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Length 33β36 cm (13-14ΒΌ in), mass 76-116 g. Breeding adult: Black forehead, nape and crown and white cheeks; the mantle, back and upper wings are grey and the collar, rump and underwing are white. The deeply forked tail is white with grey outer webs. The long bill, legs and feet are red. Non-breeding adult and immature: Forehead white, crown streaked black and white, nape black. The bill is black and the legs and feet dark red to blackish.
Common and Arctic Terns are a species pair easily confused.
On the ground the short legs of Arctic Terns give them a somewhat huddled look whilst the tail feathers in summer birds extends well beyond the folded wings. Common Terns stand taller and the tail feathers do not extend past the primaries.
On flying birds the wing of Arctic when seen against the light is really quite translucent whereas in Common only the inner primaries are translucent. Common Terns have a black wedge or notch effect on the upper outer 5/6 primaries which is absent in Arctic and the trailing black edge of the primaries is narrower and neater in Arctic.
I find the easiest way to separate them in summer is by the fact that Arctics have a very buoyant bouncy flight and are more compact in the head and neck coupled with the long tail giving the impression that the wings are well-forward on the body whereas Common Terns seem to have their wings more to the centre of the bird. The real clincher however is the beak - Common Tern has a fairly long orange-red beak with a clear black tip whilst Arctic Terns have a shorter solid blood-red beak (and are more likely to draw blood when you invade the nesting colony!!)
Written by Steve G
On real close ups, look for the colour of the crescent under the eye; black in Arctic Tern, white in Common, creating different impressions on completeness of the mask.
In the Southern Hemisphere, Arctic Tern can be confused with the Antarctic Tern. It is similar to Arctic Tern, but has longer legs and bill and less black in it wings. Also wintering Arctic Terns lack the full black cap of breeding Antarctic Terns in the Austral summer.
Separation from the similar South American Tern even more difficult. They have an all red bill like Arctic, but their overall shape and wing tip pattern is more like Common Tern. In basic (non-breeding) plumage, South American Terns still show considerable gray color on their underparts and their bill remains red.
Breeds in arctic and sub-arctic Europe, Asia, and North America. Truly phenomenal migration, by far the longest distance for any bird, allowing it to get two summers per year. Birds breeding around the North Sea (including the Farne Islands in Northumberland, and the Netherlands) first fly to a staging area in the central North Atlantic around 1000 km NNW of the Azores, feeding there for a week or two, then down the mid Atlantic to the cool waters west of South Africa to stage there for another 2-3 weeks; after that, flying ESE to the Southern Ocean south of Australia and New Zealand before turning south to the Antarctic pack-ice off Wilkes Land, East Antarctica for their second summer of the year. They then return first west off the Antarctic coast, then north by the same route as southbound; individual birds flew up to 96,000 km per year. The speed of migration is remarkable too, with one juvenile ringed as a chick on the Farne Islands, UK reaching Melbourne, Victoria, Australia in less than 4 months from fledging. Birds breeding in Greenland travelled slightly shorter routes, to the Atlantic coast of Antarctica, not going east to Australia and "only" covering up to 70,000 km.
Accidental in continental interior locations such as Kansas.
Birdforum thread discussing separation of Common vs Arctic Terns and mentioning eye crescent in post #12
Gochfeld, M., Burger, J. & Garcia, E.F.J. (2018). Arctic Tern (Sterna paradisaea). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/54026 on 7 September 2018).