This common species is unmistakable. It is one of the largest species of duck. Length can range from , wingspan ranges from and weight from .Ogilvie & Young, Wildfowl of the World. New Holland Publishers (2004), Sarkidiornis melanotos (Comb duck, Knob-billed duck). biodiversityexplorer.org Adults have a white head freckled with dark spots, and a pure white neck and underparts. The upperparts are glossy blue-black upperparts, with bluish and greenish iridescence especially prominent on the secondaries (lower arm feathers). The male is much larger than the female, and has a large black knob on the bill.
