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The breeding biology of the parasitic Black-headed Duck

Weller, M.W. 1968.

The breeding biology of the parasitic Black-headed Duck.

Living Bird, 7: 169-207.


A study was made of the breeding biology of the Black-headed Duck (Heteronetta atricapilla) with special reference to its parasitic laying. Observations of laying were made mostly at 2 marshes near General Lavalle in the Cape San Antonio region of the Province of Buenos Aires, Argentina.

The Black-headed Duck was found to be adapted to dense marsh vegetation, where it fed mainly on seeds of marsh plants and occasionally on snails and duckweeds. It is rarely seen on land but flies easily and rapidly and dives as well as dabbles for food. It forms pairs during the breeding season but becomes social during the fall and is migratory. Sex ratio seems to be about 58% males to 42%.

Courtship behavior is distinctive. Some components resemble displays of stifftails whereas others resemble those of dabblers. The main courtship display involves a Toad-call with a head-pumping movement which produces a grunt-and-whistle followed by a wing-up, tail-up display. Pair bonds are formed and tested in courtship groups in a manner similar to that of other species.

Parasitism appears to be the sole means of reproduction as no nests or brood care is known in the species. Hosts are birds that nest in dense marsh vegetation, with the highest incidence being Red-fronted Coot (Fulica rufifrons), Rosybills (Netta peposaca), Red-gartered Coot (Fulica armillata), and White-faced Ibis (Plegadis falcinellus). There was a tendency for the parasitic female to lay during the hosts’ laying period without disturbing the nest or eggs. Egg success was 18% and 64% of eggs observed on 2 study areas. Losses were due mostly to egg burial by coots. Eggs hatched in 24–25 d.

Ducklings were cared for by the host during the first 24–36 h of life. Both wild and captive ducklings left the parent host at <2 d of age. They proved impossible to keep and rear under artificial conditions because of their tendency to leave the brooding site. They showed little of the following reaction common to other species of ducks. They were remarkably precocial in the development of feeding and maintenance behavior and were not alarmed by isolation or cold.

The plumage, anatomy, and behavior of Black-headed Ducks suggest that they are most closely related to the stifftails and probably should be maintained in that tribe. However, because they share several behavior traits of the dabblers, they may be ancient birds of dual affinities.

The success of parasitism in Heteronetta seems to be due to the selection of coots and a variety of marsh birds as hosts. Such birds are more numerous and successful in nesting than most ducks. The Black-headed Duck has achieved success, not by specializations in laying behavior or egg color but by the random placement of eggs in nests containing eggs of any color. Survival of the young in the nests of these divers is possible because the young rear themselves after only a brief period of parental care. Because it is the least damaging to the host, it may be considered the most perfect of avian parasites; indeed, it is nearly commensal.

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