Ivory gull narrower os pubis, and potentially more flexibility in skull kinetic structure. Structurally, it is most similar to the kittiwakes; however, recent genetic analysis based on mtDNA sequences shows that Sabine's gull is the ivory gull's closest relative, followed by the kittiwakes, with Ross's gull and swallow-tailed gull sharing a clade with these species. "Pagophila" is maintained as a unique genus because of the bird’s morphological, behavioral and ecological differences from these species. Colloquial names from Newfoundland include slob gull (from "slob", a local name for drift ice) and ice partridge, from a vague resemblance to a ptarmigan. This species is
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