The Swan Goose (Anser cygnoides) is a large goose with a natural breeding range in Mongolia, northernmost China, and southeastern Russia. It is migratory and winters mainly in central and eastern China. The species has been domesticated, and introduced and feral populations occur elsewhere.
It is large for its genus, 81"?94 cm long (the longest Anser goose) and weighing 2.8"?3.5 kg (the second-heaviest Anser, after the Greylag Goose). It has a long neck, long, heavy black bill, brown cap and pale underparts apart from some belly streaking; the upperparts are brown and the legs are orange. The sexes are similar, although the male is larger, and juveniles are duller. The voice is a loud honking. It grazes on sedges, and rarely swims. It forms small flocks outside the breeding season.
It is a rare goose of steppes and mountain valleys, which breeds near marshes and other wetlands, laying 5"?8 eggs in a ground nest. In the winter, it grazes on steppes and stubble, sometimes far from water.
The Swan Goose was listed as endangered in the wild, the population declining due to habitat loss and excessive hunting, but new research has shown it to be not as rare as it was believed. Consequently, it was downlisted to vulnerable status in 2008.
Though the majority of domestic geese are descended from the Greylag Goose, two breeds are direct descendants of the Swan Goose: the Chinese Goose and the African Goose. These breeds have been domesticated since at least the mid-eighteenth century, and vary considerably from their wild parent in appearance, temperament, and ability to produce meat and eggs.
Charles Darwin studied goose breeds as part of his work on the theory of evolution. He noted that the external differences between Chinese geese and breeds descended from the Greylag Goose belied a rather close relationship:
"The hybrids from the common and Chinese geese (A. cygnoides), species which are so different that they are generally ranked in distinct genera, have often bred in this country with either pure parent, and in one single instance they have bred inter se."