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The common tern (''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,〔 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 Linnaeus in his ''Systema Naturae'' in 1758 under its current scientific name, ''Sterna hirundo''. The word "stearn" was used in Old English and a similar word was used by the Frisians for tern.〔 Library subscription required.〕 "Stearn" appears in the poem ''The Seafarer'', written around 1000 AD.〔 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",〔(【引用サイトリンク】work=Birdguide ) Retrieved 25 January 2012.〕 recorded from at least the seventeenth century.〔 The Scots names ''picktarnie'',〔(SND: Pictarnie )〕 ''tarrock''〔(SND: tarrock )〕 and their many variants are also believed to be onomatopoeic, derived from the distinctive call.〔 Due to the difficulty in distinguishing the two species, all the informal common names are shared with the Arctic tern.〔Cocker & Mabey (2005) pp. 246–247.〕 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''.〔Hume (1993) pp. 88–89.〕〔(【引用サイトリンク】url=http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=176888 ) Retrieved 23 January 2012.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Common tern」の詳細全文を読む スポンサード リンク
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