the remaining five states in its range (Appendices A and B). The National Heritage Status ranking for Gull-billed Terns is “Critically Imperiled” or “Imperiled” in 10 of the 14 states in its range (Table 1). The National Conservation status of Gull-billed Terns in the United States is “Apparently Secure” and its Global Heritage Status is “Secure” (NatureServe Explorer 2006).
Mexico
Gull-billed Terns are protected under the 1936 Convention between the United States and Mexico for the Protection of Migratory Birds and Game Mammals, but have no special legal status in Mexico (Secretaría de Medio Ambiente y Recursos Naturales 2002).
Central America and West Indies
Gull-billed Terns appear to have no legal status in Central America or the West Indies.
United States
Gull-billed Terns are federally protected by the Migratory Bird Treaty Act (1918) (MBTA) in the United States (16 U.S.C. 703-712; Ch. 128; July 13, 1918; 40 Stat. 755). The MBTA is the primary federal law that implements international treaties mandating the conservation and management of migratory birds jointly with Great Britain, Mexico, Japan, and Russia (16 U.S.C. 703-712, as amended). The species is listed by the U.S. Fish and Wildlife Service (USFWS) as a Bird of Conservation Concern at the national scale and in four USFWS Regions (Regions 2, 4, 5, and 8) and seven Bird Conservation Regions (BCRs 27, 30, 31, 32, 33, 36, and 37) (U.S. Fish and Wildlife Service 2008). These designations identify Gull-billed Terns as a species in need of conservation.
At the state level, Gull-billed Terns are listed as Endangered in Maryland and Threatened in Georgia, North Carolina, and Virginia (Table 1). It is a Species of Special Concern or equivalent in Alabama, California, Louisiana, Mississippi, and
Table 1. State agency status of Gull-billed Terns in North America (continental United States only), and National Heritage Status (NatureServe Explorer 2006). “No status” indicates that the state has not it given the species a specific conservation status.
State Legal Status National Heritage Status
Alabama Protected Imperiled
California Species of special concern Critically imperiled
Delaware No status Possibly extirpated
Florida No status Imperiled
Georgia Threatened Critically imperiled
Louisiana Rare animal of conservation concern Imperiled
Maryland Endangered Critically imperiled
Mississippi Species of greatest conservation need Imperiled
New Jersey No status Critically imperiled
New York No status Critically imperiled
North Carolina Threatened Vulnerable
South Carolina Species of concern Unranked
Texas No status Apparently secure
Virginia Threatened ImperiledDescription 3
Gull-billed Terns are a medium-sized (35 cm,
170–190 g) member of the Sterninae sub-family, best distinguished by the combination of its heavy black bill, black legs, and very pale gray upperparts. Compared to other medium-sized terns, it is longer-legged, has broader-based wings with longer outer primaries, and a shorter tail with a shallower fork. The following plumage information has been taken from Cramp (1985), Parnell et al. (1995), Sibley (2000), and from specimens at the University of California at Los Angeles Dickey Bird and Mammal Collection and the Natural History Museum of
Los Angeles County.
Alternate-plumaged adults have a jet black crown, are white on the head and underparts, and very pale gray on the back, upperwings, rump, and tail. In basic plumage, adults lose the black cap and appear white-headed with a small dark gray patch behind the eye and indistinct black peppering on the crown. At fledging, juveniles show a buffy wash and interior brown markings on the back feathers and wing coverts, dark markings on the tertials and rectrices, and extensively dusky primaries. The crown of juveniles is whitish with fine dark spotting, and
the patch behind the eye is gray. First alternate
(one year old) birds resemble basic-plumaged adults but show some dark in the centers of the tertials and may show some black mottling on the center and rear of the crown. Second winter birds are essentially indistinguishable from basic-plumaged adults but differ subtly in molt limits and wear in the flight feathers. Second summer birds closely resemble alternate-plumaged adults but usually retain some white on the forehead.
The two North American subspecies (G. n. aranea and G. n. vanrossemi) are, on average, morphologically distinct, with G. n. vanrossemi bigger overall. There is substantial overlap in most characteristics making it difficult to distinguish between the two subspecies (Parnell et al. 1995, Molina and Erwin 2006).
Description
Kathy Molina©4 Status Review and Conservation Recommendations for the Gull-billed Tern
Geographic Distribution
Wintering
Gelochelidon nilotica aranea.—G. n. aranea
winters south of the central Atlantic coast of
Florida, along the Gulf of Mexico coasts from
Louisiana to the Lower Laguna Madre, in Texas,
and possibly along the coasts of Tamaulipas and
Veracruz in Mexico (Fig. 1). Howell and Webb (1995)
suggest that the species occurs in winter along the
entire Gulf of Mexico and Caribbean coastlines of
Mexico, Belize (where uncommon; Jones 2003)
and Honduras. Documentation indicates that a
few birds winter in coastal Yucatan, Mexico
(Gómez de Silva 2007).
Breeding
Gelochelidon nilotica aranea.— G. n. aranea
breeds along the Atlantic coast of the United States
from Long Island, New York, south to northeastern
Florida and locally in the interior of Florida
(Fig. 1). Along the Gulf of Mexico coast,
G. n. aranea breeds from Tampa Bay, Florida,
west through coastal Alabama, Mississippi, and
Louisiana to Texas and south to Tamaulipas
(Garza-Torres and Navarro S. 2003) and possibly
Veracruz, Mexico (Fig. 1; Parnell et al. 1995).
Gull-billed Terns were absent along the eastern
coast of Yucatan, Mexico in 1992 (Rangel-Salazar
et al. 1993). Although their status and distribution
in the Caribbean is poorly known, G. n. aranea is
thought to breed sporadically and in small numbers
on Caribbean islands from the Bahamas south
to the British Virgin Islands and Anguilla
(Chardine et al. 2000).
Gelochelidon nilotica vanrossemi.—In the western
United States, G. n. vanrossemi breeds very locally
in extreme southern California at San Diego Bay
and at the Salton Sea (Fig. 1). In western Mexico,
G. n. vanrossemi breeds in the Colorado River
Delta in northeastern Baja California, as well as in
coastal Sinaloa, Nayarit and Colima (Fig. 1; Palacios
and Mellink 2007). G. n. vanrossemi has bred
sparingly and infrequently at the Guerrero Negro
saltworks in extreme northern Baja California Sur
(Fig. 1; Danemann and Carmona 2000, Palacios and
Mellink 2007).
Subspecies undetermined.—In Central America,
Gull-billed Terns possibly breed in Panama (Ridgely
and Gwynne 1989), but confirmed breeding records
are lacking and the subspecific identity of possible
breeders is unknown.
Matt Sadowski©
Geographic Distribution 5from known breeding sites but normally occur within the general breeding range of the species. Small numbers may disperse northward as vagrants in spring, summer, and fall. Gull-billed Terns occur nearly annually in Massachusetts from May to September (Veit and Petersen 1993) and have been recorded in the Canadian Maritime Provinces in July and August (Godfrey 1986). Gull-billed Terns are casual or accidental in the interior of eastern North America (McWilliams and Brauning 2000). Correa-Sandoval and Garcia-Barron (1993) considered the Gull-billed Tern to be a rare migrant in the large lagoon systems of Campeche and Yucatan, Mexico.
Gelochelidon nilotica vanrossemi.—G. n. vanrossemi migrates along the western coast of mainland Mexico and presumably the Pacific and Gulf of California coasts of the Baja Peninsula (Wilbur 1987, Erickson et al. 2001, 2004). Gull-billed Terns are casual or accidental in the interior of western North America (Monson and Phillips 1981), including interior southern California (Lehman 1994, Hamilton and Willick 1996; McCaskie and Garrett 2001, 2004, 2005).
Summer Non-breeding
Small numbers of both subspecies over-summer within the respective portions of their winter
ranges in Mexico (Howell and Webb 1995). Contreras-Balderas (1993) considered G. n. aranea as a year-round resident in Tamaulipas. In California, non-breeders (white-headed birds presumably in their second year) have been noted
at breeding colonies at the Salton Sea (KCM).
Gelochelidon nilotica vanrossemi.—G. n. vanrossemi primarily winters in Mexico along the eastern Gulf of California from the Colorado River Delta south to Sinaloa and along the Pacific coast to Nayarit, Colima and Oaxaca (Howell and Webb 1995). There are a few mid-winter records for the Salton Sea (Patten et al. 2003), the Mexicali Valley (R. A. Erickson, pers. comm.), and southern Baja California Sur (Fig. 1; Erickson et al. 2003).
Subspecies undetermined.—The non-breeding distribution of G. n. aranea and G. n. vanrossemi from southern Mexico to northern South America is poorly understood. Gull-billed Terns winter in small numbers on both coasts of Honduras (Monroe 1968), Costa Rica (Stiles and Skutch 1989) and Panama (Ridgely and Gwynne 1989). Although it is likely that migrant and wintering Gull-billed Terns on the Caribbean coast of Central and South America belong to the subspecies G. n. aranea, the subspecific identity of birds wintering on the southern Pacific Coast is unclear (Molina and
Erwin 2006). The single specimen from the Pacific
coast of Honduras appears from measurements to be G. n. aranea (Monroe 1968), so some Gull-billed Terns on the Pacific coast of Central and South America may be of this subspecies. The pattern of eastern North American and Caribbean breeding populations wintering on the southern Pacific Coast occurs in other avian taxa (Molina and Erwin 2006).
Migration and Vagrancy
Gelochelidon nilotica aranea.—G. n. aranea migrates through coastal areas of the Atlantic states and Gulf of Mexico. Migrants may be noted away
Kathy Molina©6 Status Review and Conservation Recommendations for the Gull-billed Tern
Natural History
Prieto, Mexico, although some years were
not surveyed (Molina and Garrett 2001, KCM).
Reasons for the difference in site-fidelity are
unknown, but could be in response to the number
of alternative sites or to management aimed at
reducing disturbance and predation events in
California (KCM).
Demography and limiting factors.—Few
quantitative data are available on the demography
of Gull-billed Tern populations in North America
(Parnell et al. 1995). The average clutch size ranges
from two to three eggs, with four eggs being
exceptional (Bent 1921). In Virginia, from 1994
through 1996, clutch size depended strongly on
time of season; early clutches averaged 2.47 and
late ones averaged 2.10 (n = 428; Eyler et al. 1999).
