New York Natural Heritage Program
Gull-billed Tern
Gelochelidon nilotica (Gmelin, 1789)
Birds

General Description [-]
The Gull-billed Tern is a medium-sized tern. Adults in breeding plumage display a stout black bill, black legs, black cap, and light grey back.

Identifying Characteristics [-]
Gull-billed Terns are stout, white, blunt-billed birds that feed in marshes and adjacent coastal uplands (Forbush 1939, Harrison 1983, Cramp 1985). They are similar in size to other medium-sized terns. Their 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 feathers between the eye and bill, around the eyes, to the nape. The rest of the upper parts, wings, and tail are pale gray. The side of the head, underparts, and wing linings are white. The long flight feathers that extend along the outer edge of the wings and that form the lower border of the folded wing (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 feathers covering the ears (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 wings 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). 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). Nests can be scraped in sandy barrier beaches and dunes above the high tide line, shell bars or banks, or saltmarsh islands (Bent 1921). The eggs are cryptically colored. The background color may vary from buff to olive, and the mottling is somewhat finer grained than the pattern of the Common Tern (Sterna hirundo) egg. The eggs also have a characteristic "frosty" appearance which also distinguishes Gull-billed Tern eggs from those of the Common Tern (Bent 1921, Harrison 1975).

Characters Most Useful for Identification [-]
The Gull-billed Tern is a medium-sized tern with a stout black bill, black cap, black legs, and a light grey back. The thick black bill distinguishes this species from other terns.

Best Life Stage for Proper Identification [-]
Adults in breeding plumage are easiest to identify but juveniles and birds in their winter plumage can also be distinguised from other tern species.

Behavior [-]
Gull-billed Terns nest in colonies usually among other terns or Black Skimmers. They are essentially monogamous and form long-term pair bonds (Moller 1981). See Sears (1981) for a description of the aerial and ground displays Gull-billed Terns enact during pair bonding, courtship, and before and after copulation. They also have an array of threat displays which may include opening their bill in a gape at the intruder, extending neck outward and abducting wings, or tossing their head back and forth. See Sears (1981), Cramp (1985) and Molina et al. (2009) for further description. They aggressively defend colonies from nest predators and may dive and threaten much larger predatory birds that approach their colonies such as egrets and herons (Molina et al. 2009). Once eggs are laid, both the male and female incubate them. One study in California documented roughly equal incubation times by the male and female (Molina 1999). Young are precocial, meaning they are fully developed and mobile at hatching. Gull-billed Tern chicks, do however, rely on their parents for food. Both parents feed and brood the young; the female feeds them more often (Molina et al. 2009, Lind 1963).

Diet [-]
The Gull-billed Tern's diet mostly consists of a variety of marine and terrestrial insects. In addition, they eat other invertebrates, small vertebrates, and the eggs and young of other birds. Major dietary items include arthropods, locusts, grasshoppers, dragonflies, insects, spiders, and marine life such as fiddler crabs, crustaceans, crabs, and sand bugs (Wilson 1840, Bent 1921, Sprunt 1954, Rohwer and Woolfenden 1968, Cramp 1985, Quinn and Wiggins 1990). Vertebrate fauna consumed include fish, frogs, toads, lizards, and small mammals (Bent 1921, Dement'ev and Gladkov 1951, Bannerman 1962). They are occasionally opportunistic predators on the downy chicks of other beach-nesting birds (Densmore 1990, Molina et al. 2009).
Gull-billed Tern Images
click to enlarge
The Best Time to See
Gull-billed Terns generally occur on Long Island from mid-May through August or into early September. Extreme dates are May 2 and September 17 (Levine 1998).
J F M A M J J A S O N D
Present Breeding
The time of year you would expect to find Gull-billed Tern present (blue shading) and breeding (orange shading) in New York.
Similar Species
  • Common Tern (Sterna hirundo)
    The adult Common Tern in breeding plumage has a mostly orange bill that is thinner than the Gull-Billed Tern's bill, orange legs and has a darker grey back.
  • Forster's Tern (Sterna forsteri)
    The adult Forster's Tern in breeding plumage has orange legs and orange at the base of the bill, a thinner bill, and a much longer, deeply forked tail.
  • Roseate Tern (Sterna dougallii)
    The adult Roseate Tern in breeding plumage has orange legs and orange at the base of the bill, a thinner bill and a much longer, deeply forked tail.