The terns, family Sternidae, are small to medium-sized seabirds closely related to the gulls, skimmers and skuas. They are gull-like in appearance, but typically have a lighter build, long pointed wings (which give them a fast, buoyant flight), a deeply forked tail, slender legs, and webbed feet. Most species are grey above and white below, and have a black cap which is reduced or flecked with white in the non-breeding season.
The nominate subspecies of the Common Tern is 31–35 cm (12.2–13.8 in) long, including a 6–9 cm (2.4–3.5 in) fork in the tail, with a 77–98 cm (31–39 in) wingspan. It weighs 110–141 g (3.9–5.0 oz).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, or all black, depending on the subspecies.The Common Tern’s 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, the carpal bar. 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. There is also some geographical variation, Californian birds often being in non-breeding plumage during migration.
Juvenile Common Terns 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, which normally remain in their wintering areas, resemble the non-breeding adult, 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.