Sterna hirundo

The common tern (''Sterna hirundo'') is a seabird in the family Laridae. 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 several 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.

Taxonomy

Terns 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.Wassink & Ort (1995) p. 78. 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.Snow & Perrin (1998) p. 764.

The common tern's closest relatives appear to be the Antarctic tern, followed by the Eurasian Arctic and roseate terns. Genetic evidence suggests that the common tern may have diverged from an ancestral stock earlier than its relatives. No fossils are known from North America, and those claimed in Europe are of uncertain age and species.

The common tern was first described by Carl Linnaeus in his landmark 1758 10th edition of ''Systema Naturae'' under its current scientific name, ''Sterna hirundo''. "Stearn" was used in Old English, and a similar word was used by the Frisians for the birds. Library subscription required. "Stearn" appears in the poem '' The Seafarer'', written around 1000 A.D. Linnaeus adopted this word for the genus name ''Sterna''. The Latin for swallow is ''hirundo,'' and refers here to the tern's superficial likeness to that unrelated bird, which has a similar light build and long forked tail.Hume (1993) pp. 12–13. This resemblance also leads to the informal name "sea swallow", recorded from at least the seventeenth century. The Scots names ''picktarnie'', ''tarrock'' and their many variants are also believed to be onomatopoeic, derived from the distinctive call. Because of the difficulty in distinguishing the two species, all the informal common names are shared with the Arctic tern.Cocker & Mabey (2005) pp. 246–247. There was some uncertainty whether ''Sterna hirundo'' should apply to the common tern or the arctic tern as the species are very similar and both occur in Sweden. In 1913, the Swedish zoologist Einar Lönnberg concluded that the binomial name ''Sterna hirundo'' applied to the common tern.

Four subspecies of the common tern are generally recognized, although ''S.h.minussensis'' is sometimes considered an intergrade between ''S.h.hirundo'' and ''S.h.longipennis''.Hume (1993) pp. 88–89.

Description

The nominate subspecies of the common tern is long, including a fork in the tail, with a wingspan. It weighs . 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.Hume (1993) pp. 21–29. The common tern's upper wings 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 upper wings have an obvious dark area at the front edge of the wing, the carpal bar. Terns that have not bred successfully may moult into non-breeding adult plumage beginning in June, though late July is more typical, with the moult suspended during migration. There is also some geographical variation; Californian birds are often in non-breeding plumage during migration.

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.

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 above the water in a following wind. Unless migrating, normally it stays below , and averages in the absence of a tail wind. Its average flight speed during the nocturnal migration flight is at a height of .

Moult

Juveniles moult into adult plumage in its first October; first the head, tail, and body plumage is replaced, 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 forty and ninety per cent 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 because of 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 favour mates which best show their fitness through the quality of their wing feathers. Rarely, a very early moult at the nesting colony is linked to breeding failure, both the onset of moult and reproductive behaviour being linked to falling levels of the hormone prolactin.

Similar species

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 two 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 its relative, and has a more powerful, direct flight. Arctic terns have greyer underparts than the common variety, which makes its white cheeks more obvious, whereas the rump of the common tern can be greyish in non-breeding plumage, compared to the white of its relative. The common tern develops a dark wedge on the wings as the breeding season progresses, but the wings of the Arctic stay white throughout the northern summer. All the flight feathers of the Arctic tern are translucent against a bright sky, only the four innermost wing feathers of the common tern share this property. The trailing edge of the outer flight feathers is a thin black line in the Arctic tern, but thicker and less defined in the common. The bill of an adult common tern is orange-red with a black tip, except in black-billed ''S.h.longipennis'', and its legs are bright red, while both features are a darker red colour in the Arctic tern, which also lacks the black bill tip.

In the breeding areas, the roseate tern can be distinguished by its pale plumage, long, mainly black bill and very long tail feathers.van Duivendijk (2011) pp. 200–202. The non-breeding plumage of roseate is pale above and white, sometimes pink-tinged, below. It retains the long tail streamers, and has a black bill.Olsen & Larsson (1995) pp. 69–76. In flight, the roseate's heavier head and neck, long bill and faster, stiffer wingbeats are also characteristic.Blomdahl et al. (2007) p. 340. It feeds further out to sea than the common tern. In North America, the Forster's tern in breeding plumage is obviously larger than the common, with relatively short wings, a heavy head and thick bill, and long, strong legs; in all non-breeding plumages, its white head and dark eye patch make the American species unmistakable.Olsen & Larrson (1995) pp. 103–110.

In the wintering regions, there are also confusion species, including the Antarctic tern of the southern oceans, the South American tern, the Australasian white-fronted tern and the white-cheeked tern of the Indian Ocean. The plumage differences due to "opposite" breeding seasons may aid in identification. The Antarctic tern is more sturdy than the common, with a heavier bill. In breeding condition, its dusky underparts and full black cap outline a white cheek stripe. In non-breeding plumages, it lacks, or has only an indistinct, carpal bar, and young birds show dark bars on the tertials, obvious on the closed wing and in flight.Enticott & Tipling (2002) p. 196.Sinclair et al. (2002) p. 212. The South American tern is larger than the common, with a larger, more curved red bill, and has a smoother, more extensive black cap in non-breeding plumage.Schulenberg et al. (2010) p. 154. Like Antarctic, it lacks a strong carpal bar in non-breeding plumages, and it also shares the distinctive barring of the tertials in young birds.Enticott & Tipling (2002) p. 192. The white-fronted tern has a white forehead in breeding plumage, a heavier bill, and in non-breeding plumage is paler below than the common, with white underwings. The white-cheeked tern is smaller, has uniform grey upperparts, and in breeding plumage is darker above with whiter cheeks.Grimmett et al. (1999) pp. 140–141.

