“The Common Tern (Sterna hirundo) is a seabird of the tern family Sternidae. This bird has a circumpolar distribution breeding in temperate and sub-Arctic regions of Europe, Asia and east and central North America. It is strongly migratory, wintering in coastal tropical and subtropical regions. It is sometimes known as the sea swallow.
This medium-sized tern is 34–37 cm long with a 70–80 cm wingspan. It is most readily confused within its range with the similar Arctic Tern (Sterna paradisaea), Roseate Tern (S. dougalli), Antarctic Tern (S. vittata) and South American Tern (S. hirundinacea).
Its thin, sharp bill is red with a dark tip. Its longish legs are also red. Its upperwings show a dark primary wedge, unlike the Arctic Tern, in which they are uniformly grey. Its long tail extends only to the wingtips on the standing bird, unlike Arctic and Roseate Terns, which extend past the wingtips. It is not as pale as the Roseate Tern and has longer wings" Wikipedia