Sabine's gull

Sabine's gull (Xema sabini) also known as the fork-tailed gull or xeme, is a small gull. It is the only species placed in the genus Xema. It breeds in colonies on 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's 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 that was introduced 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; it is placed within the genus Larus only when the genus is enlarged. The black bill and notched tail are almost unique within the gulls, as they are shared only with the swallow-tailed gull of the Galapagos. On the basis of this the two species were often thought to be each other's closest relatives, a hypothesis ruled out by a number of behaviour and ecological differences. Mitochondrial DNA studies confirmed that they are not closely related, and the closest relative of Sabine's gull is now thought to be the ivory gull, another Arctic species. The two species are thought to have separated around 2 million years ago, longer ago than most groups of gull species.

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. The Handbook of the Birds of the World recognises four subspecies. The nominate subspecies, X. s. sabini, breeds from the Canadian Arctic to Greenland. X. s. palaearctica (Stegman, 1934) breeds from Spitsbergen to the Taymyr Peninsula in Russia, and X. s. tschuktschorum (Portenko, 1939) breeds on the Chukotskiy Peninsula of Russia, and X. s. woznesenskii (Portenko, 1939) is found from the Gulf of Anadyr to Alaska.

Description
Sabine's gull is a small gull, 27 to(-) in length and weighing 135 to(-). The wings are long, thin and pointed with a span of between 81 and(-). The bill, which is black with a yellow tip, is around 2.5 cm long.

This species is easy to identify through its striking wing pattern. The adult has a pale grey back and wing coverts, black primary flight feathers and white secondaries. The white tail is forked. The male's hood darkens during breeding season. 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 molt pattern for gulls. Fledged birds retain their juvenile plumage through the autumn and do not start molting into their first winter plumage until they have reached their wintering grounds. Adults have their complete molt in the spring prior to the spring migration, and have a partial molt 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.

Distribution and habitat
Sabine's gulls breed in the Arctic, as they maintain a circumpolar distribution across northernmost North America and Eurasia. They migrate south to slightly warmer locations, typically in autumn, with most of the western North American population wintering at-sea in the Pacific Ocean. The birds will also head to islets and outcrops off of the South American west coast (such as the Galápagos Islands), where a consistent food supply is nourished by the cold waters of the Humboldt Current. Along their migration route, Sabine's gulls make stops along the US West Coast, and the pacific coasts of México and Central America; they have been observed in San Diego, California and along the Pacific coast of Baja California and Baja California Sur, México.

By comparison, Greenlandic and eastern American Sabine's gulls usually cross the Atlantic Ocean (by-way of the westernmost coasts of Europe) to winter off southwestern Africa in the cold waters of the Benguela Current. During their flight south, the birds will stop at different island chains, such as the Azores and Canary Islands. Occasionally, individuals of Sabine's gull can be seen on other coastlines, such as the northeastern United States and Eastern Seaboard, or even further east along coastal Western Europe, typically following autumn storms. Sabine's gull is recorded often enough inland, in North America, Europe, and even Siberia, that it has been said to exhibit "cross-continental migration" in addition to migration at sea.



Diet and feeding
The diet and feeding preferences of Sabine's gull varies by season and habitat. In the breeding season, the gulls pursue a range of freshwater and terrestrial prey on the tundra (and within boreal river deltas, estuaries and coastal wetlands), including both terrestrial and aquatic beetles, springtails, craneflies, mosquitoes, midges, flower flies (Syrphidae), molluscs, insects, arachnids, water bugs, various invertebrate larvae, crustaceans, fish, as well as nestling birds or unhatched eggs. Young chicks and eggs, while generally consumed opportunistically, may include waterfowl, black turnstones and lapland longspurs, as well as other gulls, including Sabine's gulls.