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The western subspecies of Gull-billed Tern (Sterna nilotica vanrossemi) has been identified as a focal species of management concern due to a very low population size and an extremely small breeding range. Western Gull-billed Terns only breed locally in extreme southern California in San Diego Bay and inland at the Salton Sea, which hosts the largest colony of Gull-billed Terns in the western U.S. The remaining portion of the population breeds in northwestern Mexico. Populations probably have not been very abundant historically. However, the number of nesting pairs at the Salton Sea declined from approximately 500 in 1927 to a low of 17 in 1976. Currently, there are only about 250 pairs of western Gull-billed Terns breeding in the U.S. and about 525 pairs in northwestern Mexico. Other threats include human disturbance of breeding colonies, increased predation of their nests, and pollution and pesticides in their habitats.
Date: August 2008
Source: Molina, K. C., and M. Erwin. 2006. The distribution and conservation status of the Gull-billed Tern (Gelochelidon nilotica) in North America. Waterbirds 29:271-295.
Plumage: This is a stout, white, blunt-billed tern that feeds in the marshes and adjacent coastal uplands of the southern and Gulf coasts of the U.S. (Forbush 1939, Harrison 1983, Cramp 1985). It is similar in size to other medium-sized terns. This tern's flight is usually more buoyant and gull-like than that of other terns (Vinicombe and Harris 1989). While in breeding plumage, a black cap extends from the lores, including the eyes, to the nape. The rest of the upper parts, wings, and tail are a pale pearl gray. The side of the head, underparts, wing linings, tail, and underwing coverts are white. The primaries are grayish-black underneath and frosty-gray above. The legs and feet are black. The heavy black bill lacks the sharp tip of other terns and is stouter and proportionately shorter. The sexes are similar in appearance. Winter plumage is similar to the breeding plumage except that the black cap is nearly absent, with only some remnant spotting near the rear of the crown. A blackish patch extends from the eye to the auriculars, although the extent of this is quite variable (Harrison 1983, Cramp 1985).
Recently fledged juveniles are similar to adults in winter plumage except that the head is darker with more blackish spots and the gray back and upper wing are edged in tan, giving the back and wing a buff-colored appearance when the bird is in flight. The downy young are variable in appearance but generally cream, buff, or peach colored, with darker down on the dorsal surfaces. Young usually have two dorsal stripes on the crown, nape, and back and a distinctive dark smudge behind the eye. The bill is typically light pink at hatching and darkens with age. The feet are light pink and darken to an orange-brown with age (Harrison 1983, Cramp 1985).
Vocalizations: The typical call is a nasal "tee-hee-hee" or "kat-y-did" (Bent 1921). Terns attacking terrestrial predators will frequently utter a harsh "grack" call during defensive dives (Sears 1981). Other calls are described by Bent (1921), Sears (1981), and Cramp (1985).
Eggs: The background color of the eggs may vary from buff to olive and the mottling is somewhat finer grained than the pattern of the common tern egg. The eggs have a characteristic "frosty" appearance which also distinguishes gull-billed tern eggs from the those of the common tern (STERNA HIRUNDO) (Bent 1921, Harrison 1975). The eggs of the gull-billed tern are cryptically colored, and the immediate area of the nest is not white-washed.
Diagnostic Characteristics: Lacks the long tail streamers of common terns (STERNA HIRUNDO), Forster's terns (S. FORSTERI), and roseate terns (S. DOUGALLII) terns. It is also longer-legged and broader-winged. It can be told from the sandwich tern (S. SANDVICENSIS) by its shorter beak and tail, shallower wingbeat in flight, and more upright posture while sitting.
Life History:
NESTING: Nests in single pairs, small scattered groups, or colonies; typically joins mixed species colonies with common terns, and black skimmers (RYNCHOPS NIGER), least terns (STERNA ANTILLARUM), royal terns (S. MAXIMA), sandwich terns, and/or caspian terns (S. CASPIA). Most of the gull-billed tern colonies studied in Virginia and North Carolina were small (mean = 45 birds) (Erwin 1978), but colony size ranges up to several hundred along the Gulf Coast.
Distances between nests ranges from two to 114 meters, with a mean of 21 meters (Sears, pers. comm., cited by Cramp 1985), although the inter-nest distance may vary as a function of colony size (Moller 1982). Vegetation in nesting areas is sparse, e.g., approximately 15% of the ground was covered in a North Carolina colony (Soots and Parnell 1975).
Clutch size is one to three eggs (Bent 1921, Sears 1978, Moller 1981) and occasionally four (Bent 1921, Forbush 1939, Pemberton 1927, Harrison 1978). Clutch size in one study was found to be significantly greater in larger colonies (Moller 1981). Renesting attempts usually result in smaller clutches (Sears 1978).
Display a variety of behaviors related to courtship or nest defense. Ritualized courtship behavior, for example, include aerial flights, a variety of terrestrial displays, and courtship feeding (Bent 1921, Lind 1963, Sears 1976, 1981). Defecating away from the nest may reduce the risk posed by nest predators (Sears 1978). They also frequently remove eggshells from the nest after hatching (Cullen 1960), but this response is not as strong as it with other tern species, probably because the cryptically-colored young leave the nest several days after hatching (Sears 1978).
The eggs are incubated by both the male and the female for 22-24 days (Harrison 1975), and only one brood is raised per season (Bent 1921, Forbush 1939). The major factors that determine the time at which the young leave the nest are the age of the chicks, the proximity of vegetation, and disturbance (Sears 1978). The young are tended by both parents and fledge at four to five weeks of age (Harrison 1975).
Nesting success appears to be low, with many colonies producing no young at all (Blus and Stafford 1980). The main causes of breeding failure are flooding of low-lying colonies, disturbance by humans, and predation (Clapp et al. 1983).
Habitat:
All Seasons: Coastlines, salt marshes, estuaries, lagoons, plowed fields, and less frequently along rivers, around lakes, and in freshwater marshes (Clapp et al. 1983).
Breeding: This is a marsh-nesting tern along the coast of New Jersey (Wilson 1840, Stone 1908). Bent (1921) concluded that it had been driven to nest on barrier beaches because of hunting at sites on inner dunes, saltmarshes, and islands. Nesting sites are presently confined to sandy barrier islands, beaches, sandy shores of saline lagoons and marshes, and artificially-produced dredge spoil islands (Clapp et al. 1983). Regional differences in nest sites occur, with the percentage nesting on spoil islands ranging from 28% in New Jersey to 60-80% in North Carolina and 70-84% in Texas (Clapp et al. 1983). Some nest on rooftops in Louisiana (Wiedenfeld and Swan 2000).
Nests are generally located close to landmarks, such as plants or pieces of driftwood, and are usually slight depressions with rims of dried straw and/or shell fragments (Harrison 1975, Sears 1978). Some nests are more elaborate piles of accumulated shell fragments, which may serve to provide protection from drifting sands (Sears 1978). The appearance of the nest lining varies greatly between nests and from day to day in the same nest, depending on the individual, the time available for placing the lining, and weather conditions (Sears 1976, 1978).
Nonbreeding: Gull-billed terns sleep and loaf on dikes, mudflats, and sandspits (Stiles and Skutch 1989).
Distribution:
United States: AL, CA, DE, FL, GA, LA, MD, MS, NC, NJ, NY, SC, TX, VA
Status:
NatureServe Status: Global Status: G5, Global Status Last Reviewed: 17Sep2007, Global Status Last Changed: 27Nov1996, Rounded Global Status: G5 - Secure
Other Statuses: IUCN Red List Category: LC - Least concern