Juvenile common terns are easily separated from similar-aged birds of related species. They show extensive ginger colouration to the back, and have a pale base to the bill. Young Arctic terns have a grey back and black bill, and juvenile roseate terns have a distinctive scalloped "saddle".Vinicombe et al. (1990) pp. 133–138. Hybrids between common and roseate terns have been recorded, particularly from the US, and the intermediate plumage and calls shown by these birds is a potential identification pitfall. Such birds may have more extensive black on the bill, but confirmation of mixed breeding may depend on the exact details of individual flight feathers.

Voice

Common terns have a wide repertoire of calls, which have a lower pitch than the equivalent calls of Arctic terns. The most distinctive sound is the alarm ''KEE-yah'', stressed on the first syllable, in contrast to the second-syllable stress of the Arctic tern. The alarm call doubles up as a warning to intruders, although serious threats evoke a ''kyar'', given as a tern takes flight, and quietens the usually noisy colony while its residents assess the danger. A down-slurred is given when an adult is approaching the nest while carrying a fish, and is possibly used for individual recognition (chicks emerge from hiding when they hear their parents giving this call). Another common call is a ''kip'' uttered during social contact. Other vocalizations include a ''kakakakaka'' when attacking intruders, and a staccato ''kek-kek-kek'' from fighting males.Hume (1993) pp. 68–75.

Parents and chicks can locate one another by call, and siblings also recognise each other's vocalisations from about the twelfth day from hatching, which helps to keep the brood together.

Distribution and habitat

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,Raffaele et al. (2003) p. 292. and off Venezuela in the Los Roques and Las Aves archipelagos.Hilty (2002) p. 310.

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.Harrison (1998) pp. 370–374.Cuthbert (2003) p. 4. Records from South America and the Azores show that some birds may cross the Atlantic in both directions on their migration.Lima (2006) p. 132.

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.Snow & Perrin (1998) pp. 779–782. The breeding range continues across the temperate and taiga zones of Asia, with scattered outposts on the Persian Gulf and the coast of Iran.Hume (1993) pp. 39–41. Small populations breed on islands off Sri Lanka, and in the Ladakh region of the Tibetan plateau.Rasmussen & Anderton (2005) pp. 194–195. 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.Zimmerman et al. (2010) p. 354. 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,Robertson & Heather (2005) p. 126. and this species' status in Polynesia is unclear.Watling (2003) pp. 204–205. A bird ringed at the nest in Sweden was found dead on Stewart Island, New Zealand, five months later, having flown an estimated 25,000km (15,000mi).Newton (2010) pp. 150–151.

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,Simpson & Day (2010) p. 110. 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,Hume (1993) pp. 30–37. and at altitudes up to in Armenia, and 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 an 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 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 as night-time roosts.

Behaviour

Territory

The common tern breeds in colonies which do not normally exceed two thousand pairs, but may occasionally number more than twenty thousand pairs.de Wolf, P. "BioIndicators and the Quality of the Wadden Sea" in Best & Haeck (1984) p. 362. Colonies inland tend to be smaller than on the coast. Common terns often nest alongside other coastal species, such as Arctic, roseate and Sandwich terns, black-headed gulls, and black skimmers. Especially in the early part of the breeding season, for no known reason, most or all of the terns will fly in silence low and fast out to sea. This phenomenon is called a "dread".

On their return to the breeding sites, the terns may loiter for a few days before settling into a territory, and the actual start of nesting may be linked to a high availability of fish. Terns defend only a small area, with distances between nests sometimes being as little as , although is more typical. As with many birds, the same site is re-used year after year, with a record of one pair returning for 17 successive breeding seasons. Around ninety per cent of experienced birds reuse their former territory, so young birds must nest on the periphery, find a bereaved mate, or move to another colony.Hume (1993) pp. 86–90. A male selects a nesting territory a few days after his arrival in the spring, and is joined by his previous partner unless she is more than five days late, in which case the pair may separate.

Inbreeding among close ''S. hirundo'' relatives appears to be avoided passively by immigration and dispersal rather than by kin discrimination and mate choice.

The defence of the territory is mainly by the male, who repels intruders of either sex. He gives an alarm call, opens his wings, raises his tail and bows his head to show the black cap. If the intruder persists, the male stops calling and fights by bill grappling until the intruder submits by raising its head to expose the throat. Aerial trespassers are simply attacked, sometimes following a joint upward spiralling flight. Despite the aggression shown to adults, wandering chicks are usually tolerated, whereas in a gull colony they would be attacked and killed. The nest is defended until the chicks have fledged, and all the adults in the colony will collectively repel potential predators.

Breeding

Pairs are established or confirmed through aerial courtship displays in which a male and a female fly in wide circles up to or more, calling all the while, before the two birds descend together in zigzag glides. If the male is carrying a fish, he may attract the attention of other males too. On the ground, the male courts the female by circling her with his tail and neck raised, head pointing down, and wings partially open. If she responds, they may both adopt a posture with the head pointed skywards. The male may tease a female with the fish, not parting with his offering until she has displayed to him sufficiently.Hume (1993) pp. 91–99. Once courtship is complete, the male makes a shallow depression in the sand, and the female scratches in the same place. Several trials may take place until the pair settle on a site for the actual nest. The eggs may be laid on bare sand, gravel or soil, but a lining of debris or vegetation is often added if available, or the nest may be rimmed with seaweed, stones or shells. The saucer-shaped scrape is typically deep and across, but may extend to as much as wide including the surrounding decorative material.Hume (1993) pp. 100–111. Breeding success in areas prone to flooding has been enhanced by the provision of artificial mats made from eelgrass