|
The swan goose (''Anser cygnoides'') is a rare large goose with a natural breeding range in inland Mongolia, northernmost China, and southeastern Russia. It is migratory and winters mainly in central and eastern China. Vagrant birds are encountered in Japan and Korea (where it used to winter in numbers when it was more common), and more rarely in Kazakhstan, Laos, coastal Siberia, Taiwan, Thailand and Uzbekistan. While uncommon in the wild, this species has been domesticated. Introduced and feral populations of its domestic breeds occur in many places outside its natural range. The wild form is also kept in collections, and escapes are not unusual amongst feral flocks of other ''Anser'' and ''Branta'' geese. ==Description== The swan goose is large and long-necked for its genus, wild birds being long (the longest ''Anser'' goose) and weighing or more (the second-heaviest ''Anser'', after the greylag goose, ''A. anser''). The sexes are similar, although the male is larger, with a proportionally longer bill and neck; in fact the largest females are barely as large as the smallest males. Typical measurements of the wing are in males, in females; the bill is about long in males and in females. The tarsus of males measures around .〔〔 The wingspan of adult geese is . The upperparts are greyish-brown, with thin light fringes to the larger feathers and a maroon hindneck and cap (reaching just below the eye). The remiges are blackish, as are the entire underwing and the white-tipped rectrices, while the upper- and undertail coverts are white. A thin white stripe surrounds the bill base. Apart from darker streaks on the belly and flanks, the underside is pale buff, being especially light on the lower head and foreneck which are sharply delimited against the maroon. In flight, the wings appear dark, with no conspicuous pattern. Uniquely among its genus, the long, heavy bill is completely black; the legs and feet, on the other hand, are orange as in most of its relatives. The eyes' irides are maroon. Juveniles are duller than adult birds, and lack the white bill base and dark streaks on the underside.〔〔 The voice is a loud drawn-out and ascending honking ''aang''. As a warning call, a similar but more barking honk is given two or three times in short succession.〔 The karyotype of the swan goose is 2n=80, consisting of four pairs of macrochromosomes, 35 pairs of microchromosomes, and a pair of sex chromosomes. The two largest macrochromosome pairs as well as the Z (female) chromosome are submetacentric, while the third-largest chromosome pair is acrocentric and the fourth-largest is metacentric. The W chromosomes are acrocentric too, as are the larger microchromosomes, the smaller ones probably being telocentric. Compared to the greylag goose, there seems to have been some rearrangement on the fourth-largest chromosome pair. 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「swan goose」の詳細全文を読む スポンサード リンク
|