Vol. 43, No. 6
David M. Larson
Female Brown-headed Cowbird. (Photograph by the author.)
The popular concept of brood parasites is of lazy cheats who dump eggs in other birds’ nests and scamper off on their merry way, leaving the poor suckers to take care of their eggs and young. Of course, the real story is much more complex. In the Old World, several groups of birds practice brood parasitism; some are obligate—they only breed this way—and others are facultative so brood parasitism is not strictly necessary. Obligate brood parasites can be generalists or can be restricted to a single host species. In Africa, for example, Variable Indigobirds only parasitize Jameson’s Firefinches, in which case the two species co-evolve as the host defenses change and the parasite catches up. In the New World, facultative brood parasites include cuckoos and several cavity-nesting ducks, while obligate brood parasitism is limited to cowbirds. And, of course, our North American exemplar is the Brown-headed Cowbird (Molothrus ater). This species is a generalist brood parasite, meaning that the female will lay an egg in the nests of a variety of host species. In fact, approximately 220 species of hosts have been documented, though only about 140 have fledged cowbirds (Lowther 1993).