Swallow-tailed gull and hunting over the open oceans, and migrating eastward to the coasts of Ecuador and Peru. The swallow-tailed gull is unique among the gulls in feeding exclusively nocturnally, mostly on fish and squid which rise to the surface at night to feed on plankton. It leaves the colony as a flock at dusk, with a great deal of screaming and display. Calls and displays are quite different from other gulls, most resembling the vocalisations of the black-legged kittiwake and Sabine's gull. The loudest and most commonly heard call is an alarm referred to as "rattle-and-whistle", a gurgling scream made with
Sabine's gull 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