The Greater Scaup can be distinguished from the very similar Lesser Scaup by its larger size, wing stripe that extends well into the primaries, and glossy, greenish head as an adult male in alternate plumage. Adult male Ring-necked Duck similar in alternate plumage but has quite different bill pattern, black back, grayer flanks and vertical white finger just behind black breast. The Ring-necked Duck favors freshwater habitats while the Greater Scaup prefers brackish or saltwater habitats. Very rare Tufted Duck is similar but note the distinctive tuft at the rear of the head. When the tuft is not visible, adult males have black backs. Females are very similar but usually lack white band around base of bill and have more black at the tip of the bill. The female Redhead is very similar in shape and coloration to the female Greater Scaup but it lacks the white patch at the base of the bill and has a gray wing stripe.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.