Male greater scaup are easily recognized by their white sides and black heads. Females are brown with a large white patch at the base of the bill. Both have a distinctive blue bill, which gives rise to a name commonly used by hunters—the blue bill. Telling the two scaup apart requires a close look at the head shape, and, in flight, the wing stripe.
Greaters have more rounded heads and a longer wing stripe. They are more coastal in winter, favoring less sheltered and saltier water. They are generally scarce inland except on their northern Canadian breeding grounds, but occasionally can be found in flocks of lesser scaup.
Greaters feed throughout the day, diving for mollusks and grass seeds, but plants make up only a small part of the diet in winter. Silent most of the year, the calls are nearly identical to those of lesser scaup. Recent studies suggest that the population may have declined significantly in recent decades, but the cause is not known.