Adults are 38–45 cm long, with a blue bill and yellow eyes. Adult males have a dark head with a purple sheen, a black breast, a light gray back, a black tail and white underparts. Adult females have a white band at the base of the bill and a brown head and body. They are smaller than the Greater Scaup, but may be an offshoot thereof, or they may both be descendants from a common ancestor.
There can be difficulty in distinguishing the Greater and Lesser Scaup. The differently colored sheen on the head is unreliable because light conditions vary, and these birds are often far from the water's edge.
The Lesser Scaup is best identified by its much smaller size, different head shape with a peaked hind crown, and a white wing bar that is visible only on the secondaries. (It extends onto the primaries in Greater.) The drake also shows vermiculations on the back. In North America, a large scaup flock will often have both species present.
They are a rare but apparently increasing vagrant to western Europe, where the identification also needs to exclude similar-looking hybrids. The first British record was a first-winter male at Chasewater, Staffordshire in 1987 but by 2006, over 60 had been recorded .
These birds dive and swim underwater, occasionally dabbling. They mainly eat mollusks and aquatic plants. It has been reported that both the Lesser and the Greater Scaup have shifted their traditional migration routes to take advantage of the presence of zebra mussels in Lake Erie. This may pose a risk to these birds because zebra mussels are efficient filter feeders and so accumulate environmental contaminants rapidly.
They nest late in a sheltered location on the ground near water. Their chicks usually do not hatch until July, and they are one of the later types of waterfowl to migrate south.
Although the Lesser Scaup has the largest population of any species of diving duck in North America, their population has been steadily declining since 1985, and reached an all-time low in 2005. Pollution is one cited cause of the decline, although the decline is poorly understood.