In California, clutch size in 1993 averaged 2.2 (n =
140; Parnell et al. 1995). The age of first breeding in
Europe has been reported to be at least five years,
with birds establishing non-nesting territories at
four years of age (Moller 1975). At the Salton Sea, a
few known-age birds have bred at three years and
occasionally birds presumed to be two years of age
loaf at colony sites (KCM).
Productivity estimates vary among years and
locations, and lack of consistency in the variables
measured often confound comparisons of
reproductive success across studies. In Virginia
from 1994 to 1996, Eyler et al. (1999) reported
a mean brood size of 1.99 chicks, but only
0.89 chicks per successfully hatched nest reached
fledging age. For all nests initiated, including
those lost to flooding or other causes, 0.53 chicks
per nest survived to fledging age. In California,
fledglings per pair in 2000 and 2001 for Salton Sea
colonies were 0.57 and 0.31, respectively, and for
the San Diego colony were 1.23 and 0.95,
respectively (KCM).
Although Gull-billed Terns generally have only one
brood in a breeding season, they may re-nest if the
initial clutch or brood is lost (Parnell et al. 1995).
No information on lifetime reproductive success is
available; similarly, information on the longevity
of Gull-billed Terns is scant. One bird banded as a
chick in South Carolina was recovered in Guyana,
South America when at least six years and four
months old (Clapp et al. 1982). In Europe, the
longevity record is 15 years, 10 months (Rydzewski
1978). Several 10-year old birds, banded as chicks at
the Salton Sea, have been observed in the vicinity of
their natal colonies (KCM).
Breeding
Colonies, nests, and nest spacing.— Gull-billed Tern
nesting colonies are generally small to medium in
size (< 50 pairs) and are loosely aggregated (Parnell
et al. 1995). In Texas, 90% of 136 colony sites in
2003 contained a median of one to 50 pairs, eight
sites (6%) contained 51 to 100 pairs, and six sites
(4%) contained > 100 pairs. At the Salton Sea in
California, Gull-billed Terns often form subcolonies
of 10 to 30 pairs (KCM). Nests are small shallow
scrapes typically containing little or no nest lining.
Nest rims are composed of small bits of beach
debris, (e.g. vegetation, small fish bones, bits
of plastic, pebbles, and feathers). Both members
of the pair participate in forming and maintaining
the nest scrape and rim (Cramp 1985, Parnell et al.
1995). Inter-nest distances vary from 0.3 to 20 m
(Gochfeld and Burger 1996).
Reproductive phenology.—The following breeding
information has been taken from Parnell et al.
(1995), except where noted. In California, Gull-billed
Terns generally arrive in mid-March to early April,
but may arrive as early as the first week of March
at the Salton Sea. Egg laying usually begins by mid-
April to early May in California and Baja California
(Molina and Garrett 2001) and is presumed to
be the same in western Mexico. Nesting by Gull-billed
Terns at the Salton Sea may occur as late as
mid-July through early August and is presumed
to involve re-nesting attempts. In Atlantic Coast
colonies, most egg laying begins from mid-May to
early June. Both sexes participate in incubation, a
period lasting 21-23 days beginning with the first
egg. Earliest hatching is in early May in the west;
most hatching on the Atlantic Coast occurs in June
(Eyler et al. 1999). Chicks may move away from the
nest soon after hatching, depending on the degree
of disturbance and vegetation around nest sites.
In extreme heat, parents lead very young broods
to loafing areas at the water’s edge (KCM). First
flights occur at 28–35 days. Young are fed by both
parents at least four weeks post-fledging.
Breeding site fidelity.—Breeding site fidelity of
Gull-billed Terns has been reported to be weak in
Virginia (Erwin et al. 1998b) and Europe (Cramp
1985, Parnell et al. 1995). On the Pacific Coast, Gull-billed
Terns often occupy colony sites for multiple
years. They nested for five and 10 consecutive years
at two sites at the Salton Sea (Molina 2004), 11
consecutive years (except 2000) at San Diego Bay
(R. T. Patton, pers. comm.), and since 1996 at Cerro
Natual History 7delivered to chicks in 1981 consisted of terrestrial orthopterans, arachnids, and assorted insects, while the remaining prey items were associated with marine (shrimp, crabs, and fish) or fresh or brackish water (odonates and frogs) habitats. In Louisiana, Gull-billed Terns feed on crayfish at Lake Charles in winter (Clement 1946).
Foraging Behavior
The following is from Parnell et al. (1995), except where noted. By virtue of their varied diet and their habit of exploiting insect and lizard prey in terrestrial habitats, Gull-billed Terns forage over a wide range of substrates. In terrestrial and aquatic habitats, Gull-billed Terns forage on the wing, with characteristic buoyant swoops down to the substrate to pluck prey from the surface or near surface. They may frequently hover over the surface when foraging into the wind. Aerial prey (swarming insects such as weevils or ladybird beetles) is taken directly from the air column. Unlike other terns, Gull-billed Terns rarely, if ever, plunge-dive for prey. Gull-billed Terns usually swallow small items in flight but may land to manipulate or disarticulate larger items (Molina and Marschalek 2003). Both parents feed young, although the relative contributions of the sexes have not been quantified.
Gull-billed Terns are known to kleptoparasitize breeding Black Skimmers (Rynchops niger) in Texas (D. J. Newstead, pers. comm.), Common Terns (Sterna hirundo) in New Jersey (Parnell et al. 1995), and occasionally California Least and Forster’s terns (S. forsteri) in California (Molina and Marschelek 2003, B. E. Collins, pers. comm.).
Pemberton (1922) reported foraging distances of up to 22.5 km from Gull-billed Tern nesting colonies in Texas. Regular foraging distances of up to 9 km away from the nesting colony occurred in San Diego Bay (Molina and Marschelek (2003). Gull-billed Terns were observed foraging in areas up to 76 km north of the San Diego Bay nesting colony
(B. Foster, pers. comm.), but it is unknown if these birds were active breeders.
Post-breeding Dispersal
Gull-billed Terns gather in small groups at river deltas, estuaries, sandy beaches, flooded agricultural fields, and inundated salt flats after young fledge. In California and Baja California, post-breeding movement may begin as early as mid-July with most, if not all, birds leaving breeding areas by mid- to late August (Parnell et al. 1995). Along the Atlantic Coast, Gull-billed Terns are one of the earliest of the terns to disperse from breeding colonies, often as early as late July (RME).
Predators.—Known predators of eggs or chicks of Gull-billed Terns in North America are raccoons (Procyon lotor), coyotes (Canis latrans), feral dogs (C. lupus familiaris) and cats (Felis catus), rats (Rattus spp.), Peregrine Falcons (Falco peregrinus), Laughing (Larus atricilla), California (L. californicus), Herring (L. argentatus) and Great Black-backed (L. marinus) gulls, Burrowing (Athene cunicularia) and Great Horned (Bubo virginianus) owls, and ghost crabs (Ocypode quadrata) (Blus and Stafford 1980, Parnell et al. 1995, Eyler et al. 1999, O’Connell and Beck 2003). Other potential predators of eggs or chicks could be red foxes (Vulpes vulpes), skunks (Spilogale putorius and Mephitis mephitis), ground squirrels (Spermophilus spp.), Western (L. occidentalis), Yellow-footed (L. livens) and Heermann’s (L. heermanni) gulls, large herons (Ardeidae), American Kestrels (F. sparverius), Common Ravens (Corvus corax), and American Crows (C. brachyrhynchos) (Parnell et al. 1995, Gonzalez-Bernal et al. 2003).
Diet.—The diet of Gull-billed Terns, composed of vertebrate and invertebrate prey, is broader than most other species of terns. They are opportunistic feeders on a variety of terrestrial and aquatic animals; invertebrate prey of terrestrial and aquatic origin appears to dominate their diets. At San Diego Bay, small marine invertebrates, primarily mole crabs (Emerita analoga), and small fish were the dominant prey items delivered by adults to chicks, comprising 43% and 25%, respectively, of all deliveries observed in 2002 (Molina and Marschalek 2003). Additional diet items included common side-blotched (Uta stansburiana) and western fence (Sceloporus occidentalis) lizards, insects, and small chicks of Black-necked Stilts (Himantopus mexicanus), Killdeer (Charadrius vociferous), western Snowy Plovers (C. alexandrinus nivosus), and California Least Terns (Sternula antillarum browni). At the Salton Sea, 49% of prey items delivered to chicks in 2001 were insects [orthopterans (mainly crickets), odonates, and hymenoptera], with small fish (Tilapia) comprising 41% (KCM). Other food items delivered, listed in descending order of importance, were amphibians, crayfish, and small Black-necked Stilt chicks. Additional prey items taken at the Salton Sea include common side-blotched lizards and periodically abundant insects such as cicadas, butterflies, weevils, and ladybird beetles. In the Gulf of California and mainland Mexico, Gull-billed Terns feed on fiddler crabs (Uca spp.) and farmed shrimp (KCM). In Virginia, small marine invertebrates (primarily fiddler crabs), fish, and insects (primarily large odonates and orthopterans) were the dominant prey items fed to chicks in 1995 and 1996 (Erwin et al. 1998a). In Lavaca Bay, Texas, Quinn and Wiggins (1990) reported that 47.5% of prey 8 Status Review and Conservation Recommendations for the Gull-billed Tern
Population Estimates and Trends
above and difficult to account for inter-annual
movements. In no single year have all states with
colonies been surveyed simultaneously, and during
the years 2000 to 2004 most states have reported
counts for only one year (Table 2). To estimate total
numbers in recent years (2000–2004), we summed
the mean counts for all states with breeding colonies
and the highest count for each state during this
period (Table 2). The resulting population estimates
for G. n. aranea for 2000–2004 are 3608 pairs (sum
of mean counts) and 4432 pairs (sum of high counts).
Although Gull-billed Terns are known to breed
in the large lagoon system of Laguna Madre in
Tamaulipas, Mexico (Garza-Torres and Navarro
S. 2003), no historic or recent population estimates
are available. A preliminary survey in 2005 in the
state of Veracruz did not locate active colony sites
(EM); no other comprehensive survey in Mexico has
been conducted.
Gelochelidon nilotica vanrossemi.—The earliest
available assessment of the historical population
size of G. n. vanrossemi in the U.S. is Pemberton’s
(1927) estimate of 500 pairs nesting at the Salton
Sea’s south end in 1927. By 1937, fewer than 200
pairs nested there (Grinnell and Miller 1944). This
decline continued through the 1950s and 1960s with
60 pairs in 1952, 75 pairs in 1957, 40-50 pairs in 1959,
and just a few pairs through the 1960s (Remsen
1978). By 1976 only 17 pairs nested at the Salton
Sea (McCaskie 1976) and twice this number may
have nested in 1977 (Remsen 1978). During the
1980s, the largest count reported was a minimum of
75 pairs in 1986 (McCaskie 1986).
