Brood parasitism is an awkward term to describe an interaction between two species in which, as in predator-prey relationships, one species gains at the expense of the other. Brood parasites "prey" upon parental care, and the victimized species usually have reduced breeding success, partly because of the additional cost of caring for alien eggs and young, and partly because of the behavior of both adult and young brood parasites which may directly and adversely affect the survival of the victim's own eggs or young.
In all there are about 107 species found among five taxonomic groups:
- Black-headed Duck (Heteronetta artricapilla) of South America, the only waterfowl and the only precocial species completely brood parasitic;
- the honeyguides (family Indicatoridae) of Africa and southern Asia (of which all 17 species are known or thought to be brood parasitic);
- three lineages within the cuckoos (the 3 species of the American Neomorphinae, 4 species of Clamator among the Phaenicophaeinae, and all 57 species of Old World Cuculinae are known or thought to be brood parasitic),
- African finches of the family Viduidae comprising all 19 indigobirds (genus Vidua) and the Cuckoo-Finch (Anomalospiza imberbis); and
- the five cowbirds (genus Molothrus) of the New World Icteridae.