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Bryan Brillhart Photography Present The “Dig IB” Bird Of The Week, The Gull-Billed Tern

Oct 15, 2020 10:48AM ● By Paul Spear

An Imperial Beach Hidden Gem and Bird of the Week, Bryan Brillhart Photography present the “Dig IB” Bird of the Week, the Gull-Billed Tern 

Gull-billed Tern (Gelochelidon nilotica)

This week’s “Bird of the Week” is the Gull-billed Tern (Gelochelidon nilotica), a seabird of the 

 tern family, Sternidae. They are a fairly large tern with a short thick gull-like bill, broad wings, long legs, and a robust body. The summer adult has grey upperparts, white underparts, a black cap, strong black bill and black legs. The call is a characteristic ker-wik. It is 33–42 cm (13–17 in) in length and 76–91 cm (30–36 in) in wingspan. Body mass ranges from 150–292 g (5.3–10.3 oz). In winter, the cap is lost, and there is a dark patch through the eye. Besides the thick bill that gives it its name, this tern has a relatively stocky build and broad wings. They are typically seen in leisurely flight over marshes, hawking for insects in the air or swooping down to take prey from the water or the ground, but unlike typical terns, they rarely dive into water for fish. On the ground, they walk better than most terns. They are widespread in warmer parts of the world, but local in North America, mainly in southeast. Generally found only in small numbers

 Gull-billed terns are far less numerous on the Atlantic Coast today than they were historically. Human disturbance and loss of nesting sites are among likely causes. They have begun nesting on rooftops in some Gulf Coast areas, and have colonized in southern California, apparently from western Mexico, beginning to nest at Salton Sea in 1920s and at San Diego in 1980s.

Gull-billed terns habitat includes salt marshes, fields, and coastal bays. They are restricted to seacoasts in North America (except in Florida and at Salton Sea, California), but does most of its foraging over marshes, pastures, farmland, and other open country just inland from the coast. Nests are mostly on beaches, and islands. Reportedly, they used to nest more often in salt marshes, but they have abandoned those sites because of human persecution.

Gull-billed terns forage by flying slowly into the wind, dipping to the surface of land or water to 

 pick up items, or by catching flying insects in the air. Sometimes they forage while walking on the ground, but rarely plunges into water. Their diet is mostly insects, feeding on a wide variety of insects, caught on the ground, in the air, or at the surface of water. They also eat spiders, crabs, shrimp, mollusks, earthworms, marine worms, small fish, lizards, frogs, toads, rodents, and small birds. They may be a threat to young chicks.

After mating, females lay 2-3, sometimes 1-4, pale buff, spotted with dark brown eggs. Incubation is by both parents (although females may do more), in about 22-23 days. Gull-billed terns are colonial breeders. Colonies are usually small, not as densely packed as those of many terns. Breeding behavior includes some aerial displays, but much of courtship display takes place on ground, involving elaborate posturing, bill-pointing, and males feeding females. Nest site is on open ground, and sometimes on gravel roofs. Nest (built by both sexes) is a shallow depression, often with a rim of soil, and the addition of some plant material and debris.

Young leave the nest a few days after hatching, moving to dense plant cover if nearby. Both parents bring food for young. Age at first flight is about 4-5 weeks. Young may remain with their parents 3 months or more, beginning their southward migration with them.

I have seen increased numbers of this aggressive tern this year and I am concerned for the young Snowy Plovers, and Least Terns nesting in the estuary since Gull-billed Terns have been known to dive for these young vulnerable chicks as part of their diet. Look for this species diving for food in our own Tijuana Estuary. Until next week, good birding!


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Bryan Brillhart Photography

Bryan Brillhart, Bryan Brillhart Photography


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