In 1986, Gull-billed Terns colonized a single site on
the California coast at the saltworks in southern
San Diego Bay, which became part of the San Diego
Bay National Wildlife Refuge (NWR) in 1999. This
colony increased to 30 pairs in 1992, varied between
eight and 20 pairs through the remainder of the
1990s, and steadily climbed from approximately 24
pairs in 2000 to 54 pairs in 2006 (R. T. Patton, pers.
comm.). The species has not established colonies at
any other location in coastal California or away from
the Salton Sea. During the early 1990s, an average
of 120 pairs of Gull-billed Terns nested at the Salton
Sea and San Diego Bay NWR (Clapp et al. 1993,
Parnell et al. 1995). From 1997 to 2004, an average
of 146 pairs nested in California at these two sites
(Table 3).
Gull-billed Terns are not considered to be abundant
anywhere in their North American range (Parnell
et al. 1995, Gochfeld and Burger 1996). There
are few historical population estimates for either
subspecies in North America, especially for
Mexico. When state census data are available,
they often have not been collected simultaneously
throughout an entire region. We compiled published
and unpublished results of surveys conducted
throughout the U.S. and western Mexico to gain
a more complete and updated understanding of
current population levels for both subspecies in
North America over the last two decades.
Estimates
Spendelow and Patton (1988) estimated 5400
Gull-billed Terns for the entire U.S. from 1976
to 1982, which when converted using Erwin’s
(1979) pairs-to-adults conversion factor of 0.66,
yields 3563 pairs. This estimate excluded birds in
California (Spendelow and Patton 1988) and may
underestimate the population in Florida (Parnell
et al. 1995); thus underestimating the entire
U.S. population.
Gelochelidon nilotica aranea.—For the
southeastern U.S. (excluding Virginia and the
Florida Gulf of Mexico coasts), Clapp et al. (1983)
reported 3472 Gull-billed Terns from 1972 to 1979.
For ease of comparison, we use Erwin’s (1979)
pairs-to-adults conversion factor to convert Clapp
et al.’s (1983) estimate of individuals into 2292 pairs.
Portnoy et al. (1981) estimated 1314 Gull-billed
Terns in North Carolina south to Atlantic Florida;
Kress et al. (1983) equated this figure to 650 pairs.
Clapp and Buckley (1984) estimated a total of
3019 pairs of Gull-billed Terns along the southern
Atlantic and Gulf of Mexico coasts between 1976 and
1983. For the north and mid-Atlantic Coast in 1982,
Buckley and Buckley (1984) reported a probable
1000 pairs; yielding 4019 pairs for the entire eastern
U.S. when combined with Clapp and Buckley’s
(1984) total. In summary, population estimates for
the G. n. aranea subspecies in the U.S. from 1976 to
1983 ranged from about 3563 to 4019 pairs.
The lack of systematic survey efforts in all states
with G. n. aranea colonies makes it difficult to
compare recent population sizes with those reported
Population Estimates and Trends 9
Table 2. Number of breeding pairs and colony sites of G. n. aranea in the continental United States for years in which comprehensive state-wide censuses were conducted.
Year No. of No. of Avg. Pairs/ Pairs Colonies Colony
Atlantic Coast
New York
1977 0 0 —
1985 2 2 1
1995 2 1 —
2003 11 3 4
New Jersey
1977 19 4 5
1985 17 3 5
1995 18a 3 6
2001 92a 5 18
Virginia
1977 729a 11 66
1984 413 11 38
1993 265 15 18
1998 310 15 21
2003 293 16 18
North Carolina
1977 621 21 30
1985 174 4 44
1995 249 10 25
2001 258 7 37
South Carolina
1976 154a 4 39
1988 254 10 25
1995 165 8 21
2003 239 7 34
Georgia
1995 80 1 —
2003 54 1 —
Florida
1975 534 2 267
1980–1985b 75c 6–8 —
2000 17 3 6
Gulf Coast
Alabama
1976d 23 1 —
2001e 87 3 29
2002e 50 1 —
2003e 9 1 —
2004e 85 3 28
Mississippi
1976d 2 1 —
1994f 0 0 —
1995f 0 0 —
1996f 2 1 —
1997f 1 1 —
1998f 0 0 —
1999f 0 0 —
2000f 0 0 —
2001f 0 0 —
2002f 5 1 —
Year No. of No. of Avg. Pairs/ Pairs Colonies Colony
Mississippi (cont’d)
2003f 2 1 —
2004f 150 1 —
Louisiana
1976d 154 4 39
1990g 161 3 54
1991h 30 1 —
1992h 350 2 175
1993h 650 3 217
1994h 290 4 73
1995h 400 3 133
1996h 173 4 43
1997h 248 11 23
1998h 1120 5 224
1999h 590 5 118
2001i 440 4 110
Texasj
1973c 2187 27 81
1974c 688 17 41
1975c 1289 23 56
1976 1098 21 52
1977 1632 32 51
1978 2034 30 68
1979 2267 38 60
1980 1810 33 55
1981 2046 39 53
1982 2123 40 53
1983 4661 33 141
1984 2416 47 51
1985 1926 42 46
1986 1075 32 34
1987 1946 38 51
1988 1243 36 35
1989 1150 40 29
1990 2868 37 78
1991 913 21 44
1992 1372 35 39
1993 1553 34 46
1994 3706 41 90
1995 2553 28 91
1996 914 37 25
1997 1576 36 44
1998 2293 41 56
1999 846 29 19
2000 2791 39 72
2001 1840 36 51
2002 2565 39 66
2003 1292 29 45
d Portnoy 1977
e R. B. Clay, pers. comm.
f M. P. Stevens, pers. comm.
g Martin and Lester 1990
h G. D. Lester, pers. comm.
i Michot et al. 2004
j U.S. Fish and Wildlife
Service 2004
a Breeding pairs estimated from counts of individuals by multiplying individuals by 0.667.
b Smith and Alvear 1997
c Minimum estimate due to
incomplete state survey coverage.10 Status Review and Conservation Recommendations for the Gull-billed Tern
Historical estimates of Gull-billed Terns breeding along the Pacific coast of Mexico are unavailable. A total of 367 potential nesting sites were surveyed in 2003, with an estimated total of 376 nesting pairs of Gull-billed Terns distributed among seven colonies in western Mexico (EM, EP). In 2005, Palacios and Mellink (2007) and KCM documented 550 to 551 breeding pairs among six of the seven breeding locations documented in 2003 and at one additional site. Combined with the number of pairs in California, 737 to 808 pairs of G. n. vanrossemi appear to have nested in western North America in 2003 and 2005 (Table 3).
Trends
Gelochelidon nilotica aranea.—The numbers of breeding Gull-billed Terns along the northern Atlantic coast are small, but seem to be stable or increasing slightly, while numbers along the mid- and southern Atlantic coasts have declined since the mid- to late 1970s (Table 2). In Virginia, partial surveys in 1975 and 1976 recorded high totals of 1485 and 1333 pairs, respectively, and a complete survey in 1977 recorded 729 pairs (Brinker et al. 2007, 2008). In North Carolina, declines occurred between 1975 and 1976 and the mid-1980s, with numbers remaining at about a third of the state’s mid-1970s levels (D. H. Allen, pers. comm.). In Florida, a relatively large number of breeding Gull-billed Terns (534 pairs in 1975) appeared to have dwindled to just a few pairs, although comprehensive surveys of the state have been infrequent (Table 2; Smith and Alvear 1997). The number of pairs in South Carolina appears to be stable or increasing slightly, while the population in Georgia may be declining (Table 2). In Maryland and Delaware, the species is possibly extirpated. The number of pairs breeding in Delaware has historically been small, and no breeding has been documented since 1991 (D. B. Carter, pers. comm.). The number of pairs in Alabama, Mississippi, and Louisiana are small, but apparently stable or increasing slightly (Table 2; Portnoy 1977, R. B. Clay and M. P. Stevens, pers. comm.). In Texas, where the largest known breeding concentrations in North America occur, Gull-billed Tern numbers appear to have remained stable overall since the early 1970s (Table 2).
Gelochelidon nilotica vanrossemi.—Although declines were apparent from the late 1930s through the late 1970s at the Salton Sea, the small number of breeding pairs in California seems to have remained stable since the early 1990s (Table 3; KCM). In 1986, Gull-billed Terns increased their numbers slightly by colonizing one coastal site at San Diego Bay. No trend information is available for Mexico (KCM).
Table 3. Number of breeding pairs and colony sites of G. n. vanrossemi in the Pacific Coast of the United States and Mexico in years surveys were conducted.
Year No. of No. of Avg. Pairs/ Pairs Colonies Colony
United States
Californiaa
1992 136 4 34
1993 131 4 33
1994 113 4 28
1995 92 3 27
1996b 155 3 55
1997 162 3 53
1998 131–133 3 —
1999 112–122 3 —
2000 135–142 4 —
2001 173 2 87
2002c 97–101 2 —
2003 187–192 4 —
2004 157 2 79
2005 252–257 5 —
Mexicod,e
Baja California
2003 183 2 92
2004 234 2 —
2005 274 2 137
Baja California Sur
2003 14 1 —
2005 10 1 —
Sonora
2003 0 0 —
2005 0 0 —
Sinaloa
2003 15 1 —
2005 26–27 2 14
Nayarit
2003 122–152 2 —
2005 185 2 93
Colima
2003 15 1 —
2005 55 5 11
a Data for Salton Sea: 1992 to 2001 from Molina 2004; 2002 to 2005 from KCM. Data for San Diego: 1992 to 2005 from R. T. Patton, pers. comm.
b Data from Salton Sea colonies only.
c Data from San Diego and some Salton Sea colonies.
d Data from Palacios and Mellink 2007, except Baja California (2004) from KCM.
e Palacios and Mellink 2007 also surveyed Jalisco, Michoacán, Guerrero, Oaxaca, and Chiapas but reported no breeding.Monitoring Activities 11
Monitoring Activities
and may include Gull-billed Terns. New Jersey,
North Carolina, and Georgia conduct periodic
surveys of breeding waterbird colonies including
Gull-billed Terns (C. D. Jenkins, S. E. Cameron,
and J. S. Calver, pers. comm.). In Florida, the only
monitoring efforts conducted recently for Gull-billed
Terns were those directed toward the state
breeding bird atlas (J. A. Rodgers, pers. comm.).
