The Common tern (Sterna hirundo) is a seabird that has a circumpolar distribution. it has four subspecies breeding in temperate and subarctic regions of Europe, Asia, and North America. Its large population and huge breeding range mean that this species is classed as being of least concern, although numbers in North America have declined sharply in recent decades.
Known for its attractive plumage and graceful flight, Common terns have a slender body and a smoothly rounded head, and long pointed wings. Their breeding plumage is light silvery-gray upperparts and clear black outer primaries on its wingtips. Outside the breeding season, the birds keep some of their distinguishable black cap, but their forehead and face turn white. Their legs and bill turn black, losing their orange-red coloration. Male and female are similar in appearance throughout the year. Juvenile Common terns have pale grey upper wings with a dark carpal bar. The crown and nape are brown, and the forehead is ginger, wearing to white by autumn. The upper parts 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.
Common terns occur from northern Canada to the Caribbean Sea in the south, throughout Europe, Northern Asia, the Middle East and North Africa. Outside of the mating season, these birds winter along the coasts of South America and Central America, along Africa’s coast and the Arabian Peninsula, as well as on islands in the Indian Ocean and throughout Australasia and much of Southeast Asia. They breed on sand spits, beaches and low-lying inshore islands. They also nest inland near slow-flowing rivers, and lakes in open country. This species favors areas that have close shallow waters where they can fish. During migration, they are mainly seen along coasts, at freshwater inland lakes and in estuaries. During winter, they are mostly coastal, in warm tropical and subtropical waters.
Common terns are diurnal birds and they live in colonies with no clear hierarchy among them, with all seeming to be equal. Although Common terns all migrate and live together, the family unit is responsible to feed and care for its own eggs and chicks, and individuals will often defend feeding territories. This species forages by flying above water and hovering, often plunge-diving to catch prey under the surface. It will also swim as it picks up food from the surface of the water or just below it. A highly migratory bird, it leaves the breeding grounds for the wintering sites typically from August to October, soon after all the chicks fledge. These birds communicate mostly by means of their unusual, hoarse voices, having three different and distinct calls. Communication during mating is mainly visual and/or tactile.
Common terns are monogamous, which means they mate with only one partner during a breeding season. At the time of courtship, which starts in April, males establish their territories within the colony before commencing what is known as "courtship feeding," where a male brings a fish to a female to court her. Courtship displays include the male posturing followed by the pair circling each other. Eggs are laid from April to June. Common terns usually breed in huge colonies, though there may be some isolated pairs. Both parents construct their nest on bare ground, sometimes with low vegetation surrounding it or on dead floating vegetation. 2-3 eggs are laid and are incubated by both parents for 21-25 days. Chicks leave their nest after several days, but remain nearby and their parents still feed them. They fledge at around 22-28 days old, remaining in the family group for at least another two months. Common terns become reproductively mature at 3 years of age.
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