Description
This is a fairly large and powerful tern, similar in size and general appearance to a Sandwich Tern, but the short thick gull-like bill, broad wings, long legs and robust body are distinctive. 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 like a Forster's Tern or a Mediterranean Gull. Juvenile Gull-billed Terns have a fainter mask, but otherwise look much like winter adults.
Juvenile Sandwich Terns have a short bill, and are frequently mistaken for Gull-billed Tern where the latter species is uncommon, such as North Sea coasts.
Read more about this topic: Gull-billed Tern
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