State breeding bird atlases documenting Gull-billed
Tern colonies were published for New York (Bull
1964), New Jersey (Walsh et al. 1999), Maryland
(Brinker 1996), Mississippi (Gandy and Turcotte
1970), and Louisiana (Michot et al. 2004), and an
atlas is currently under development in Alabama
(KCM). A preliminary survey in the state of
Veracruz in Mexico in 2005 yielded no colony sites
(EM). Comprehensive surveys in coastal eastern
Mexico for G. n. aranea have not been conducted.
To facilitate coordinated comprehensive survey
efforts, the USFWS sponsored the development of
field identification cards (in English and Spanish)
and a bi-national workshop in 2003 to address a
standardized survey protocol for G. n. vanrossemi.
The USFWS also sponsored the first comprehensive
surveys for G. n. vanrossemi in Mexico in 2003 and
2005, which combined with annual monitoring in
California, resulted in the first range-wide surveys
for this subspecies.
Breeding Bird Survey
The Breeding Bird Survey (Sauer et al. 2008) does
not adequately monitor this species because survey
routes do not adequately represent Gull-billed Tern
breeding areas in coastal estuarine habitats (KCM).
Gull-billed Terns were not included in Breeding
Bird Survey trend analysis by Price et al. (1995).
Regional and State Surveys
There are no coordinated breeding or winter
surveys of Gull-billed Terns throughout their entire
range in the United States or in North America.
The Gulf of Mexico coasts of Alabama, Mississippi,
and Louisiana were occasionally surveyed as a
region; this survey was last conducted in 1976
(Portnoy 1977). Known Gull-billed Tern colony sites
are monitored annually in California and Virginia
(RME, KCM). In Texas, Mississippi, Alabama,
South Carolina, and New York, Gull-billed Tern
colonies are monitored annually as part of each
state’s comprehensive waterbird nesting surveys
(P. Glass, M. P. Stevens, R. B. Clay, T. M. Murphy,
and M. R. Wasilco, pers. comm.). Louisiana conducts
comprehensive surveys of nesting waterbirds at
least every four years (P. L. Leberg, pers. comm.),
Matt Sadowski©
12 Status Review and Conservation Recommendations for the Gull-billed Tern
Habitat Requirements
Winter and Summer
Non-breeding Seasons
Wintering Gull-billed Terns along the Gulf of
Mexico Coast are generally found in estuaries, salt
and freshwater marshes, canals, and ponds. Away
from the coast, this species is commonly observed
in flooded agricultural fields (rice and crayfish
impoundments). In western Mexico, Gull-billed
Terns are usually found in bays and estuaries with
extensive tidal flats, agricultural fields, canals
and drains, salinas (salt mines), and aquaculture
(primarily shrimp) impoundments (KCM) and use
exposed dikes for courtship displays (EP). Away
from the coastal lowlands on the Pacific Coast, small
numbers of Gull-billed Terns are regularly found in
winter near Laguna Sayula in Jalisco, an ephemeral
wetland at the extreme southwest corner of the
Mexican Plateau (Howell 1999).
Breeding Season
In North America, Gull-billed Terns typically
nest on barrier islands, dredged-material islands,
constructed islets or isolated levees in wildlife,
salt extraction, and aquaculture impoundments,
shell bars and islands in open marshes, abandoned
causeways, natural islets in shallow tidal and
brackish lagoons, and sand and shellbars in river
deltas (Parnell et al. 1995). Nesting substrates
include bare sand, gravel, crushed shell, and silty
clay soils. Nest sites generally lack vegetation,
but when present, it is usually low and sparse. In
Texas, Gull-billed Terns have nested in dense areas
of the grasses Paspalum and Monanthochloe (J.
K. Wilson, pers. comm.) and in Virginia, the largest
colony is on wrack (drifted rafts of dead Spartina)
on a low island in a salt marsh. In western Mexico,
Gull-billed Terns nest on low islands with mangrove
or cactus and on mud flats with salt marsh
vegetation. Gull-billed Terns have occasionally
nested on gravel rooftops in coastal Texas (P. Glass,
pers. comm.), Louisiana (Purrington 2002),
and Florida (Coburn 1996).
Inland, nesting occurs on natural and constructed
islands in saline and freshwater lakes, reservoirs,
and impoundments, and on abandoned oil and gas
causeways (Parnell et al. 1995, Molina and Garrett
2001, Molina 2004). In Florida, Gull-billed Terns
have nested on sand fill in phosphate mine pits
(Smith and Gore 1996). In the Mexican Central
Plateau, one or two pairs (of either subspecies) have
been observed nesting above 2200 m elevation at
Lake Xochimilco and Lake Texcoco; these are the
only North American breeding records significantly
above sea level (Molina and Erwin 2006).
Whether on the coast or inland, colony sites are
typically located near optimal foraging habitats,
which include the shallow margins of bays,
rivers, and marshes, exposed mudflats, the tidal
margins of sandy beaches, agricultural fields and
drains, wildlife, salt extraction, and aquaculture
impoundments, sandy lake shores, and open
shrublands.
Kathy Molina©
Threats 13
Habitat Loss and Degradation
Vegetation succession and erosion can alter the
suitability of colony sites on dredged-material
islands along the Gulf of Mexico (Chaney et al.
1978) and Atlantic coasts (Parnell and Soots 1979).
Changes in the distribution of ground-nesting
waterbirds from 1977 to 1995 in New Jersey,
Virginia, and North Carolina coincided with changes
in dredging policy along the mid-Atlantic (Erwin
et al. 2003). Since the 1980s, competing demands
by coastal communities for sand augmentation for
beaches has slowed the rate of replenishment of
dredged-material islands allowing the establishment
of dense and woody vegetation or the erosion and
disappearance of former colony sites (Erwin et al.
2003). In North Carolina, the number of Gull-billed
Terns nesting on dredged-material islands declined
from 524 pairs in 1977 to only 128 pairs in 1995
(Erwin et al. 2003). Parnell et al. (1997) attributed
the low nesting site fidelity and high rates of site
turnover exhibited by Gull-billed Terns in
North Carolina from 1977 to 1995 to the
degradation of colony sites due to vegetation
succession and erosion.
Increases in predator populations on Atlantic Coast
barrier islands are believed responsible for the
diminished suitability of these sites for Gull-billed
Terns and other colonial ground nesting birds since
the 1980s, especially in Virginia (Erwin et al. 2001,
2003) where, although the number of colonies has
increased, the size of each and the overall number
of breeding Gull-billed Terns has declined (Erwin et
al. 2003). Fragmentation of larger colonies into more
numerous but smaller ones does not necessarily
increase successful reproduction as smaller colonies
may be less resistant or resilient to predation and
human disturbances (Sears 1979, Wittenberger and
Hunt 1985).
Diminishing freshwater and agricultural inflows to
the Salton Sea have resulted in lower water levels
and the bridging of once isolated islands, rendering
them accessible to mammalian predators and
unsuitable for nesting. Water levels may be reduced
even further by conservation measures under
the Imperial Irrigation District’s water transfer
program (J. A. Bartel, pers. comm.).
Threats
Nesting attempts by Gull-billed Terns at Isla
Montague in Mexico were consistently interrupted
by regular tidal inundations throughout the 1993
and 1994 breeding seasons (Peresbarbosa and
Mellink 2001). During the 2004 and 2005 breeding
seasons, complete colony failures were attributed to
non-storm related tidal inundations (KCM). Since
the completion of upstream dams and diversions
on the Colorado River in the 1930s and 1940s, this
estuary island no longer receives the sediment
load that prevailed prior to the damming of the
river. Lacking such sediment replenishment, it is
subjected to the erosion forces of Gulf of California
tides (Alvarez-Borrego 2001).
The recent large scale conversion of estuarine
habitats to commercial aquaculture (shrimp and
oyster farms) in northwestern mainland Mexico
(Páez-Osuna et al. 2003) may reduce or degrade
available mudflats for foraging, while providing
novel and concentrated food sources during the
breeding season (Molina et al. 2009). Shrimp are
generally harvested in the fall and winter, removing
this food source for wintering Gull-billed Terns
(Molina et al. 2009). Since 1989, the extent of
wetlands converted to aquaculture in Sonora has
increased some 3000% to encompass nearly 7500
ha of shrimp farms; in Sinoloa, approximately 1300
ha have been dedicated to aquaculture (Instituto
Tecnologico de Sonora 2004). Loss of estuarine
habitat in Mexico has also occurred through the
construction of marinas and other tourism-related
development and saltworks (KCM).
Overutilization
Overutilization (such as egging and over-hunting)
is not a known threat to Gull-billed Terns.
14 Status Review and Conservation Recommendations for the Gull-billed Tern
Inadequacy of Existing
Regulatory Mechanisms
Despite the protections and status designations denoting conservation concern at the Federal, state, and BCR scale, the species has continued to show declines in recent decades in the Atlantic Coast region, particularly in Virginia, North Carolina, Maryland and Florida (Table 2; Smith and Alvear 1997, R. B. Clay, pers. comm., RME). Populations in Texas have remained large and stable despite the lack of additional regulatory measures, apparently due to the large number of dredged material nesting sites in this state (KCM).
Other Natural or Manmade Factors
Storm events and other natural disturbances.— Colony sites on the Atlantic Coast are frequently flooded by abnormally high spring tides as well as storm events, resulting in high rates of nest loss, particularly at marsh nesting sites (Erwin et al. 1998b). Major hurricanes have had dramatic adverse impacts to nesting habitats in the Gulf of Mexico, as many sites have simply disappeared (W. J. Vermillion, pers. comm.). Spring tides regularly wash out entire nesting colonies in the Gulf of California at Isla Montague in Baja California and Isla El Rancho in Sinaloa, Mexico (Peresbarbosa and Mellink 2001, X. Vega, pers. comm.). Gull-billed Terns failed to re-nest at Isla Montague and Isla El Rancho in 2004 and 2005 after late April high tides flooded first attempts (KCM).
