Large sized duck, 56-76 cm. Whitish head, neck and underparts and
contrasting blackish upperside distinctive, crown and hindneck blackish
with black speckles on the rest of head and neck, green, bluish-purple and
bronze glossed upperpart, wing-coverts and secondaries narrow black bar
on sides of breast Male : having prominent knob on top of bill and which
becoming larger in breeding season.