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.

The swan goose was formally described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae under the binomial name Anas cygnoid.

[9] The upperparts are greyish-brown, with thin light fringes to the larger feathers and a maroon hindneck and cap (reaching just below the eye).

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.

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 clutch is usually 5–6 but sometimes up to 8 eggs, which are laid in a shallow nest made from plants, placed directly on the ground, often on a small knoll to keep it dry.

Around late August/early September, the birds leave for winter quarters, where they gather in small groups to moult their worn plumage.

[2][3] The swan goose was uplisted from Near Threatened to Vulnerable on the IUCN Red List in 1992 and further to Endangered in 2000, as its population is declining due to habitat loss and excessive hunting and (particularly on the Sanjiang Plain in China) egg collecting.

They vary considerably from their wild parent in appearance, temperament, and ability to produce meat and eggs; the most conspicuous feature is the prominent bill knob[3][12][13] and upright posture.

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,[14] have often bred in this country with either pure parent, and in one single instance they have bred inter se.