Pesticides or other contaminants.—There is limited information available on the exposure of Gull-billed Terns to contaminants. Residue levels of DDE in 11 Gull-billed Tern eggs collected in South Carolina in 1972 ranged from 0.28 to 10.71 μg per g (wet wt.) (Blus and Stafford 1980). Two eggs from the 1972 sample containing the highest residue levels (8.75 and 10.71 μg per g) had abnormal and fragile shells. The ranges of DDE residues in Gull-billed Tern eggs from South Carolina sampled in 1974 (n =14 eggs) and in 1975 (n = 5 eggs) declined to 0.18-1.34 μg per g and 0.14-0.38 μg per g (wet wt.), respectively (Blus and Stafford 1980). The mean eggshell thickness for the 1972, 1974, and 1975 samples ranged between 0.220 mm to 0.227 mm and was not significantly different from the mean thickness of 0.228 mm for four pre-1947 Gull-billed Tern eggs (Blus and Stafford 1980). Residues of polychlorinated biphenyls, oxychlordane, dieldrin, and trans-nonachlor were low or undetectable in Gull-billed Tern eggs sampled between 1972 and 1975 (Blus and Stafford 1980).
Disease and Predation
No information exists regarding disease or parasites (Parnell et al. 1995). Gull-billed Terns seemed unaffected by the large outbreaks of botulism, cholera, and other diseases that occurred at the Salton Sea during the 1990s (KCM).
Low reproductive success of Gull-billed Terns in Virginia is in part attributed to predation on eggs and chicks by gulls and Great Horned Owls (Eyler et al. 1999, O’Connell and Beck 2003). O’Connell and Beck (2003) reported that 77% of 133 eggs among 64 Gull-billed Tern nests in Virginia were lost to predation by Herring and Great Black-backed gulls. Nesting by Gull-billed Terns on traditional barrier island sites has become more limited in New Jersey and Virginia due to red fox and raccoon expansions (Erwin et al. 2001).
Receding water levels have caused traditional nesting sites at the Salton Sea to become increasingly accessible to mammalian predators, resulting in complete breeding failures at the Morton Bay colony in 2004 and 2005 (KCM). The colonization of the Salton Sea and subsequent breeding by California Gulls in 1996 adversely affected Gull-billed Tern nesting and fledging success (Molina 2004). From 1997 to 2001, Gull-billed Terns ceased nesting at the Obsidian Butte colony and on two islands at the Sonny Bono Salton Sea NWR while these sites were occupied by California Gulls (Molina 2004). Gull-billed Terns reoccupied the Obsidian Butte colony site in 2004 when California Gulls ceased to nest there (KCM).
Feral dogs and cats and introduced rats are also threats to colonies in or close to urban environments. At the San Diego Bay NWR, Gull-billed Terns do not nest on isolated islands, but instead occupy sites among a network of easily accessible earthen levees where an aggressive predator control program reduces mammalian predation (B. E. Collins, pers. comm.).Threats 15
The geometric mean concentration of selenium for six Gull-billed Terns eggs collected from the Salton Sea in 1991 was 4.10 ppm (dry wt., range = 3.4 to 5.3; D. J. Audet, pers. comm.) below the threshold for lowered egg hatchability (Skorupa and Ohlendorf 1991). The concentration of total DDT from one Gull-billed Tern egg from San Diego Bay, California was considered elevated at 2.9 ppm (wet wt.), but below levels associated with reproductive impairment in other species (C. A. Roberts, pers. comm.). 2.9 ppm of DDT is approaching levels of severe effects in sensitive species (Blus 1984). The total PCB concentration in that egg was below the 2.9 ppm threshold at 1.8 ppm (wet wt.; C. A. Roberts, pers. comm.). Concentrations of arsenic, cadmium, chromium, copper, mercury, nickel, and zinc in the San Diego egg were below threshold levels (C. A. Roberts, pers. comm.).
The April 20, 2010 Deepwater oil rig disaster off the Louisiana coast dramatically demonstrated the actual and potential impacts of oil contamination on a wide range of coastal waterbirds, including Gull-billed Terns. Although they are less dependent on prey from marine and estuarine waters than other tern species, Gull-billed Terns along northern and eastern Gulf Coast, Florida, and the southeast Atlantic barrier islands are certainly at risk from oil-contaminated sand and marsh nesting and feeding habitats.
Population size and colony distribution.—The small population size and the low number and sizes of Gull-billed Tern breeding colonies in North America increases their vulnerability to catastrophic habitat change, human disturbances, flooding events, predation, displacement by other nesting species, and other natural and anthropogenic threats.
This is particularly true for G. n. vanrossemi, for which recent breeding is known at only one coastal California site, one to four sites in the Salton Sea, and six to eight widely separated sites in western Mexico. From 1992–2004, 65–90% of California’s annual breeding population nested at the Salton Sea, indicating a high degree of population consolidation in one area (Table 3). Two other
areas of consolidation occur in western Mexico,
at Cerro Prieto and Isla Montague in Baja California and at Laguna Pericos in Nayarit.
These few sites support a high percent of the
G. n. vanrossemi population (KCM). G. n. aranea colonies are also small and localized on the Atlantic Coast with an area of consolidation from Virginia
to South Carolina (RME).
Introduced species.—Predation by feral dogs and cats and introduced rats is discussed in the
Disease and Predation section (above). The encroachment of invasive plants, salt cedar (Tamarisk spp.) and common reed (Arundo and Phragmites spp.), degrades nesting habitat at Salton Sea colonies (KCM).
Conflicts with other species.—In San Diego County, California, Gull-billed Terns have been observed to prey upon the eggs and chicks of two species listed under the Endangered Species Act (ESA), the threatened western Snowy Plover and endangered California Least Tern, since 2001 (R. T. Patton, pers. comm.). The highest number of observed predation events was in 2003, when 52 chicks of California Least Terns and western Snowy Plovers were taken. Additional predation was documented by the presence of California Least Tern chick leg bands in Gull-billed Tern nests (J. A. Bartel, pers. comm.). Not all predation events are directly observed, and more events are suspected (R. T. Patton,
pers. comm.).
Between 1993 and 1995, conflicts with California Least Terns and western Snowy Plovers in southern California resulted in the lethal control of six Gull-billed Tern adults under the MBTA (T. E. Tate-Hall, pers. comm.). No additional lethal control of Gull-billed Terns in response to such conflict has been authorized or reported. Little information is currently available on the overall impact of predation by Gull-billed Terns on California Least Terns and western Snowy Plovers. Implementation of regular or long-term lethal control of Gull-billed Terns to protect California Least Terns and Snowy Plovers may affect the stability or growth of the Gull-billed Tern population in southern California.
Gull-billed Tern predation on Least Terns is not as evident in areas outside of southern California. Gull-billed Terns frequently share nesting colonies with Least Terns at Isla Montague, with no evidence of such predation (KCM). Gull-billed Terns have been noted to forage near a colony of Least Terns in Gulfport, Mississippi and predation of one chick was documented (Densmore 1990). Gull-billed Terns were observed to swoop on Least Tern chicks in Florida but capture was never observed (Smith and Gore 1996). Gull-billed Terns didn’t include avian prey in their diet in Virginia (Erwin et al. 1998a).
Other interspecific interactions.—Black Skimmers have damaged and/or caused Gull-billed Tern nests to be abandoned, and were suspected of inflicting lethal lacerations to Gull-billed Tern chicks at San Diego Bay (R. T. Patton, pers. comm.). At the Salton Sea, loafing Brown (Pelecanus occidentalis) and American White (P. erythrorhynchos) pelicans caused high rates of nest loss and abandonment of several Gull-billed Tern colonies (KCM). In early spring 2007, Caspian Terns (Hydroprogne caspia) began nesting on islands at the Salton Sea typically used by breeding Gull-billed Terns, precluding the later arriving Gull-billed Terns from nesting (C. C. Schoneman, pers. comm.). In Virginia, competition with Herring and Great Black-backed gulls for nest sites among higher elevation habitats force Gull-billed Terns and other small larids to nest in flood prone areas (O’Connell and Beck 2003).16 Status Review and Conservation Recommendations for the Gull-billed Tern
Aggressive anti-predator defenses of Gull-billed Tern are well known (Sears 1978, Parnell et al. 1995) and may benefit colony associates that show less aggressive defense (Pius and Leberg 1997, 1998). Black Skimmers may benefit from the aggressive nest defense of Gull-billed Terns by nesting in or near Gull-billed Tern colonies (Burger and Gochfeld 1990, Pius and Leberg 2002). In Florida, Gull-billed Terns frequently co-occupy nesting colonies with Least Terns (Smith and Gore 1996). In mixed species colonies, the smaller Least Tern may also benefit from G. n. aranea’s aggressive response to predators.
Other conflicts.—The Gull-billed Tern’s ability to use a variety of terrestrial habitats when foraging resulted in conflicts with military aircraft operations at the Naval Base, Coronado and Naval Outlying Landing Field, Imperial Beach in San Diego Bay. In 2004, two foraging adults were lethally removed near an active runway on the Naval Base; and in 2007, one adult was lethally removed at the Naval Outlying Landing Field (T. E. Tate-Hall, pers. comm.). These birds were removed under the authority of a Bird Airstrike Hazard permit under the MBTA (T. E. Tate-Hall, pers. comm.).
In Mexico, Gull-billed Terns forage extensively over commercial shrimp farms during harvest. Although lethal control of predators is not legally authorized in Mexico, it does occur (KCM); however, data on potential impacts are unavailable.
Disturbance to nesting sites.—Human disturbances, especially when frequent or prolonged, threaten reproductive success by exposing eggs and young to opportunistic predators or to lethal temperatures (Parnell et al. 1995). Gull-billed Tern chicks are highly precocial and will move long distances from the nest site, frequently over water, when disturbed repeatedly, potentially resulting in mortality from drowning or from immobilization in soft silty substrates (KCM). Human and pet disturbances at nesting colonies are potentially severe in Florida (Smith and Gore 1996) and Alabama, and are increasing in Virginia and North Carolina (Parnell et al. 1997). Management to reduce human disturbance has likely contributed to the presence, and in some cases persistence, of Gull-billed Tern colonies on NWRs and other managed state and federal lands (KCM).
Flickr/marj_k©Management and Conservation 17
Predator Management
Predator control, whether by lethal or non-lethal
measures (e.g. fencing, predator relocation, etc.)
can directly benefit Gull-billed Terns and other
ground nesting colonial waterbirds at nearby
colonies. In Virginia, control (removal) of foxes and
raccoons is conducted on selected barrier islands
(RME). Control of avian and mammalian predators
is implemented annually at the San Diego Bay
NWR for recovery of California Least Terns and
western Snowy Plovers, incidentally benefiting
Gull-billed Terns (B. E. Collins, pers. comm.). A
mammalian predator control program is proposed
to protect Gull-billed Tern colonies at the Sonny
Bono Salton Sea NWR (C. C. Schoneman, pers.
comm.). Currently, predator control is occurring at
waterbird colonies located on private lands in the
Salton Sea area (C. C. Schoneman, pers. comm.).
