This 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 is not a very close relative of these, but rather the product of convergent evolution in the ancestors of the stiff-tailed ducks. It is a small, dark duck, the male with a black head and mantle and a paler flank and belly, and the female pale brown overall.
They live in swamps lakes and marshes in northern Chile, Paraguay, and northern Argentina, feeding by dabbling on water plants and insects. The black-headed duck is an obligate brood parasite (meaning the female does not build a nest). It lays its eggs in the nests of other birds, instead, earning it the nickname cuckoo duck. The hosts are (particularly) rosy-billed pochard (Netta peposaca), other ducks, coots (Fulica species), and occasionally even gulls (such as the brown-hooded gull) and birds of prey. Unlike some cuckoos, neither the chicks nor adults destroy the eggs or kill the chicks of the host. Instead, after a 21-day incubation, the ducklings fledge and after a few hours are completely independent, leaving their broodmates and fending for themselves, because the brood parasitic passerines (family Viduidae and genus Molothrus) are altricial, while black-headed duck ducklings are precocial.
- BirdLife International (2012). "Heteronetta atricapilla". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
- Livezey, Bradley C. (1986). "A phylogenetic analysis of recent anseriform genera using morphological characters" (PDF). Auk. 103 (4): 737–754.
- McCracken, Kevin G.; Harshman, John; McClellan, David A. & Afton, Alan D. (1999). "Data Set Incongruence and Correlated Character Evolution: An Example of Functional Convergence in the Hind-Limbs of Stifftail Diving Ducks" (PDF). Systematic Biology. 48 (4): 683–714. doi:10.1080/106351599259979. PMID 12066296. Archived from the original (PDF) on 14 June 2011.
- David Attenborough, Nature of the Cuckoo Duck, The Life of Birds, BBC Episode 9, 19 minutes ff.