The lesser scaup (Aythya affinis) is a small North American diving duck that migrates south as far as Central America in winter.

[3][4] The scientific name is derived from Ancient Greek aithuia an unidentified seabird mentioned by authors including Hesychius and Aristotle, and Latin, affinis "related to", from its resemblance to the greater scaup.

Downy hatchlings look much like those of related species, with dark brown upperparts and pale buff underparts, chin, supercilium and back spots.

Females, juveniles and drakes in eclipse plumage are hard to identify; there is considerable overlap in length between the two species, but greater scaup are usually noticeably more bulky.

In the greater scaup drake, the forehead is usually quite massive (especially in North American and East Asian populations), whereas the nape presents a smooth shallow curve and may appear almost straightly sloping.

The lesser scaup drake presents the opposite shape, with a less bulging forehead and a nape that looks strongly curved or even angular due to the small crest.

Notable breeding concentrations, with more than half a million birds at the height of the season, can be found in Alaska, in the woodlands of the McKenzie River valley and on the Old Crow Flats.

These birds migrate south (mostly via the Central and Mississippi Flyways) when the young are fledged and return early spring, usually arriving on the breeding ground in May.

Lesser scaup typically travel in flocks of 25–50 birds and winter mainly on lakes, rivers and sheltered coastal lagoons and bays between the US–Canada border and northern Colombia, including Central America, the West Indies and Bermuda.

In the extreme southeast and southwest of the breeding range—the Rocky Mountains region of the northwestern United States and the southern Great Lakes—lesser scaup are present all-year; it is not clear whether the breeding birds are replaced by migrants from the far north in winter, or whether the local populations do not migrate, or whether both local and migrant birds are found there in winter.

Vagrant lesser scaup have also been recorded on the Hawaiian Islands Japan, possibly China, and—for the first time on 18 January 2000—in the Marianas, as well as in Ecuador, Suriname, French Guiana, Trinidad and Venezuela (in winter), and Greenland (in summer).

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 the zebra mussel (Dreissena polymorpha) in Lake Erie, which was accidentally introduced in the 1980s and has multiplied enormously.

This may pose a risk to these birds because zebra mussels are efficient filter feeders and so accumulate environmental contaminants rapidly.

When nesting starts, the males aggregate while they moult into eclipse plumage, leaving the task of incubation and raising the young to the females alone.

[3][19] Before the start of the population decline (see below), about 57% of the lesser scaup nests failed each breeding season because the female was killed or the eggs were eaten or destroyed.

Due to the wide breeding range and the fact that the rate of decline, though remarkable, is still not threatening in respect to the enormous overall numbers, the lesser scaup is classified as a Species of Least Concern by the IUCN.

On the other hand, the narrow time frame in which lesser scaup breed and raise their young may be tied to some specific ecological conditions—such as abundance of key food, without the ducks being able to adapt.

[citation needed] However, it seems that greater scaup eats larger food items on average,[18] and the species are sympatric in part of their range and presumably have been for millennia without any problems due to competition.