Chain-link and electric fencing is used at East Lake
colonies at the Lower Rio Grande Valley NWR
in Texas to exclude mammalian predators from
waterbird nesting areas (D. S. Stolley, pers. comm.).
Electric fencing is also used at Sonny Bono Salton
Sea NWR to exclude predators, primarily raccoons.
Habitat Management
Federal and state wildlife agencies and conservation
organizations implement a variety of management
actions to protect Gull-billed Tern colonies. At
some colony locations, signs are posted or barriers
are constructed to eliminate human disturbance.
At a few colonies in Virginia, New Jersey, Texas,
and California, experiments have been attempted
with electric and traditional fencing to exclude
mammalian predators. These efforts, and others to
control the encroachment of vegetation at nest sites,
have reduced threats of habitat modification and
human disturbance (Smith and Gore 1996).
The Sonny Bono Salton Sea NWR manages water
levels in freshwater impoundments to avoid shallow
depths that allow access by mammalian predators,
manages invasive vegetation (i.e., Tamarisk) to
maintain open nesting habitat, and restricts public
access to colonies to reduce human disturbance.
The effectiveness of these management actions
is evidenced by the early season abandonment
of unmanaged colonies on adjacent private lands
due to predator disturbances, and the subsequent
relocation of these failed colonies to managed
habitat in NWR impoundments (C. C. Schoneman,
pers. comm.).
Isla Montague lies within the protective core zone
of the Rio Colorado Delta Biosphere Reserve, but
has no habitat management specifically directed
at nesting Gull-billed Terns. Neither does Isla El
Rancho, in Bahia Santa Maria, which is included in
the Gulf of California Island Park System and the
Santa Maria Bay Ecosystem Management Program
(KCM). Sites in Nayarit and Colima also lack
habitat management (EP).
Management and Conservation
18 Status Review and Conservation Recommendations for the Gull-billed Tern
Artificial Nesting Habitat
In 2005, at the Sonny Bono Salton Sea NWR, the USFWS experimented with a small floating raft in a freshwater impoundment to augment existing nesting habitat (C. C. Schoneman, pers. comm.). Gull-billed Tern decoys and a sound system to broadcast recorded Gull-billed Tern colony vocalizations were placed on the raft. No Gull-billed Terns nested on the raft in 2005 but at least five pairs nested on the raft in 2006, although no nests successfully hatched (C. C. Schoneman, pers. comm.). Gull-billed Tern fledglings and parents also used the raft late in the season for loafing and roosting. Successful nesting by Gull-billed Terns occurred in 2007 with approximately 30 nests established on the raft (KCM). In 2006, a second and larger nesting platform, also using decoys and recordings, was constructed on stilts in the Salton Sea (M. A. Ricca, pers. comm.). This platform was not used by Gull-billed Terns but was colonized by four pairs of nesting Black Skimmers. In 2007, a maximum of 28 Gull-billed Tern nests were observed on the platform, with minimal nest and fledgling success (KCM). Ramps were installed on the raft and elevated plaforms to assist chicks to nest sites if they fell from or left the site before fully fledging off. Strong winds and waves damaged the integrity of ramps at both sites, requiring modification and annual maintenance
(C. C. Schoneman, pers. comm.). Although the
value of artificial nesting platforms is unclear,
their potential benefit of providing nesting habitat free from human disturbance and mammalian predators merits further study (C. C. Schoneman, pers. comm.).
Education
Apart from signs at various colony sites and interpretive information provided at some federal and state wildlife refuges, there is little or no outreach specifically relating to Gull-billed Terns. On the east coast, the Virginia Coastal Bird Partnership, a program involving agencies, research institutions, and conservation organizations, was formed in 1993 to monitor waterbirds and educate the public about them; Gull-billed Terns have been a specific focus of this group (RME). In southern California, Gull-billed Terns have received a negative public perception because of their observed predation on two species listed under the ESA (KCM). Discussions with managers of the listed species’ habitats are needed to develop management actions benefiting all three species.Conservation Recommendations 19
(2) Minimize disturbance to nesting colonies by
conducting surveys outside the colony, wherever
possible, using observers in blinds or vehicles or
observing from a distance to prevent flushing.
(3) Count fledging and near-fledging age young
approximately three weeks after the first chick has
hatched on the colony to measure breeding success.
(4) Explore the potential for conducting surveys
of Gull-billed Terns in the species’ winter range
from the Gulf of Mexico coast states south
through Mexico, Central America, the Caribbean,
and northern South America to determine winter
distribution and the overlap in wintering by
both G. n. aranea and G. n. vanrossemi and to
aid in determining conservation needs on the
wintering grounds.
(5) Use field identification cards of Gull-billed Terns
to train surveyors to identify both breeding and
wintering terns and to estimate the approximate
age of pre-fledged young.
Habitat Management and Protection
Protection of nesting and foraging habitat of Gull-billed
Terns is vital to the long-term survival of the
species. Conservation programs aimed at reducing
or reversing the impacts of river channelization,
changes in sediment deposition, beach erosion,
sea level increases, regional water transfers, and
other landscape level perturbations will ensure
the long-term viability of Gull-billed Tern habitat.
Because most Gull-billed Terns spend their annual
cycle in the United States and either Mexico,
Central America, or the Caribbean, conservation
efforts will require multi-national cooperation.
Laws protecting habitat range-wide and prohibiting
take in countries outside the United States require
better enforcement. Increased international
communication and cooperation between biologists
may help refine conservation strategies.
(1) Evaluate active and historic colony sites to
identify opportunities to enhance or protect
colonies. Develop and implement colony specific
management plans to improve colony security and
reproductive success and to reduce threats.
Our recommendations for range-wide
conservation practices for both G. n. aranea and
G. n. vanrossemi emphasize monitoring, habitat
management and protection, and research.
These recommendations are in priority order
within each section.
Monitoring
(1) Conduct breeding population surveys focused
specifically on Gull-billed Terns. Small colony size
and unique nest sites, substrates, and seasonality
may cause colonies to be overlooked during multi-species
surveys and aerial waterbird surveys may
fail to distinguish Gull-billed Tern nests from those
of other terns.
(a) Standardize survey methodology recognizing
that it may need to be modified for location, size,
distribution, and habitat of an individual colony.
(b) Report abundance in number of pairs
whenever possible, so that data are comparable
across regions. Measure “number of adults”
rather than “number of nest attempts”
given the species’ low nest site tenacity and
re-nesting ability.
(c) Coordinate the seasonal timing of Gull-billed
Tern surveys within regions.
(d) Conduct range-wide surveys every three
to five years as Gull-billed Tern populations
may fluctuate inter-annually and site fidelity
is relatively low. More frequent surveys (i.e.,
annually) for the subspecies G. n. vanrossemi
are recommended.
(e) On state, federal, and other managed
conservation lands, survey Gull-billed Tern
colony sites annually for presence (or absence) of
nesting birds. If the colony consists only of Gull-billed
Terns, conduct a single survey to coincide
with the peak incubation period. If other species
are present, conduct a visit timed specifically for
Gull-billed Terns.
(f) Conduct baseline surveys along the Gulf
of Mexico Coast in Mexico to determine
breeding distribution, colony status, and
abundance of G. n. aranea. Once this has
been determined, conduct periodic surveys
as described in (d), above.
Conservation Recommendations
20 Status Review and Conservation Recommendations for the Gull-billed Tern
(5) Develop population viability models for Gull-billed and California Least terns and western Snowy Plovers to assess each species’ population health and to evaluate the effects of Gull-billed Tern predation.
(6) Conduct long-term studies to determine age and sex-specific mortality, fecundity rates, and lifetime reproductive success for both subspecies.
(7) Investigate specific foraging patterns of
Gull-billed Tern adults.
(8) Investigate winter distribution and ecology of colonies breeding in the United States and Mexico through banding, auxiliary marking, and telemetry.
(9) In conjunction with surveys in Mexico and Central America, take measurements and collect specimens or tissues of breeding birds to delineate the southern limits of the breeding ranges of both G. n. aranea and G. n. vanrossemi and to determine the subspecific identity of breeding birds in
Central America.
(10) Investigate contaminants in Gull-billed Terns
that may result from foraging in agricultural areas.
(11) Investigate the value of artificial nesting platforms.
(12) Determine nest success of roof-nesting
Gull-billed Tern populations to determine the importance of this substrate to breeding populations.
(13) Gather information on lethal control of
G. n. vanrossemi at shrimp farms in western Mexico to determine effects to the subspecies’ population.
Conclusion
The current population of the subspecies G. n. aranea in the United States is estimated to be approximately 3610 pairs, with over 60% occurring in Texas. This species is possibly extirpated in Maryland and Delaware. The number of pairs has declined in Virginia, North Carolina, Florida, and possibly Georgia, but appears stable in Texas. The subspecies G. n. vanrossemi has 737 to 808 breeding pairs in western Mexico and California. Trend information for G. n. vanrossemi is unavailable because of the lack of data from Mexico prior to 2003. The main causes of population declines in North America are disturbance of nesting colonies, loss of natural nesting islands, and development or modification of upland foraging habitats.
(a) Reduce erosion of nesting islands by supplementing with crushed shell, gravel, or sand (0.2-0.4 mm) and/or installing riprap borders where necessary and feasible.
(b) Manage vegetation growth to heights of less than 12 cm and densities of less than 14% cover, as indicated by Parnell et al. (1995), to provide suitable open space for Gull-billed Tern colonies.
(c) Assess active colonies to determine the need for predator management Design and implement an integrated predator management program using non-lethal (e.g., fencing) and, where necessary, lethal predator control measures in coordination with state and federal agencies. Implement measures to control feral and domestic pets that impact colony nesting success.
(2) Seek long-term protection for all colony sites through land acquisition or conservation easements or agreements.
(3) Seek long-term protection of upland foraging habitats through land acquisition or conservation easements or agreements.
(4) In coordination with ESA recovery teams, design and implement a multi-species management strategy for Gull-billed and California Least terns, western Snowy Plovers, and other ground-nesting waterbirds in coastal southern California.
