UKBirds

Common Tern

Sterna hirundo

The Common Tern is a seabird of the tern family Sternidae. This bird has a circumpolar distribution, its four subspecies breeding in temperate and subarctic regions of Europe, Asia and North America. It is strongly migratory, wintering in coastal tropical and subtropical regions. Breeding adults have light grey upperparts, white to very light grey underparts, a black cap, orange-red legs, and a narrow pointed bill. Depending on the subspecies, the bill may be mostly red with a black tip or all black. There are a number of similar species, including the partly sympatric Arctic Tern, which can be separated on plumage details, leg and bill colour, or vocalisations.

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, and it readily adapts to artificial substrates such as 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. Up to three eggs may be laid, their dull colours and blotchy patterns providing camouflage on the open beach. Incubation is by both sexes, and the eggs hatch in around 21-22 days, longer if the colony is disturbed by predators. The downy chicks fledge in 22-28 days. Like most terns, this species 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.

Eggs and young are vulnerable to predation by mammals such as rats and American mink, and large birds including gulls, owls and herons. Common Terns may be infected by lice, parasitic worms, and mites, although blood parasites appear to be rare. 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. Despite international legislation protecting the Common Tern, in some areas populations are threatened by habitat loss, pollution or the disturbance of breeding colonies.

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.

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 10-30 m (30-90 ft) above the water in a following wind. Unless migrating, normally it stays below 100 m (300 ft), and averages 30 km/h (18.6 mph) in the absence of a tail wind. Its average flight speed during the nocturnal migration flight is 43-54 km/h (27-33 mph) at a height of 1,000-3,000 m (3,200-9,700 ft).

The juvenile starts moulting into adult plumage in its first October; the head, tail and body plumage is replaced first, mostly by February, then the wing feathers. The primaries are replaced in stages; the innermost feathers moult first, then replacement is suspended during the southern winter (birds of this age staying in their wintering areas) and recommences in the autumn. In May to June of the second year a similar moult sequence starts, with a pause during primary moult for birds that return north, but not for those that stay in the winter quarters. A major moult to adult breeding plumage occurs in the next February to June, between 40-90 percent of feathers being replaced. Old primary feathers wear away to reveal the blackish barbs beneath. The moult pattern means that the oldest feathers are those nearest the middle of the wing, so as the northern summer progresses, a dark wedge appears on the wing due to this feather ageing process.Terns are unusual in the frequency in which they moult their primaries, which are replaced at least twice, occasionally three times in a year. The visible difference in feather age is accentuated in the greater ultraviolet reflectance of new primaries, and the freshness of the wing feathers is used by females in mate selection. Experienced females tend to accept mates which best show their fitness through the quality of their wing feathers. Rarely, a very early moult at the nesting colony may be linked to breeding failure, both the onset of moult and reproductive behaviour being linked to falling levels of the hormone prolactin.

Most populations of the Common Tern are strongly migratory, wintering south of their temperate and subarctic Northern Hemisphere breeding ranges. First summer birds usually remain in their wintering quarters, although a few return to breeding colonies some time after the arrival of the adults. In North America, the Common Tern breeds along the Atlantic coast from Labrador to North Carolina, and inland throughout much of Canada east of the Rocky Mountains. In the United States, some breeding populations can also be found in the states bordering the Great Lakes, and locally on the Gulf coast. There are small, only partially migratory, colonies in the Caribbean; these are in The Bahamas and Cuba, and off Venezuela in the Los Roques and Las Aves archipelagos.

New World birds winter along both coasts of Central and South America, to Argentina on the east coast and to Northern Chile on the west coast. Records from South America and the Azores show that some birds may cross the Atlantic in both directions on their migration.

The Common Tern breeds across most of Europe, with the highest numbers in the north and east of the continent. There are small populations on the north African coast, and in the Azores, Canary Islands and Madeira. Most winter off western or southern Africa, birds from the south and west of Europe tending to stay north of the equator and other European birds moving further south. The breeding range continues across the temperate and taiga zones of Asia, with scattered outposts on the Persian Gulf and the coast of Iran. Small populations breed on islands off Sri Lanka, and in the Ladakh region of the Tibetan plateau. Western Asian birds winter in the northern Indian Ocean, and S. h. tibetana appears to be common off East Africa during the northern hemisphere winter. Birds from further north and east in Asia, such as S. h. longipennis, move through Japan, Thailand and the western Pacific as far as southern Australia. There are small and erratic colonies in West Africa, in Nigeria and Guinea-Bissau, unusual in that they are within what is mainly a wintering area. Only a few Common Terns have been recorded in New Zealand, and this species' status in Polynesia is unclear. A bird ringed at the nest in Sweden was found dead on Stewart Island, New Zealand, five months later, having flown an estimated 25,000 km (15,000 mi).

As long distance migrants, Common Terns sometimes occur well outside their normal range. Stray birds have been found inland in Africa (Zambia and Malawi), and on the Maldives and Comoros islands; the nominate subspecies has reached Australia, the Andes, and the interior of South America. Asian S. h. longipennis has recent records from western Europe.

The Common Tern breeds over a wider range of habitats than any of its relatives, nesting from the taiga of Asia to tropical shores, and at altitudes up to 2,000 m (6,500 ft) in Armenia, and 4,800 m (15,500 ft) in Asia. It avoids areas which are frequently exposed to excessive rain or wind, and also icy waters, so it does not breed as far north as the Arctic Tern. The Common Tern breeds close to freshwater or the sea on almost any open flat habitat, including sand or shingle beaches, firm dune areas, salt marsh, or, most commonly, islands. Flat grassland or heath, or even large flat rocks may be suitable in a island environment. In mixed colonies, Common Terns will tolerate somewhat longer ground vegetation than Arctic Terns, but avoid the even taller growth acceptable to Roseate Terns; the relevant factor here is the different leg lengths of the three species. Common Terns adapt readily to artificial floating rafts, and may even nest on flat factory roofs. Unusual nest sites include hay bales, a stump 0.6 m (2 ft) above the water, and floating logs or vegetation. There is a record of a Common Tern taking over a Spotted Sandpiper nest and laying its eggs with those of the wader. Outside the breeding season, all that is needed in terms of habitat is access to fishing areas, and somewhere to land. In addition to natural beaches and rocks, boats, buoys and piers are often used both as perches and night-time roosts.