Greater Scaup

The Greater Scaup (Aythya marila) is a medium-sized diving duck with a circumpolar distribution, breeding across the subarctic regions of North America, Europe, and Asia. Often called "bluebills" by observers due to their distinctive blue-grey bills, they are highly gregarious birds. During the winter months, they travel south to form vast, dense flocks on large inland bodies of water and coastal estuaries. These massive aggregations are colloquially known as "rafts." While closely resembling the Lesser Scaup, the Greater Scaup is slightly larger, stouter, and generally prefers open, brackish, or saltwater environments during the winter.

Distinguishing the Greater Scaup from the very similar Lesser Scaup (Aythya affinis) is one of the classic identification challenges in North American and Eurasian birding.

Key Field Marks

• Head Shape: The Greater Scaup features a smoothly rounded, dome-like head without the distinct tuft or "peeked" crown at the rear of the head seen on the Lesser Scaup.
• Plumage (Breeding Male): Displays a dark, iridescent head that usually shines with a green gloss in favorable light (though it can occasionally look purple). The breast and tail are black, while the back and sides are a clean, very pale grey, almost appearing white from a distance.
• Plumage (Female): Overall dark brown with a prominent, well-defined white patch wraps around the base of the bill.
• Bill: Broad, heavy, and pale bluish-grey with a small black tip (called the "nail"). The nail of the Greater Scaup is wider than that of the Lesser Scaup.

In-Flight Identification

In flight, both sexes show a bold white stripe running along the trailing edge of the wing. In the Greater Scaup, this brilliant white stripe extends nearly the entire length of the wing, stretching well into the outer primary feathers. In contrast, the Lesser Scaup's white stripe is restricted mostly to the inner secondary feathers.

Breeding Range

During the spring and summer breeding seasons, Greater Scaup inhabit mid-to-high latitude wetlands, particularly tundra ponds, gravelly lakes, and slow-moving rivers in muskeg and subarctic regions of Alaska, Canada, Scandinavia, and Siberia.

Wintering Range & Migration

They are medium-to-long-distance migrants, moving south in late autumn. Unlike Lesser Scaups, which prefer freshwater marshes and smaller ponds, wintering Greater Scaups are marine-oriented. They concentrate along coastal bays, estuaries, lagoons, major river deltas, and the Great Lakes, where they can find deep water and high densities of shellfish.

Feeding Habits

Greater Scaup are skilled divers, routinely submerging to depths of 2 to 6 meters to forage. They use their large, webbed feet for propulsion and grab prey with their wide bills. Their winter diet consists primarily of aquatic invertebrates, including blue mussels, clams, oysters, and aquatic insect larvae. On breeding grounds, they supplement this animal matter with seeds, leaves, and stems of pondweeds, sedges, and muskgrass.

Social Behavior & Rafting

Extremely social outside the breeding season, Greater Scaup gather in massive flocks that can number in the thousands. These rafts of ducks float tightly packed together on open water, providing safety from aerial predators like eagles.

Vocalization and Breeding

Generally silent during winter and migration. During spring courtship, males make a soft, low cooing whistle, while females produce a harsh, raspy "scaup, scaup" vocalization, from which their common name is derived. They nest on the ground, hidden in dense vegetation near water, laying 6 to 11 olive-grey eggs.