(5) Given the importance of the Salton Sea to G. n. vanrossemi, ensure that restoration and management plans for the Salton Sea address the subspecies’ long-term habitat needs.
(6) Reduce or limit lethal control of Gull-billed Terns at aquaculture farms in northwestern Mexico.
(7) Provide informational signs and other outreach material at colony sites to reduce human disturbance.
Research
(1) Develop standardized monitoring and analytical protocol for trend analysis in range-wide surveys.
(2) Investigate the subspecies taxonomy of
Gull-billed Terns.
(3) Develop techniques to identify and establish alternative nesting sites in southern California for
G. n. vanrossemi to decrease conflicts with California Least Terns and western Snowy Plovers.
(4) Use video monitoring techniques at co-occurring
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Research/Monitoring: Research focused on Gull-billed
Terns has not been conducted in Alabama.
Comprehensive surveys were conducted in the state
in 1976 and 1983. Since 2001, state-wide censuses
have been conducted annually (R. B. Clay, pers.
comm.). Field work for a state Breeding Bird Atlas
is currently underway (R. B. Clay pers. comm.).
Conservation/Management Activities: Dauphin and
Pelican islands are privately owned, while Gaillard
Island is an active dredge disposal site owned by
the state. No conservation or management activities
specific to nesting waterbirds occur at these sites
(R. B. Clay, pers. comm.).
State Status: Protected (Alabama Natural Heritage
Program 2008).
Natural Heritage Rank: S2—Imperiled (NatureServe
Explorer 2006).
Habitat Conditions: In Alabama, nesting occurs on
undeveloped, low elevation sandy shoal islands
sparsely vegetated with dune grasses and on a
dredged-material island that provides bare nesting
substrate (R. B. Clay, pers. comm.).
Threats: All three islands are of low relief and
breeding efforts may be swamped by the surge
of early season tropical storms and hurricanes
(R. B. Clay, pers. comm.). Human disturbance
can be significant on Dauphin and Pelican islands
(R. B. Clay, pers. comm.).
a See Appendix C for contact information for contributors
to the state and regional summaries.
UNITED STATES
Alabama
Summary: Imhof (1976) considered Gull-billed
Terns uncommon summer residents, with breeding
restricted to islands in the southwestern part of
the state, in Mobile Bay and the Mississippi Sound
in Mobile County. They occur more commonly as
migrants on Alabama’s Gulf of Mexico Coast (> 75
birds observed on 6 Aug 1960) with small numbers
wintering in upper Mobile Bay and the Mississippi
Sound (Imhof 1976). In recent years, small colonies
of Gull-billed Terns (totaling only a few dozen
breeding pairs) have been found on Gaillard Island,
a dredged material island in Mobile Bay, and on two
natural barrier islands, Dauphin and Pelican islands,
in the Mississippi Sound (Fig. A-1).
Population Trends: Breeding Gull-billed Terns were
apparently never abundant in Alabama during the
1900s. Small numbers of Gull-billed Terns were
first documented from Petit Bois Island, near
the Mississippi/Alabama border, on 4 July 1913,
where only about 15 or 20 pairs were believed to
be nesting (Howell 1924). Three active nests were
reported at Cedar Point in 1956 (Imhoff 1976) and
23 individuals were reported on Dauphin Island
in 1976 (Portnoy 1977). More recently, as many as
87 pairs have nested in Alabama (Table A-1; R. B.
Clay, pers. comm.). Although breeding numbers are
highly variable, an average of about 50 pairs nested
annually at one to three colonies from 1988–2004
(Fig. A-1; Tables 2, A-1).
Appendix A: State and Regional Summaries of
Gelochilidon nilotica aranea Status Within the
Breeding Range in the U.S., Mexico, and Caribbeana
Table A-1. Number of Gull-billed Tern breeding pairs at colonies in Mobile County, Alabama, 1988–2004
(R. B. Clay, pers. comm.). Dash (—) indicates data are insufficient to discern site availability, occupancy, or
extent of survey coverage for a particular site and year.
Colony 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004
Dauphin I. — — — — — — — — — — — — — 15 0 0 30
(west end)
Gaillard I. 59 57 72 3 34 90 12 58 39 14 75 10 2 5 0 9 20
Pelican I. — — — — — — — — — — — 35 50 67 50 0 35
Totals 59 57 72 3 34 90 12 58 39 14 75 45 52 87 50 9 70
Appendix A 27
Delaware and Maryland
Summary: Gull-billed Terns have bred sporadically and in very small numbers in at least two known sites in Delaware. At Rehobeth Bay and Assawoman Wildlife Refuge, between 1958 and 1991 (Hess et al. 2000). Gull-billed Terns are virtually extirpated in Maryland and are considered rare summer residents with records primarily from Worcester County in the extreme southeastern portion of the state. The species has nested at South Point dredged-material islands, Clam Harbor Tumps, Oyster Island, Big Bay Marsh, and Ocean City spoil, all in Worcester County (Brinker 1996). The numbers of breeding pairs in Maryland peaked in the 1950s and 1960s, and since 1986, the state has seen intermittent breeding by only a single pair. Unsuitability of sand and shell dredged-material due to island erosion or vegetation succession has been a factor in the virtual extirpation of this species from Maryland.
Population Trends: Breeding in Delaware was first documented in 1958, when three clutches were collected at Rehobeth Bay (Hess et al. 2000). Gull-billed Terns possibly bred at Assawoman
Figure A-1. Locations and sizes of Gull-billed Tern colonies in Alabama, 2004.Wildlife Area in 1958; breeding was documented there in 1959 and in 1989–1991. No breeding was documented from 1983–1987 and there has been no documented nesting since 1991 (Hess et al. 2000).
Gull-billed Terns were never numerous historically in Maryland (Brinker 1996). The species colonized the South Point dredged-material islands in 1945 and continued to breed there until the early 1960s. During the 1950s, 25 to 30 pairs were recorded but the number of breeding pairs was considerably lower than that in most years. During the 1970s, the maximum number of breeding pairs was three and the species ceased to breed in the state from 1977–1983. From 1985–1988, nine, 33, one, and zero pairs nested, respectively, and were limited to a single site in any one year (Brinker 1996). The location of the last confirmed breeding was Big Bay Marsh, where possibly one pair has bred intermittently from 2000–2004 (D. F. Brinker, pers. comm.).
Research /Monitoring: Breeding Bird Atlas field work was conducted from 1983–1987 in Delaware (Hess et al. 2000) and Maryland (Brinker 1996). Census and monitoring efforts in Delaware are unknown but are conducted irregularly in Maryland.28 Status Review and Conservation Recommendations for the Gull-billed Tern
Presently, most nesting occurs in the St. John’s River estuary in Duval County and in Tampa Bay (Fig. A-2). Most Gull-billed Terns depart Florida
for the winter, but small numbers remain on
the Atlantic coast north to Jacksonville and on the
Gulf coast north to the Tampa Bay area (Stevenson
and Anderson 1994).
Population Trends: No comprehensive survey data are available for the state but a strong downward trend is evident. Ogden (1975) reported a total of 534 pairs nesting near Merritt Island in Brevard County and the St. John’s River estuary in Duval County in 1975. In 1976 and 1977, census estimates from only one or the other of these two Atlantic coast sites were available, obscuring any indication of trend. Gull-billed Terns were not reported during a study of 40 select dredged-material islands in 1977 (Schreiber and Schreiber 1978). In Franklin County, Smith et al. (1993) reported two to three nests on the St. George Island causeway from 1990–1992.
By 2000, populations declined from 1970’s levels (to as low as 17 pairs) at the larger Atlantic coast colonies (Merritt and Bird islands, Tables 2, A-2).
Research/Monitoring: No research studies specific to Gull-billed Terns have been conducted. A Breeding Bird Atlas conducted from 1986–1991 confirmed Gull-billed Terns in only 10 of the state’s 1028 quadrangles, indicating they are rare and extremely localized breeders in Florida (Florida Fish and Wildlife Conservation Commission 2003). Local coastal censuses were conducted in 1975, 1980–1985, and in 2000. A comprehensive state-wide census for this species has not been undertaken.
Conservation/Management Activities: One of the two largest historic colonies where Gull-billed Terns were known to have nested within Merritt Island NWR and is protected from private development. There is no active management specifically directed at Gull-billed Terns.
State Status: No status
Natural Heritage Rank: S2—Imperiled (NatureServe Explorer 2006).
Habitat Conditions: In Florida, Gull-billed Terns nest on coastal beaches, natural estuarine islands, coastal marshes, dredged-material islands, islands in freshwater lakes, and sand fill at phosphate mining areas. Nesting substrates are sandy and sparsely vegetated. Gull-billed Terns forage in coastal and freshwater marshes, open uplands, shrub-dominated fields, and along coastal beaches (Smith and Gore 1996).
Threats: Loss, modification, and degradation of coastal habitats, including human disturbance, flooding, and succession of vegetation at colony sites are thought to be significant threats to the species (Smith and Gore 1996).
Conservation /Management Activities: No active conservation or management measures are
currently conducted.
State Status: None in Delaware; Endangered in Maryland (Maryland Department of Natural
Resources 2007).
Natural Heritage Rank: SHB, S2N—Possibly extirpated in Delaware; S1—Critically Imperiled in Maryland (NatureServe Explorer 2006).
Habitat Conditions: In Delaware, breeding habitat consisted of salt marshes from Indian River Inlet north to Bombay Hook NWR. In Maryland, nesting was on dredged-material islands in barren areas of sand and shell. The South Point dredged-material islands in Maryland became unsuitable due to vegetation succession by the early 1960s, and Clam Harbor Tumps and Oyster Island have eroded to below mean sea level (Brinker 1996). Big Bay Marsh, Maryland, is a narrow (2–3 m) shell beach of such low elevation that it hardly extends above wind-driven storm tides. In Maryland, Gull-billed Terns foraged in open salt marshes, fields and along beaches (Brinker 1996).
Threats: None described in Delaware. In Maryland, succession of vegetation on some dredged-material islands is believed to render them unsuitable for nesting (Brinker 1996). The limited number of available nesting sites in reasonable proximity to feeding areas and the loss of two previously used nesting islands due to erosion are believed to have contributed to their apparent extirpation from the state (Brinker 1996).
