Harlequin ducks are small diving sea ducks. Males weigh about 650 g (1.4 lbs); females about 575 g (1.3 lbs). Males are striking, with a slate blue body, white bands and collars bordered by black lines on chest and neck, and a large crescent in front of the eye. They have a white spot behind eye and a white stripe along their neck. A black streak bordered by white and chestnut run along the top of the head, and the flanks are chestnut brown. Their wings are dark brown with an iridescent blue patch on the trailing inner edge and a few white spots or stripes close to the body.