|
The Arctic tern (''Sterna paradisaea'') is a seabird of the tern family, Sternidae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe, Asia, and North America (as far south as Brittany and Massachusetts). The species is strongly migratory, seeing two summers each year as it migrates along a convoluted route from its northern breeding grounds to the Antarctic coast for the southern summer and back again about six months later. Recent studies have shown average annual roundtrip lengths of about for birds nesting in Iceland and Greenland〔 and c. for birds nesting in the Netherlands.〔 These are by far the longest migrations known in the animal kingdom. The Arctic tern flies as well as glides through the air, performing almost all of its tasks in the air. It nests once every one to three years (depending on its mating cycle); once it has finished nesting it takes to the sky for another long southern migration. Arctic terns are medium-sized birds. They have a length of and a wingspan of .〔 They are mainly grey and white plumaged, with a red beak (as long as the head, straight, with pronounced gonys) and feet, white forehead, a black nape and crown (streaked white), and white cheeks. The grey mantle is 305 mm, and the scapulae are fringed brown, some tipped white. The upper wing is grey with a white leading edge, and the collar is completely white, as is the rump. The deeply forked tail is whitish, with grey outer webs. The hindcrown to the ear-coverts is black. Arctic terns are long-lived birds, with many reaching thirty years of age. They eat mainly fish and small marine invertebrates. The species is abundant, with an estimated one million individuals. While the trend in the number of individuals in the species as a whole is not known, exploitation in the past has reduced this bird's numbers in the southern reaches of its range. The Arctic tern was known as sea swallow describing their slender shape as they swoop over the water. == Distribution and migration == The Arctic tern has a continuous worldwide circumpolar breeding distribution; there are no recognized subspecies. It can be found in coastal regions in cooler temperate parts of North America and Eurasia during the northern summer. While wintering during the southern summer, it can be found at sea, reaching the northern edge of the Antarctic ice.〔 The Arctic tern is famous for its migration; it flies from its Arctic breeding grounds to the Antarctic and back again each year, the shortest distance between these areas being . The long journey ensures that this bird sees two summers per year and more daylight than any other creature on the planet.〔 One example of this bird's remarkable long-distance flying abilities involves an Arctic tern ringed as an unfledged chick on the Farne Islands, Northumberland, UK, in the northern summer of 1982, which in October 1982, just three months from fledging, reached Melbourne, Australia. Assuming a direct route of flight, the distance covered would have been more than .〔 Another example is that of a chick ringed in Labrador, Canada, on 23 July 1928. It was found in South Africa four months later.〔 A 2010 study using tracking devices attached to the birds showed that the above examples are not unusual for the species. In fact, it turned out, previous research had seriously underestimated the annual distances travelled by the Arctic tern. Eleven birds that bred in Greenland or Iceland covered on average in a year, with a maximum of . The difference from previous estimates is due to the birds' taking meandering courses rather than following a straight route as was previously assumed. The birds follow a somewhat convoluted course in order to take advantage of prevailing winds.〔 The average Arctic tern lives about thirty years, and will, based on the above research, travel some 2.4 million km (1.5 million mi) during its lifetime. A 2013 tracking study of half a dozen Arctic terns breeding in the Netherlands〔 shows average annual migrations of c. . On their way south, these birds roughly followed the coastlines of Europe and Africa.〔 Having rounded the southern tip of Africa, they then turned east, some flying approximately halfway to Australia before again turning south to eventually reach Wilkes Land in the north-eastern Antarctic. One bird flew several hundred kilometres along the south coast of Australia before turning south for the Antarctic, while one flew along the entire south coast of Australia, passing between Australia and Tasmania. Having reached the Melbourne area, it turned south and flew in an arc to Wilkes Land in the north-east Antarctic, passing the south-western tip of New Zealand's South Island en route. Once back in the Netherlands, this bird had journeyed c. , the longest migration yet recorded for any animal. Arctic terns usually migrate sufficiently far offshore that they are rarely seen from land outside the breeding season.〔 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「Arctic tern」の詳細全文を読む スポンサード リンク
|