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Lesser Scaup
Aythya affinis
– Family
Authors: Austin, Jane E., Christine M. Custer, and Alan D. Afton
Revisors: Anteau, Michael J., Jean-Michel DeVink, and David N. Koons

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Figure 1. Distribution of the Lesser Scaup.
Adult male breeding (Alternate) Lesser Scaup; Hillsboro, Washington Co., OR; March.

This 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, and numbers of this species breeding in the Prairie Pothole Region have increased over the past several decades. Breeders favor large seasonal and small semipermanent wetlands and lakes with emergent vegetation.

This is a late fall migrant, one of the last waterfowl to leave an area at freeze-up. Throughout fall and winter, Lesser Scaup form large flocks on rivers, lakes, and large wetlands. Individuals also winter in estuaries and marine habitats of the Gulf and Pacific coasts, and large rafts of this duck have been observed wintering offshore in the Gulf of Mexico during some winters. Indeed, the wintering distribution of this Scaup is farther south than other Aythya species—into Mexico, Central America, Puerto Rico, 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 southern portions of the Prairie Pothole Region in mid-May. Individuals form new pair bonds during spring migration each year, and nest in late May through June. Females build nests on the ground near or over water, as well as in uplands, unlike other diving ducks. Only the female incubates; her mate leaves during mid- or late-incubation. Ducklings hatch synchronously, spending less than one day in the nest before they follow the female to water, and they fledge by late August or September. Annual nest success and productivity vary with female age, body condition, timing of breeding, predation, spring weather, and water conditions. Adults and ducklings are mainly carnivorous, consuming aquatic invertebrates (mainly crustaceans, insects, and mollusks) during the breeding season and throughout the annual cycle.

Despite its abundance and broad distribution, this species historically has received less study than other North American Aythya. Diet, parasitology, and breeding biology in the southern portion of the breeding range have received the most attention. In response to concerns over a potential decline of this species (Afton and Anderson 2001), research efforts have accelerated since 1998. Information for birds breeding in northern regions has become available, but large spatial-scale studies of breeding Lesser Scaup largely are lacking. Other detailed investigations also have included studies of spring and fall migration, winter ecology, and contaminant biology.

Our knowledge of population size and trends is confounded by 1) unknown biases in the waterfowl breeding population survey because timing of the survey does not always match that of Lesser Scaup migration and breeding, and 2) the inability to separate Greater (Aythya marila) and Lesser Scaup in survey data (Afton and Anderson 2001).

Despite increased research efforts on Lesser Scaup in the 2000s, there is no clear single explanation for declines in this species. In the late 2000s population estimates suggested increases; however, owing to uncertainty with the survey, it is not clear if this recent trend is a result of annual survey bias, population stabilization, or population increase.