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Lesser Scaup
Aythya affinis
– Family
Authors: Austin, Jane E., Christine M. Custer, and Alan D. Afton

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Adult male Lesser Scaup; Oregon, February.
Figure 1. Distribution of the Lesser Scaup.
Adult female Lesser Scaup

The Lesser Scaup, a medium-sized black and white diving duck, is one of the most abundant and widespread of North American ducks. Its core nesting habitats are in boreal forests and parklands from central Alaska through Manitoba. Breeders favor small seasonal and semipermanent wetlands and lakes with emergent vegetation. The Lesser Scaup is a late fall migrant; the last migrants leave an area at freeze-up. Throughout fall and winter, this species forms large flocks on rivers, lakes, and large wetlands and consumes primarily mollusks, crustaceans, and aquatic insects. Most individuals winter on freshwater bays and wetlands, as well as in estuaries and marine habitats of the Gulf and Pacific Coasts. Lesser Scaup are encountered farther south than other Aythya species—into Mexico, Central America, and the Caribbean.

This species is among the latest of migrant waterfowl to move north in spring; small migrant flocks often are still moving through the southern portions of the Prairie Pothole Region in mid-May. Individuals form new pair bonds during spring migration each year, and they nest in late May through June. Females build nests on the ground near or over water, as well as in uplands, unlike other div-ing ducks. Only the female incubates; her mate leaves during mid- or late incubation. Ducklings hatch together in 1 day, follow the female to water after no more than a day in the nest, and they fledge by late August or September. Annual nest success and productivity vary with female age, predation, and water conditions. Adults and ducklings are mainly carnivorous, consuming aquatic invertebrates and mollusks during the breeding season.

Despite its abundance and broad distribution, this species has not been studied as extensively as other North American Aythya have been. Diet (e.g., Bartonek and Murdy 1970, Afton and Hier 1991, Afton et al. 1991), parasitology (e.g., Bush and Holmes 1986, McLaughlin and McGurk 1987), and breeding biology in the southern portion of the breeding range (e.g., Rogers 1964, Hammell 1973, Afton 1984, Dawson and Clark 1996) have received the greatest study, but comparative information for birds breeding in northern regions is largely lacking. Detailed investigations also have included studies of postbreeding ecology (Austin 1983) and aspects of fall migration (Afton and Hier 1987). Little information is available on winter ecology or spring migration. Our knowledge of population size and trends is confounded by the inability to separate Greater (Aythya marila) and Lesser scaup in survey data, and by unknown biases in the Breeding Population Survey.