Gelochelidon nilotica

Formerly (AOU 1983, 1998) included in the genus Sterna but separated on the basis of genetic data that correspond to plumage patterns (Bridge et al. 2005) (AOU 2006).

Breeding range includes southern California (San Diego Bay, Salton Sea), western coast of Mexico (Sonora, Sinaloa, Baja California, Gulf of California), Atlantic and Gulf coasts of North America from New York (Long Island; scarce north of Maryland) south to Florida and west to southern Texas (also inland); and probably also in Tamaulipas and Veracruz in eastern Mexico, the Bahamas, and Virgin Islands (Anegada, probably Sombrero, formerly Cockroach Bay); in South America (southwestern Ecuador, and from central Brazil south to northern Argentina); and in the Old World from northern Europe, central Russia, southern Mongolia, and eastern China south to northwestern Africa,Asia Minor, Iran, India, Sri Lanka, and southern China; and in Australia (Sprunt 1954, Parnell et al. 1995, AOU 1998).

Nonbreeding range in the Americas includes coastal areas from Nayarit, the Gulf Coast, and southern Florida south through Middle America and the West Indies to Peru and northern Argentina (AOU 1998). In the Old World, the nonbreeding range extends from tropical Africa, Persian Gulf, India, Southeast Asia, eastern China, and the Philippines south to southern Africa, Java, and Borneo; also Australia and Tasmania (AOU 1998).

This species is represented by a large number of breeding occurrences (subpopulations).

This species was historically decimated by egg collectors and plume hunters. Via and Duffy (1992) concluded that loss of upland foraging sites and island nesting sites has probably been the greatest threat, although predation and competition with gulls, and human disturbance at colonies have also contributed to declines. Other threats include beach erosion and development.

Main threats to populations in North America include loss of natural nesting islands through beach erosion or perturbations to estuarine functions, development or modification of upland habitats near breeding areas that may be important for foraging, and disturbances to colonies by humans and feral or human-subsidized predators (Molina and Erwin 2006).

HABITAT LOSS: Destruction of marshes and nesting sites has probably been the most important problem in the past for gull-billed terns and other waterbirds (Erwin 1980). The past effects of marsh ditching on nesting and foraging are unknown, as are the historical population levels before market hunting and marsh drainage. Today, however, most northeastern marshes are either publicly owned or subject to laws that, at least in theory, protect them from further damage.

ENVIRONMENTAL CONTAMINATION: At the mouths of major estuaries, terns may be exposed to a wide variety of water-borne contaminants, including crude oil and other petroleum products. Analyses of eggs from South Carolina, however, indicated that levels of hydrocarbons were insufficient to induce eggshell thinning and reproductive failure (Blus and Stafford 1980).

HUMAN DISTURBANCE: Human disturbance may include a wide variety of human activities ranging from walking near nesting colonies to more severe forms of disturbance, such as vehicular traffic and slaughter of waterbirds (Parnell et al. 1988). Disturbance may result in desertion of nesting areas or exposure of eggs and chicks to extreme temperatures, rain, wind, and numerous predators, particularly gulls. Human disturbance has its most disastrous effects following hatching, because chicks may leave the nest prematurely and suffer excessive mortality (Sears 1978). Erwin (1980) compared barrier island nest sites in New Jersey with those in Virginia and found that the New Jersey sites were less frequently used by terns because of human disturbance. While there are no guidelines for the minimum distance at which gull-billed terns will flush, Erwin (1989) has proposed a minimum distance of 200 m for posting colonies of black skimmers and common terns, species with which gull-billed terns often nest. However, because these birds are extremely sensitive to disturbance during the nesting period (Sears 1978), even these distances may be insufficient.

PREDATION/COMPETITION: Both pet and feral dogs may pose a problem. Potential mammalian predators include raccoons, red foxes, and rats (Rattus spp.). Suspected rat predation on the eggs and nests was reported in two studies (Sears 1978, Blus and Stafford 1980). Nocturnal avian predators of common terns that are also likely to prey on gull-billed terns include great horned owls and black-crowned night-herons (Nisbet 1975, Morris and Wiggins 1986). Potential diurnal avian predators in the northeast include, laughing gulls, herring gulls, and great black-backed gulls (Buckley and Buckley 1972). Blus and Stafford (1980) suggested that predation by laughing gulls caused nearly complete nesting failure from 1972-75 in South Carolina. Other avian predators include the fish crow and northern harrier. During a period of population decline in Virginia (1975-90), there was an almost a ten-fold increase in the numbers of herring and great black-backed gulls, species which may usurp nesting sites or prey on gull-billed terns (Williams et al., in press). Nesting common terns have abandoned the barrier islands for a more protected, human-made island in the Chesapeake Bay (Williams et al., in press) and this may have left the remaining gull-billed terns at greater risk from predation.