NATURAL HISTORY

WESTERN GULL-BILLED TERN } Gelochelidon nilotica vanrossemi
FAMILY: Laridae

DESCRIPTION: The gull-billed tern is a medium-sized, heavy-billed, black-capped seabird about 33 to 38 centimeters long, with wide wings that are pale gray above. It appears stocky and its body is very white with a short, slightly forked tail. While standing, its relatively long, black legs are noticeable, and its stout, all-black bill is distinctive. Depending on plumage and age, coloration and markings may vary.

HABITAT: Gull-billed terns on the coast of North America generally nest on barrier islands or islands of dredged material, on levees or constructed islets, in open marshes, on natural islets in shallow tidal and brackish lagoons, and in river deltas. They nest on a variety of substrates from gravel to clay soil, usually in sites with little or no vegetation.
In western Mexico, they nest on mud flats with salt marsh vegetation or on low islands with cactus or mangrove. At inland sites, terns nest in freshwater and saline lakes on constructed and natural islands, impoundments, and abandoned oil and gas causeways. Colony sites are generally located near optimal foraging habitats, including shallow edges of marshes, bays, rivers, and exposed mudflats, the tidal edges of sandy beaches, agricultural fields and drains, impoundments, sandy lake shores, and open shrublands.

RANGE: The western gull-billed tern occurs in Southern California and western Mexico. In the United States, it breeds at San Diego Bay and the Salton Sea; in western Mexico, it breeds in the Colorado River delta in northeastern Baja California and in coastal Sinaloa, Nayarit, and Colima. In Baja California on the Pacific coast, it has infrequently bred at the Guerrero Negro saltworks in extreme northern Baja California Sur.

MIGRATION: This tern generally migrates from California to wintering habitats along the eastern Gulf of California from the Colorado River delta south to Sinaloa, and southward along the Pacific Coast of Mexico to Nayarit, Colima, and Oaxaca. A few midwinter records exist for bird in the Salton Sea, the Mexicali Valley, and southern Baja California Sur in Ensenada de La Paz.

BREEDING: Gull-billed terns nest in colonies. They arrive in California in March and April and generally begin egg-laying by mid-April to early May. At the Salton Sea, nesting may occur as late as mid-July through early August and likely involves re-nesting attempts. Both the male and female construct and maintain the nest and are active in incubation, which takes from 21 to 23 days from the laying of the first egg. The earliest hatching for western gull-billed terns occurs in early May. Soon after hatching, chicks may move away from the nest based on the presence of vegetation and disturbance levels around the nest site. Chicks younger than five days often leave the nest but remain nearby and are fed by the parents.

LIFE CYCLE: Young terns may remain dependent on parents into migration, as juveniles may not reach full feeding efficiency for some months after fledging. Little is known about gull-billed terns’ longevity. Records exist for banded birds being re-detected at six years and 10 years of age, with an exceptional longevity record in Europe of 15 years.

FEEDING: Gull-billed terns are opportunistic foragers that feed on a wide variety of prey including insects, crabs, lizards, fish, small chicks, and other prey taken from the ground, air, and/or vegetation.

THREATS: This tern is threatened by habitat destruction and modification, predation, human disturbance, a population control project at one of only two U.S. breeding sites, military take, take for aquaculture, competition with other species, poor water quality and contaminants, flooding, global climate change, and poor ocean conditions.

POPULATION TREND: Rangewide, there are only 495 to 1042 breeding pairs of this subspecies, only 154 to 263 of which breed in the United States. This bird has only two U.S. breeding sites and 12 breeding sites in Mexico.

Western gull-billed tern photo © Kimball L. Garrett