The Thayer's Gull (Larus thayeri) is a large gull native to North America that breeds in the Arctic islands of Canada and primarily winters on the Pacific coast, from southern Alaska to the Gulf of California, though there are also wintering populations on the Great Lakes and the upper Mississippi River. This species has occurred as accidental vagrants to Tamaulipas in Mexico 1, Denmark and other parts of western Europe, and Japan 3.

Intermediate between American Herring Gull and Iceland Gull in habit and appearance and at times considered conspecific with either or both species, the Thayer's Gull in nonbreeding plumage has a pale gray mantle, with obvious blackish wingtips, and extensive brown streaking on the head and neck. The head, neck, breast, belly, and underwings are primarily white, and the legs are pink. There is a red spot on the lower mandible, and the color of the iris is generally dark. The Thayer's Gull reaches a length of 58"?63 cm, with a wingspan of 130"?140 cm and a weight of approximately 7.2"?1.5 kg (del Hoyo et al. 1996). In summer, the head and neck are white, with the bill turning bright yellow with a larger red spot on the lower mandible. Juvenile gulls are brown, with black bills, and black legs which quickly fade to adult pink.

During winter, it is found in small numbers among mixed flocks of large gulls, though it may gather in large numbers in certain locations. In summer, it is found on the tundra of high Arctic islands. These gulls with lay 3 bluish or greenish eggs in nests lined with grass, moss or lichens. Their voice consists of mostly mewing and squealing notes.

There is continuing debate about the taxonomic status of this species, and some authorities consider the Thayer's Gull to be the dark-mantled form of the Iceland Gull, with Kumlien's Gull (variously treated as a subspecies of either Thayer's or Iceland Gulls) as an intermediate example, forming a cline rather than separate species. The American Ornithologists' Union considered Thayer's Gull a subspecies of American Herring Gull from 1917 until 1973, when they determined it was a separate species from Herring Gull. While numerous papers have since been written suggesting downgrading this species to a subspecies or even a morph of Iceland Gull, the AOU as well as all North American field guides continue to treat Thayer's Gull as a separate species. The British Ornithologists' Union follows the publication Birds of North America in lumping the three as forms of Iceland Gull.

Both the common and species names honor ornithologist John Eliot Thayer.