Game Birds of the World

The South American black-headed duck is not a typical stifftail, inasmuch as it lacks a lengthened tail, rarely if ever dives for food, and has a bill shape that more closely resembles that of dabbling ducks. More interestingly, perhaps, it is the only species of waterfowl that is a social parasite; no nest or incubating female of the black-headed duck has ever been discovered, and it is believed that they invariably deposit their eggs in the nests of other marsh birds. The blackheaded duck is fairly common on the marshes of northern Argentina, north to southern Brazil, where it occurs with several other breeding duck species as well as three species of coots, of which the latter are especially common hosts of its parasitic behavior.

When in breeding condition, males acquire a bluish bill color, which is reddish basally, but show no evident seasonal differences in plumage. Like the ruddy duck, males of this species seem to have a very limited courtship display repertoire. Pair-bonding appears to be weak or absent. The number of eggs laid by a female in the course of a breeding season is unknown. Frequently two eggs are deposited in a single host's nest, at about the time that the host female is completing her clutch, but before incubation gets under way. More than a dozen species of birds have thus been reportedly parasitized by black-headed ducks. However, the hatching success is very low. Only the fairly short incubation period (about 21 days) and the precocial condition of the newly hatched young ensure the survival of the ducklings, which are able to fend for themselves when only a few days old. The fledging period has been estimated at ten weeks.