Florida
Summary: Florida’s Breeding Bird Atlas (Florida Fish and Wildlife Commission 2003) considers Gull-billed Terns to be rare and highly localized breeders in the state. Breeding populations in Duval and Brevard counties totaled several hundred pairs in the mid-1970s, but overall numbers of birds as well as occupied colony sites have since declined, likely a result of modification and degradation of coastal habitats. The first Florida nest was recorded in 1932 at Pensacola in Escambia County (Weston 1933); however, an egg set collected in 1892 from Anna Maria Island in Manatee County predates the Pensacola nest. Except for a colony in the interior of Palm Beach County and on islands in Lake Okeechobee, nesting by Gull-billed Terns has been restricted to the northern half of the state. Gull-billed Terns have nested in Duval and Brevard counties on the Atlantic Coast, in Bay and Franklin counties in the Panhandle, in the Tampa Bay area in Pinellas and Hillsborough counties (Florida Fish and Wildlife Conservation Commission 2003), and inland, at the phosphate mines of Polk and Hillsborough counties (Smith and Gore 1996). Appendix A 29
Table A-2. Number of breeding pairs of Gull-billed Terns at colonies in Florida 1973–1977, 1979, 1985, 1995, 1998–2000a. Dash (—) indicates data are insufficient to discern site availability, occupancy, or extent of survey coverage for a particular site and year. Data from Clapp et al. 1983, Florida Fish and Wildlife Commission 2003, Loftin and Sutton 1979, Ogden 1974, 1975, 1979, Portnoy et al. 1981, Smith and
Alvear 1997.
Colony 1973 1974 1975 1976 1977 1979 1985 1995 1998 1999 2000
Duval Co.
Bird Islands — 325 249 121 180 0 0 0 0 0 0
Huguenot Memorial Park — — — — — — — 25 0 0 10
Mayport Naval Station — — — — — 67 0 0 0 0 0
Third Bird Island — — — — — — — — — 2 0
Brevard Co.
Merritt Island NWR 171 89 285 0 150 0 13 0 0 0 0
Franklin Co.
Apalachicola, Bird Island — — — — — — — — 17 18 6
Hillsborough Co.
TPA Island 3D — — — — — — — — 1 0 1
Totals 171 414 534 121 330 67 13 25 18 20 17
Figure A-2. Locations and sizes of Gull-billed Tern colonies in Florida, 2000.30 Status Review and Conservation Recommendations for the Gull-billed Tern
Georgia
Summary: Breeding populations of Gull-billed Terns in Georgia have historically been small. In recent years only a single island, Little Egg Island Bar, a small, wind-swept and wave-washed sand bar at the mouth of the Altahama River (Kale et al. 1965), has been occupied; about 50 to 100 pairs nest annually on this state-owned island (Clapp et al. 1983). Ericksen (1926) described the first breeding record of four Gull-billed Tern nests on Oysterbed Island in 1926. Gull-billed Terns nested formerly at the mouth of the Savannah River in Chatham County. Burleigh (1958) considered the species to be a scarce summer resident on the coast of Georgia.
Population Trends: Little quantitative data are available for Gull-billed Terns in Georgia prior to the 1990s. During the 1990s, fewer than 100 pairs nested at Little Egg Island Bar (Fig. A-3;
Tables 2, A-3). The 2003 census count of 54 pairs at this site may indicate a recent decline (Tables 2, A-3;
B. Winn, pers. comm.).
Table A-3. Numbers of breeding pairs of Gull-billed Terns in McIntosh County, Georgia 1993, 1995, 1999 and 2003 (B. Winn, pers. comm.).
Colony 1993 1995 1999 2003
Little Egg Island Bar 65 80 50 54
Research/Monitoring: No research studies specific to Gull-billed Terns have been conducted. Comprehensive surveys were not conducted prior to 1993. Since then waterbird colony censuses
have been conducted every two to four years
(B. Winn, per. comm.).
Conservation/Management Activities: Only one site, Little Egg Island Bar, is presently active and occurs within a state Protected Natural Area. Management activities at this site include the posting of protective signage to prevent public access year round (B. Winn, pers. comm.). Georgia recently completed its comprehensive plan for the conservation of wildlife and identified the Gull-billed Tern as a high priority species (Georgia Department of Natural Resources 2005).
State Status: Threatened (Georgia Department of Natural Resources 2005).
Natural Heritage Rank: S1—Critically imperiled (NatureServe Explorer 2006).
Habitat Conditions: Given the relatively large tidal fluctuations characteristic of Georgia’s coast, the extent of suitable nesting habitat for terns; open and sandy islands, is highly dynamic and often ephemeral in nature (Georgia Department of Natural Resources 2005).
Threats: Accelerated urban growth and development and parallel increases in levels of disturbance by recreationists are believed to be the largest threat to colonial breeding birds in the state (B. Winn, pers. comm.). Predation of eggs and chicks by Laughing Gulls may limit Gull-billed Tern reproductive success (B. Winn, pers. comm.).
Figure A-3. Locations and sizes of Gull-billed Tern colonies in Georgia, 2003.Appendix A 31
Research/Monitoring: Few studies specifically directed toward Gull-billed Terns have been undertaken in Louisiana. Leberg et al. (1995) examined colony site use by seabirds on dredged-material islands and Mallach and Leberg (1999) examined nest site selection and success among breeding tern and skimmer colonies in the Atchafalaya Delta. Pius and Leberg (1997, 2002) examined aggression and nest spacing in
Gull-billed Tern and Black Skimmer colonies
and the influence of Gull-billed Terns on nest site choice by Black Skimmers.
Colonial waterbird monitoring in Louisiana has been conducted irregularly in the past. Comprehensive surveys were conducted in 1976, 1978, 1983, 1990, and 1993–1999. Beginning in 2001, comprehensive surveys have been conducted every four years (P. L. Leberg, pers. comm.). Coastal surveys are conducted aerially, either by helicopter or fixed wing aircraft. Rooftop colonies, which may contain up to 100 to 200 pairs (R. D. Purrington, pers. comm.), are generally not included in coastal survey efforts (P. L. Leberg and T. C. Michot,
pers. comm.); surveys of these colonies are conducted opportunistically.
Conservation/Management Activities: All recent colony sites occur on state lands and are protected from development.
State Status: Rare Animal of Conservation Concern (Louisiana Department of Wildlife and Fisheries 2008).
Natural Heritage Rank: S2—Imperiled (NatureServe Explorer 2006).
Habitat Conditions: In Louisiana, Gull-billed Terns nest primarily on barrier beaches, saltmarsh shell berms, and dredged-material islands, and are almost always in close association with Black Skimmers (Portnoy 1977). Dredged-material islands that receive regular seasonal deposits of dredged-material were thought to be increasingly important to nesting Gull-billed Terns and other species (Leberg et al. 1995). Islands in inland lakes and artificial substrates, such as gravel rooftops, are used infrequently. Gull-billed Terns forage over coastal marshes, canals, ponds, and lakes (Lowery 1974, R. D. Purrington, pers. comm.).
Threats: Inundation of estuarine and barrier islands by high or storm tides and loss of nesting habitat as a result of vegetation succession adversely impact nesting success. Human disturbance is believed responsible for the premature desertion of rooftop colonies in 2004 (R. D. Purrington, pers. comm.).
Louisiana
Summary: Lowery (1974) considered Gull-billed Terns to be fairly common permanent residents in Louisiana; least numerous in summer, and almost strictly coastal in occurrence. Although Gull-billed Terns are observed in winter in the rice prairie region of Acadia Parish, generally within 30 miles of the coast (W. J. Vermillion, pers. comm.), there is a record farther inland (> 50 miles of the coast) of five individuals over lakes on the Louisiana State University campus in Baton Rouge in September 1965 during Hurricane Betsy (Lowery 1974). Portnoy (1977) considered Gull-billed Terns to be uncommon breeders in Louisiana. The species breeds very locally along the Gulf of Mexico Coast in the Atchafalaya Delta in St. Mary and Terrebonne parishes, and east to islands in Breton and Chandeleur sounds in Plaquemines and St. Bernard parishes. A few Gull-billed Terns are reported to have bred near Sabine Lake at the Louisiana-Texas border (Portnoy 1977) and at Fearman Lake (Martin and Lester 1990). Purrington (2001, 2002) reported recent breeding attempts from the rooftops of several buildings in downtown New Orleans, though success in such colonies is generally low. Most of the recent colonies occur on the central coast, near Marsh Island and Atchafalaya Bay (Fig. A-4; Michot et al. 2004, P. L. Leberg, pers. comm.).
Population Trends: No comprehensive census data are available for the state prior to 1976; however, Louisiana’s small Gull-billed Tern population appears to have been relatively stable since the 1990s. Historical records summarized by Portnoy (1977) suggested that Gull-billed Terns had been nesting in very small numbers in the Mississippi River Delta since 1837 and on the western coast since 1906. During the 1960s and early 1970s, up to 11 nests and 27 birds were documented from Grassy Island and the Chandeleur Islands, respectively, in St. Bernard Parish (Portnoy 1977). During the first comprehensive survey of the eastern Gulf of Mexico Coast in 1976, Portnoy (1977) reported 154 pairs (or incubating birds, each assumed to represent an active nest and thus a pair) in four colonies in Louisiana (Table 2), although Clapp et al. (1983) felt Portnoy may have overlooked some nesting areas. Martin and Lester (1990) reported 161 pairs among three colonies during their coastal aerial census in 1990. Results of annual aerial surveys conducted by Linscombe and Vermillion (W. J. Vermillon, pers. comm.) from 1991–1999 ranged from 30 to 1120 incubating adults (Tables 2, A-4). In a survey of dredged-material islands in the Atchafalaya Delta Wildlife Management Area, Leberg et al. (1995) estimated a maximum of 300 to 400 pairs from
1992–1995. Although this survey involved only the central portion of the coast, it is felt to have encompassed most of the known sites. 440 pairs
were reported in a 2001 coastal census
(Table A-4; T. C. Michot, pers. comm.).32 Status Review and Conservation Recommendations for the Gull-billed Tern
Table A-4. Number of breeding pairs of Gull-billed Terns at colonies along the coast of Louisiana, 1976, 1990–1999 and 2001. Dash (—) indicates data are insufficient to discern site availability, occupancy, or extent of survey coverage for a particular site and year. Topographic quad names follow the Louisiana Natural Heritage Program (LNHP) colony identification numbers. Data from Martin and Lester 1990, Michot et al. 2004, Portnoy 1977, G. D. Lester, pers. comm.
Colony 1976 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2001
Iberia Parish
LNHP 439-Bayou Lucien — — — — — 10