Cepphus grylle

From the Cornell Birds of North America website: "Recent treatments list 5 subspecies (Cramp 1985, Nettleship 1996), only 2 of which occur in North America. Based on similarities of wing-covert pattern, overall whiteness in subadult and Basic plumages, and presence or absence of white tips to secondaries in Juvenal plumage, 2 groups seem apparent: arctic breeders (C. g. mandtii, including C. g. ultimus) and all others".