| Sterna paradisaea
All Terns are considerable migrants and the Arctic Tern is the greatest migrant on Earth (by distance, anyway). It breeds as far north as there is land in the Arctic and travels as far south as there is open water in the Antarctic.
Identification
Terns as a family are related to Gulls, but have a generally sharper, lighter shape. They usually have a long, deeply forked tail and a long, pointed bill. In flight, they look lighter than gulls, with the wings more bent, more at ease but less robust. Terns are like gulls which have died and gone to heaven.
The Arctic Tern is a member of the Sterninae, the pelagic terns. It is a fairly average size member of the family with a wing span of 66-77 cm and is 33-39 cm in length, including tail streamers of 7-12cm. Sexes are similar. In breeding plumage, it's bill is dark red, it has a full black cap and the outer edges of the primaries are dark with the primaries as a whole showing a translucency when seen against the sky. The tail streamers are also refreshed as the bird comes into breeding plumage. These are often worn or broken in the course of the year and so are often not as distinct in no-breeding plumage. Additionally, the bill changes to black and the cap recedes leaving the forehead pale.
The most likely confusion species is the Common Tern. In breeding plumage, the Common Tern usually has a black tip to the bill and lacks the long tail streamers. The outer primaries of the common tern are greyer and less translucent. There isn't much in life better than lying on your back in the sand looking up at terns in a blue sky to determine the translucency of the wing!. It is easy to tell Arctic Tern from the rarer Roseate Tern in breeding plumage as the latter has a pinkish breast.
Outside of breeding plumage, these three birds do have greater similarity. The Roseate Tern can be separated as the outer primaries darken, producing a visible wedge in the upper wing. Separating Common from Arctic is more difficult, leading to the usage of 'Commic' Tern for an incompletely identified individual. On close view, it may be possible to decide as the Common Tern is generally greyer, having darker secondaries and a dark carpal bar.
In the Antarctic, the primary confusion species is, of course, the Antarctic Tern. The easist distinction is that terns in breeding plumage will not be Arctic Terns. Where Antarctic Terns aren't breeding, they are, nevertheless, larger than Arctic Terns with a heavier, brighter bill and, overall, a heavier jizz.
Lifestyle
Arctic Terns breed at high latitudes across the northern hemisphere (from about 50 degrees to above 80 degrees) in waterside locations (generally on coasts and inshore islands but also occasionally beside rivers) and often colonially, including mixed colonies with other terns. Outside the breeding season, the Arctic Tern is a marine species, though apparently happy to migrate across inland areas. Diet is fish, crustaceans and insects. Fishing terns are a pleasure to watch as they float in the air and hover over the water before diving upon their prey. Their calls are high and repeated, particularly when courting. As with other terns, this often involves passing fish between mates. Both share nesting duties, with one to three eggs in a clutch. Incubation is 20-24 days with fledging after an equivalent period.
Numeric Data derived from BWP
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