The White-winged Scoter is most distinctive in flight. Its large white wing patches and dark plumage are diagnostic. At rest, the dark plumage is similar to other scoters; sometimes, the white wing patches are visible. Adult male White-winged Scoters have a distinctive white patch around the eye. Female Black Scoters have darker crowns with a contrasting paler face, lack the white spots on the face, and have rounder heads. Female Surf Scoters are very similar to female White-winged Scoters but have darker crowns and more obvious bills.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.