The knob-billed duck (Sarkidiornis melanotos) i also known as comb duck. This common species is unmistakable. It is one of the largest species of duck. Length can range from 56 to 76 cm (22 to 30 in), wingspan ranges from 116 to 145 cm (46 to 57 in) and weight from 1.03 to 2.9 kg (2.3 to 6.4 lb). 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. 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. If seen at a distance, they can also be mistaken for a fulvous whistling-duck (Dendrocygna bicolor) or a female maned duck (Chenonetta jubata). The former is more vividly colored, with yellowish and reddish brown hues; the latter has a largely dark brown head with white stripes above and below the eye. However, knob-billed ducks in immature plumage are rarely seen without adults nearby and thus they are usually easily identified too.


Habitat and Distribution

It breeds in still freshwater swamps and lakes in the tropics. It is largely resident, apart from dispersion in the wet season.


Feeding

This duck feeds on vegetation by grazing or dabbling and to a lesser extent on small fish, invertebrates, and seeds. It can become a problem to rice farmers.


Breeding

African birds breed during and after the rainy season and may not breed if the rain is scanty. Knob-billed ducks nest mainly in tree holes, also in tall grass. They line their nests with reeds, grass, or feathers, but not down. Males may have two mates at once or up to five in succession. They defend the females and young but not the nest sites. Unmated males perch in trees and wait for opportunities to mate. Females lay 7 to 15 yellowish-white eggs. Several females may lay in a single `dump nest` containing up to 50 eggs.


Calls and Songs

The knob-billed duck is silent except for a low croak when flushed