Knob-billed Duck - Description and Systematics

Description and Systematics

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.

The Knob-billed Duck is silent except for a low croak when flushed.

There are two easily distinguished subspecies., in fact, some taxonomists consider them to be distinct species:

• Knob-billed Duck (Sarkidiornis melanotos melanotos) (Pennant, 1769) (also called Nakta in South Asia) or Old World Knob-billed Duck (Sarkidiornis melanotos melanotos) from the Old World

Larger; flanks lighter (light grey, in females sometimes whitish)

• Comb Duck (Sarkidiornis melanotos sylvicola) (Ihering & Ihering, 1907) from South America

Smaller; flanks darker (black in males, medium grey in females).

• Male Knob-billed Duck

• Female Knob-billed Duck

• Female Comb Duck

Uncertainty surrounds the correct systematic placement of this species. Initially, it was placed in the dabbling duck subfamily Anatinae. Later, it was assigned to the "perching ducks", a paraphyletic assemblage of waterfowl most of which are intermediate between dabbling ducks and shelducks. As the "perching ducks" were split up, the Knob-billed Duck was moved to the Tadorninae or shelduck subfamily. In addition, Some taxonomists separate the two subspecies into distinct species.

Analysis of mtDNA sequences of the cytochrome b and NADH dehydrogenase subunit 2 genes, however, suggests that it is a quite basal member of the Anatidae, vindicating the earliest placement. But its closest living relatives cannot be resolved to satisfaction without further study.