Extremely rare diving duck. Breeding male has a dark green gloss to the head, but often seems dark-headed in poor lighting. Non-breeding males are duller, but have pale eyes. Females and immatures have dark brown heads and dark eyes. 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. Can be difficult to separate from Ferruginous Duck, with which it sometimes hybridizes; note less extensive white wing panels in present 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.