The Pigeon Guillemot is most similar in plumage to the Black Guillemot but their ranges only overlap in Alaska. In all plumages, the Pigeon Guillemot has dark wing linings and a dark wedge intruding into the white wing patch. In breeding plumage the male White-winged Scoter is vaguely similar but has a differently shaped bill and profile, white secondaries rather than coverts, and some white on the face. Murres and razorbill lack the white wing patches.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.