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. Basic and juvenile plumage Marbled Murrelets are somewhat similar but lack the wing patches, have shorter necks, and smaller bills.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.