The common tern[2] (Sterna hirundo) 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.
Taxonomy
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,[3] and webbed feet.[4] 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.[3]
The common tern's closest relatives appear to be the Antarctic tern,[5] 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.[6] No fossils are known from North America, and those claimed in Europe are of uncertain age and species.[5]
The common tern was first described by Linnaeus in his Systema Naturae in 1758 under its current scientific name, Sterna hirundo.[7] The word "stearn" was used in Old English and a similar word was used by the Frisians for tern.[8] "Stearn" appears in the poem The Seafarer, written around 1000 AD.[8] 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.[9] This resemblance also leads to the informal name "sea swallow",[10] recorded from at least the seventeenth century.[9] The Scots names picktarnie,[11] tarrock[12] and their many variants are also believed to be onomatopoeic, derived from the distinctive call.[9] Due to the difficulty in distinguishing the two species, all the informal common names are shared with the Arctic tern.[13]
Four subspecies of the common tern are generally recognized, although S. h. minussensis is sometimes considered to be an intergrade between S. h. hirundo and S. h. longipennis.[14][15]
| Subspecies | Breeding range | Distinctive features |
|---|---|---|
| S. h. hirundo (Linnaeus, 1758) | Europe, North Africa, Asia east to western Siberia and Kazakhstan, and North America.[16] | Differences between the American and Eurasian populations are minimal. American birds have a slightly shorter wing length on average, and the extent of the black tip on the upper mandible tends to be less than in birds from Scandinavia and further east in Eurasia. The proportion of black on the bill is at its minimum in the west of Europe, so British breeders are very similar to American birds in this respect.[16] |
| S. h. minussensis (Sushkin, 1925) | Lake Baikal east to northern Mongolia and southern Tibet.[17] | Paler upper body and wings than S. h. longipennis, black-tipped crimson bill.[17] |
| S. h. longipennis (Nordmann, 1835) | Central Siberia to China, also Alaska.[16] | Darker grey than the nominate subspecies, with shorter black bill, darker red-brown legs, and longer wings.[16] |
| S. h. tibetana (Saunders, 1876) | Himalayas to southern Mongolia and China.[16] | Like the nominate subspecies, but bill is shorter with broader black tip.[16] |
Description
The nominate subspecies of the common tern is 31–35 cm (12–14 in) long, including a 6–9 cm (2.4–3.5 in) fork in the tail, with a 77–98 cm (30–39 in) wingspan. It weighs 110–141 g (3.9–5.0 oz).[16] 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.[18]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.[19] 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.[19] 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.[16]
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.[16]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.[20]
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 (33–98 ft) above the water in a following wind. Unless migrating, normally it stays below 100 m (330 ft), and averages 30 km/h (19 mph) in the absence of a tail wind.[5] Its average flight speed during the nocturnal migration flight is 43–54 km/h (27–34 mph)[21] at a height of 1,000–3,000 m (3,300–9,800 ft).[5]
Moult
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% of feathers being replaced.[16] 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.[18]
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.[22] Experienced females tend to accept mates which best show their fitness through the quality of their wing feathers.[23] 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.[24]
Similar species
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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.[25] The Arctic tern has greyer underparts than the common, 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 its relative. The common tern develops a dark wedge on the wings as the breeding season progresses, but the wings of 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.[25][26] 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.[19] 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.[25]
In the breeding areas, the roseate tern can be distinguished by its pale plumage, long, mainly black bill and very long tail feathers.[26] 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.[27] In flight, the roseate's heavier head and neck, long bill and faster, stiffer wingbeats are also characteristic.[28] It feeds further out to sea than the common tern.[27] 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.[29]
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. Identification may be aided by the plumage differences due to "opposite" breeding seasons. 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.[30][31] 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.[32] 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.[33] 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.[34] The white-cheeked tern is smaller, has uniform grey upperparts, and in breeding pumage is darker above with whiter cheeks.[35]
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".[19] 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.[16]
Voice
The common tern has 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.[36] A down-slurred keeur 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.[36]
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.[37][38]
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.[20] 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.[39] There are small, only partially migratory, colonies in the Caribbean; these are in The Bahamas and Cuba,[40] and off Venezuela in the Los Roques and Las Aves archipelagos.[41]
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.[20][39] Records from South America and the Azores show that some birds may cross the Atlantic in both directions on their migration.[42][43]
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.[44] The breeding range continues across the temperate and taiga zones of Asia, with scattered outposts on the Persian Gulf and the coast of Iran.[45] Small populations breed on islands off Sri Lanka,[46][47] and in the Ladakh region of the Tibetan plateau.[48] Western Asian birds winter in the northern Indian Ocean,[20][49] and S. h. tibetana appears to be common off East Africa during the northern hemisphere winter.[50] 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.[20] 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.[45] Only a few common terns have been recorded in New Zealand,[51] and this species' status in Polynesia is unclear.[52] 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).[53]
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;[54] the nominate subspecies has reached Australia,[34] the Andes, and the interior of South America.[32][55] Asian S. h. longipennis has recent records from western Europe.[56]
The common tern breeds over a wider range of habitats than any of its relatives, nesting from the taiga of Asia to tropical shores,[57] and at altitudes up to 2,000 m (6,600 ft) in Armenia, and 4,800 m (15,700 ft) in Asia.[44] 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.[57] 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.[58] Common terns adapt readily to artificial floating rafts, and may even nest on flat factory roofs.[57] 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.[59] 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
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