Extremely rare diving duck. Both sexes have a dark head that has a green gloss in good lighting, but which usually appears dark brown or black. Boldly patterned chestnut-and-white sides unique among ducks; compare the brown sides of Ferruginous Duck and pale sides of Common Pochards, both similar species. During the colder months, often found singly or in small groups within non-breeding flocks of more common duck species. Breeds on sheltered, well-vegetated lakes; prefers large open bodies of water in the winter.