Breeding adults have a white head, rump, tail and underparts and a pale gray back and upper wings. The wingtips are black with white spots known as "mirrors" and the rear edge of the wing is white. The underwing is grayish with dark tips to the outer primary feathers. The legs and feet are normally pink but can have a bluish tinge, or occasionally be yellow. The bill is yellow with a red spot on the lower mandible. The eye is bright, pale to medium yellow, with a bare yellow or orange ring around it. In winter, the head and neck are streaked with brown.
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It has no song but has a variety of cries and calls. The "long call" is a series of notes during which the head is dipped then raised. The "choking call" is produced during courtship displays or territorial disputes. Juvenile birds emit high-pitched plaintive cries to elicit feeding behavior from a parent, and may also emit a clicking distress call when a parent suddenly flies off.
The breeding range extends across the northern part of North America from central and southern Alaska to the Great Lakes, and north-east coast of the USA south to North Carolina. It breeds over most of Canada apart from the southwest and Arctic regions.
Birds are present all year in southern Alaska, the Great Lakes and north-east USA but most birds winter to the south of the breeding range as far as Mexico with small numbers reaching Hawaii, Central America and the West Indies. Vagrants have reached Columbia and Venezuela and there is a report from Ecuador and another from Peru. The first European record was of a bird ringed in New Brunswick which was caught on a boat in Spanish waters in 1937 and there are have a number of additional records from Western Europe since 1990. The first British record was in 1994 in Cheshire.
It has a varied diet including marine Invertebrates such as Mussels, crabs, sea urchins, and squid, fish such as capelin, alewife and smelts, insects and other birds including their chicks and eggs. It often feeds on carrion and human refuse. Food is plucked from the surface of the shore or sea or is caught by dipping underwater or by shallow plunge-diving.