Comb Duck

Comb DuckThe Comb Duck (Sarkidiornis melanotos), formerly known as the Knob-billed Duck, is an unusual, pan-tropical duck, found in tropical wetlands in sub-Saharan Africa, Madagascar and south Asia from Pakistan to Laos and extreme southern China. It also occurs in continental South America south to the Paraguay River region in eastern Paraguay, southeastern Brazil and the extreme northeast of Argentina[1], and as a vagrant on Trinidad.
It is the only known species of the genus Sarkidiornis. The supposed extinct "Mauritian Comb Duck" is based on misidentified remains of the Mauritian Shelduck (Alopochen mauritianus); this was realized as early as 1897[2] but the mistaken identity can still occasionally be found in recent sources.

Description and systematics
This common species is unmistakable. 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 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.[3]
The adults are unmistakeable. Immature Comb 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 Australian Wood 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, Comb Ducks in immature plumage are rarely seen without adults nearby and thus they are usually easily identified too.[4]
The Comb Duck is silent except for a low croak when flushed.[5]
There are two easily-distinguished subspecies:[4]
* Common Comb Duck (also called Nakta in South Asia[6]) or Old World Comb Duck (Sarkidiornis melanotos melanotos) from the Old World
Larger; flanks lighter (light grey, in females sometimes whitish)
* South American Comb Duck (Sarkidiornis melanotos sylvicola) from South America
Smaller; flanks darker (black in males, medium grey in females)