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Length 48â€“56 cm (19-22 in), wingspan 127-140 cm, weight 570-780 g. The largest tern, 50% heavier than the next largest (Royal Tern and Great Crested Tern).
Widespread but patchy breeding distribution in temperate and subtropical regions of Europe (north to 66Â°N in the Baltic Sea), Asia, Africa, North America (north to 62Â°N in the Great Slave Lake in Northwest Territories), Australia and New Zealand.
Migratory in the temperate Northern Hemisphere, wintering south to the tropics (including the West Indies and northern South America, where it does not breed); resident or dispersive in the subtropics and the temperate Southern Hemisphere.
There are records most years for the British Isles.
This is a monotypic species, in spite of its near-cosmopolitan range. Some authorities formerly recognised two or three subspecies, with nominate H. c. caspia in the Old World, H. c. imperator in the New World, and H. c. strenua in Australasia.
It was in the past (and is still by some authorities) included in the genus Sterna.
Large lakes (fresh or salt water) and shallow, sheltered sea coasts; avoids exposed and deep-water ocean coasts. Breeds on sandy coasts and islands.
The diet includes fish, insects, eggs and young birds.
They are ground nesters, colonially and singly; 1-3 pale blue green eggs, heavily spotted brown, are laid in a bare scrape.
The call is a loud croak.
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