Common Tern

The Common Tern is a seabird of the tern family. Breeding adults have pale grey upperparts, very pale grey underparts, a black cap, orange-red legs and a narrow pointed bill that can be mostly red with a black tip. The upperwings are pale grey but as the summer wears on the dark feather shafts of the outer flight feathers become exposed and a grey wedge appears on the wings. The rump and tail are white and on a standing bird the long tail extends no further than the folded wingtips unlike the Arctic and Roseate Terns in which the tail protrudes beyond the wings. There are no significant differences between the sexes. In non-breeding adults the forehead and underparts become white, the bill is all black or black with a red base and the legs are dark red or black. The upperwings have an obvious dark area at the front edge of the wing. Terns that have not bred successfully may start moulting into non-breeding adult plumage from June but late July is more typical with the moult suspended during migration. Juveniles have pale grey upperwings with a dark carpal bar. The crown and nape are brown and the forehead is ginger wearing to white by autumn. The upperparts are ginger with brown and white scaling and the tail lacks the adult's long outer feathers. Birds in their first post-juvenile plumage resemble non-breeding adults but have a duskier crown, dark carpal bar and often very worn plumage. By their second year, most young terns are either indistinguishable from adults or show only minor differences such as a darker bill or white forehead.

There are several terns of a similar size and general appearance to the Common Tern. A traditionally difficult species to separate is the Arctic Tern and, until the key characteristics were clarified, distant or flying birds of the 2 species were often jointly recorded as "commic terns". Although similar in size, the two terns differ in structure and flight. The Common Tern has a larger head, thicker neck, longer legs and more triangular and stiffer wings than the Arctic Tern and has a more powerful and direct flight. The Arctic Tern has greyer underparts than the Common Tern which make its white cheeks more obvious whereas the rump of the Common Tern can be greyish in non-breeding plumage compared to the white of the Arctic Tern. The Common Tern develops a dark wedge on the wings as the breeding season progresses but the wings of the Arctic Tern stay white throughout the northern summer. All the flight feathers of the Arctic Tern are translucent against a bright sky but only the 4 innermost wing feathers of the Common Tern share this characteristic. The trailing edge of the outer flight feathers is a thin black line in the Arctic Tern but it is thicker and less defined in the Common Tern. The bill of an adult Common Tern is orange-red with a black tip and its legs are bright red while both are a darker red colour in the Arctic Tern which also lacks the black bill tip.

The Common Tern is an agile flyer, capable of rapid turns and swoops, hovering and vertical take-off. When commuting with fish, it flies close to the surface in a strong head wind but up to 100 feet above the water in a following wind.

The Common Tern has a circumpolar distribution and breeds in temperate and sub-arctic regions of Europe, Asia and north America. It is strongly migratory and winters in coastal tropical and subtropical regions.

Breeding in a wider range of habitats than any of its relatives, the Common Tern nests on any flat, poorly vegetated surface close to water including beaches and islands. It also readily adapts to artificial floating rafts. The nest may be a bare scrape in sand or gravel but it is often lined or edged with whatever debris is available. Eggs and young are vulnerable to predation by mammals such as rats and American Mink and large birds including gulls, owls and herons.

Like most terns, the Common Tern feeds by plunge-diving for fish, either in the sea or in freshwater, but molluscs, crustaceans and other invertebrate prey may form a significant part of the diet in some areas.

Date: 7th June 2009

Location: Siikalahti near Parikkala, Etelä-Karjala, Finland