The Breeding Biology of Captive Black-Headed Ducks and the Behavior of Their Young

Abstract

The behavior patterns of captive Black-headed Ducks (Heteronetta atricapilla) at the Wildfowl Trust, Slimbridge, Gloucester, were studied for three consecutive breeding seasons to investigate the breeding biology of this parasitic species. The birds fed mostly in the early morning and swam most actively in the evening, while they mainly rested on land during midday. In the evening, both males and females patrolled or skulked near nest sites of other birds, apparently searching for a suitable host to parasitize. The existence of pair-bonds during the breeding season was fully established; copulation and egg-laying were observed. Two Black-headed Duck eggs were returned to the nest of a Rosy-billed Pochard (Netta peposaca) to determine the post-hatching behavior of the young and the role of the host bird. One Black-headed duckling hatched before any of the host's own clutch and left the nest on its own one day after hatching. The second duckling, which hatched with the young pochards, accompanied the family to water and returned to the female for warmth and protection (at increasingly infrequent intervals) for two days. These findings emphasize the uniqueness of the species and reinforce the case for placing Heteronetta in a tribe of its own. The Black-headed Duck (Heteronetta atricapilla) frequents freshwater marshes in the temperate regions of South America, ranging from central Argentina to Bolivia and across the continent from central Chile to southern Brazil. Breeding has been recorded in Argentina, Chile, and central Paraguay (Delacour 1959, Weller 1967, Todd 1979). Since Phillips (1925) first remarked about the lack of information on the Black-headed Duck, studies of its morphology and behavior have revealed its singularity; even its taxonomic position is still disputed. Similarities in anatomy and certain behavior patterns link Heteronetta to the Oxyurini, with whom they are most commonly classified. However, Blackheaded Ducks also exhibit a number of behavioral characteristics of the Anatini, and may be descendants of a primitive group from which both tribes evolved (Weller 1968a). The parasitic breeding behavior of the Blackheaded Duck is considered unique among waterfowl. Several species of Anatidae, most notably Redheads (Aythya americana) and Ruddy Ducks (Oxyurajamaicensis) occasionally lay eggs in other birds' nests (Friedmann 1932), but they usually hatch and rear their own young. The Black-headed Duck, however, neither builds a nest nor incubates its eggs, but leaves these duties entirely to host birds. The female tends to lay during the host's laying period, thereby making full incubation more likely, and usually adds her egg to the host's nest with little or no damage to the main clutch. Black-headed Duck eggs have been found in nests of a wide range of birds, including birds of prey (e.g., Chimango Caracara, Milvago chimango) and gulls (e.g., Common Black-headed Gull, Larus ridibundus). However, the preferred hosts appear to be coots (Fulica rufifrons and F. armillata), probably because their nests are common in marshy areas (Weller 1968a). Although the Black-headed Duck in its natural environment has been studied extensively by Weller (1967, 1968a, b), the species' unobtrusive nature and preferred habitat have made it difficult to observe its breeding behavior and a number of points still require clarification. We describe here the diurnal activities of captive Black-headed Ducks during the breeding season, including their pre-laying and pre-copulatory behavior, as well as the behavior of ducklings that hatched in the nest of a Rosy-billed Pochard (Netta peposaca). MATERIALS AND METHODS In 1974 the Wildfowl Trust collected eggs of Black-headed Ducks in Argentina in order to establish a captive breeding population of this species at Slimbridge, Gloucestershire, England. The five males and two females from