An extremely rare diving duck, the breeding male displays a dark green gloss on its head, though it can appear dark-headed in poor lighting conditions. Non-breeding males are duller with pale eyes, while females and immatures have dark brown heads and dark eyes. This duck is uniquely marked by its bold chestnut-and-white sides, which set it apart from similar species like the Ferruginous Duck with its brown sides and the Common Pochard with its pale sides.