Black-headed Duck

The black-headed duck is the most basal living member of its subfamily, and it lacks the stiff tail and swollen bill of its relatives. Overall much resembling a fairly typical diving duck, its plumage and other peculiarities indicate it may not be a very close relative of the other stiff-tailed ducks, but rather the product of convergent evolution from the ancestors of the stiff-tailed ducks. The black-headed duck is about 35 cm (14 in) long. Males weigh an average of about 510 g (18 oz) and females 565 g (20 oz). The species has a distinctive shape, with a long body and disproportionately short wings. Its bill has a black maxilla and an orange mandible; the maxilla has a rosy patch at its base during the breeding season. Adult males have a glossy black head and upper neck and a white chin. Their upperparts are deep brownish black with cinnamon or pale rufous speckles and vermiculation. The folded wing shows two white bars. Their underparts are whitish with brown mottling and appear silvery. Adult females have a dark brown head and neck and a whitish throat, and are otherwise like the male. Immature birds are similar to the female but have more rufous upperparts, more yellowish underparts, and a pale eye stripe.

The black-headed duck has two separate ranges. One is the western side of central Chile. The other is from extreme southern Bolivia south through central Paraguay to central Argentina and east through southern Uruguay into southernmost Brazil. Undocumented sight records in the Falkland Islands lead the South American Classification Committee of the American Ornithological Society to treat it as hypothetical there. It mostly inhabits freshwater marshes whose dominant plant is the sedge Scirpus californicus, and outside the breeding season may be found in lakes, water-filled ditches, and sometimes flooded fields.

The black-headed duck feeds by diving. Though little information is available about its diet, a major component appears to be seeds of Scirpus californicus, and snails a minor component.