The knob-billed duck (Sarkidiornis melanotos), or African comb duck, is a duck found in tropical wetlands in Sub-Saharan Africa, Madagascar and the Indian Subcontinent from northern India to Laos and extreme southern China.
This common species is unmistakable. It is one of the largest species of duck. 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. Young birds are dull buff below and on the face and neck, with dull brown upperparts, top of the head and eyestripe. Knob-billed ducks are generally larger in size when compared to comb ducks, and flanks are usually lighter (light grey, in females sometimes whitish).
Immature knob-billed ducks look like a large greyish female of the cotton pygmy goose (Nettapus coromandelicus) and may be difficult to tell apart if no other birds are around to compare size and hue. However, knob-billed ducks in immature plumage are rarely seen without adults nearby and thus they are usually easily identified too.
The knob-billed duck is silent except for a low croak when flushed.