Scaup are medium-sized diving ducks belonging to the genus Aythya and are native to the Northern Hemisphere. They are distinguished by their compact bodies and forage by completely submerging themselves underwater. They are sometimes colloquially known as “Bluebills” in North America, a reference to the bluish-gray color of their bills. The term scaup refers to two widespread, closely related species.
Greater Scaup and Lesser Scaup
The term “Scaup” encompasses two distinct species: the Greater Scaup (Aythya marila) and the Lesser Scaup (Aythya affinis). Distinguishing them can be challenging, as both males display a dark head and chest, a pale gray back, and whitish flanks. The most reliable visual difference is the shape of the head, which is especially noticeable in males. The Greater Scaup has a more rounded, full head, with the highest point near the front of the crown.
In contrast, the Lesser Scaup’s head appears more peaked or angular, with the highest point located toward the rear of the crown. A second key field mark is the white wing stripe, visible when the birds are in flight. The Greater Scaup’s white secondary feathers extend into the primary feathers, creating a long, bright white stripe across the wing. The Lesser Scaup’s stripe is notably shorter, limited almost entirely to the secondary feathers.
Although the Greater Scaup is slightly larger, size is often impossible to judge unless both species are observed side-by-side. Their habitat preferences differ, with Greater Scaup favoring larger, open coastal bays and estuaries during winter. Lesser Scaup are more frequently encountered on smaller, inland freshwater bodies, such as reservoirs and ponds. The Greater Scaup also possesses a wider bill with a larger black “nail” at the tip.
Ecology and Behavior
Scaup secure their food by propelling themselves beneath the water’s surface, unlike dabbling ducks that only tip their forequarters. Their powerful legs are set far back on their bodies, making them agile swimmers underwater but giving them a labored gait on land. They are capable of diving up to 23 feet (7 meters), using their feet for propulsion to reach food lying on the bottom.
Their diet consists mostly of aquatic invertebrates, which they extract from the muddy substrate. Mollusks, particularly mussels and clams, form a large part of their winter diet when they are on coastal waters. In freshwater environments, they consume a variety of crustaceans, insect larvae, and some plant material.
Scaup are highly gregarious birds, often congregating in massive groups called “rafts” on open water. These rafts sometimes number in the thousands.
Scaup undertake long-distance migrations between their northern breeding grounds and southern wintering areas. They are among the last waterfowl species to depart for the winter, with migration often occurring in late fall. The breeding season takes place in the far north, across the tundra and boreal regions. The female builds a down-lined nest concealed in dense vegetation near water.
Population Status and Conservation
Although both scaup species are widespread, their overall populations have experienced a significant long-term decline in North America since the 1980s. This decline has been more pronounced in the Lesser Scaup, which is the more abundant species. Wildlife surveys often combine the counts for both species, making it challenging to track the status of each one individually.
Several factors contribute to the population trends, including changes in the quality of their breeding and migratory habitats. A reduction in key food sources, such as specific amphipods in northern lakes, is a leading theory for the decline in reproductive success. Environmental contamination also poses a threat, as scaup are vulnerable to accumulating heavy metals and pollutants from their diet of bottom-dwelling invertebrates.
Conservation efforts focus on improving water quality to support healthy invertebrate populations along migratory corridors. Protecting and restoring nesting habitat in the Prairie Pothole Region and the Western Boreal Forest is a priority. Monitoring the effects of introduced species, such as zebra mussels, is ongoing, as the mussels can alter scaup feeding habits and expose them to concentrated toxins.