Sabine's gull ( or ) (Xema sabini) is a small gull. It is usually treated as the only species placed in the genus Xema, though some authors include it with other gulls in a wide view of the genus Larus. It has also been known historically as fork-tailed gull or xeme (from the genus name). It breeds in colonies on Arctic coasts and tundra, laying two or three spotted olive-brown eggs in a ground nest lined with grass. Sabine's gull is pelagic outside the breeding season. It takes a wide variety of mainly animal food, and will eat any suitable small prey.
Taxonomy
Sabine's gull was formally described in 1819 by the naturalist Joseph Sabine under the binomial name Larus sabini. Sabine based his description on specimens that had been collected by his brother Captain Edward Sabine, who had accompanied Captain John Ross on a voyage to look for the Northwest Passage. The birds were found breeding on low-lying islands off the west coast of Greenland in July 1818. Sabine's gull is now the only species placed in the genus Xema; the genus was described in 1819 by the zoologist William Leach in an appendix to Ross's account of the voyage. The genus name Xema appears to be an invented name without meaning.
Sabine's gull is usually treated as comprising a monotypic genus;
Geographical variation is slight; birds from Alaska are slightly darker and perhaps bigger. Most authorities recognise no races, but a few recognise four based on size and mantle (back) colour.
Description
right|thumb|Adult flying in Iceland
thumb|Sabine's gull flying at the fjord [[Trygghamna in Spitsbergen]]
Sabine's gull is a small gull, in length and weighing . The wings are long, thin and pointed with a span of between . The bill, which is black with a yellow tip, is around long.
This species is easy to identify through its striking wing pattern, though at long range it can be confused with immature black-legged kittiwakes. The adult has a pale grey back and wing coverts, four black outer primary flight feathers, and white inner primaries and secondaries. The white tail is slightly forked. The adult's hood darkens during breeding season to dark grey with a narrow black collar at the base of the hood. Young birds have a similar tricoloured wing pattern, but the grey is replaced by brown, and the tail has a black terminal band. Juveniles take two years to attain full adult plumage. Sabine's gulls have an unusual moult pattern for gulls. Fledged birds retain their juvenile plumage through the autumn and do not start moulting into their first winter plumage until they have reached their wintering grounds. Adults have their complete moult in the spring prior to the spring migration, and have a partial moult in the autumn after returning to the wintering area, a reversal of the usual pattern for gulls. They have a very high-pitched and squeaking call.