Traditionally, the Bay-winged Cowbird (Agelaioides badius) has been considered the "primitive," non-parasitic cowbird and has been known as Molothrus badius. Friedmann's (1929) view of cowbird taxonomy, and of the evolution of brood parasitism within the icterid blackbirds, had rooted phylogenies of the cowbirds with this species. The Bay-winged Cowbird sometimes is a nest parasite, a behavior considered by Friedmann to be a precursor to brood parasitism. Bay-winged Cowbirds, however, incubate their own eggs and rear their own young, often with helpers -- and often care for eggs and young of the Screaming Cowbird (Molothrus rufoaxillaris), a brood parasite that almost exclusively parasitizes Bay-winged Cowbirds. Friedmann (1929) considered these two species closely related, but this is not the case (Lanyon 1992); regardless of phylogenetic relationships, these two species are tightly bound together in their reproductive biology.
The host-parasitic relationship between Bay-winged and Screaming cowbirds was first realized on 12 April 1873 by W. H. Hudson (1870-1874, 1920) when he collected, from a flock of Bay-winged Cowbirds, young Screaming Cowbirds (which were molting from their "Bay-winged" juvenal plumage to the "Screaming Cowbird" adult plumage); Hudson then understood that the "extra" eggs in Bay-winged Cowbird nests must be those of the Screaming Cowbird. Until then, the large numbers of eggs in the nests had generated many questions of their source (e.g., Sternberg 1869). Because these two species show great similarity in-the-nest, information of both species are presented sometimes for comparative purposes in this account.
The source of information for much of this account has been the work of Rosendo M. Fraga (1972, 1979, 1983, 1984, 1986, 1988, 1991, 1992, 1996, 1998) who has looked the biology of this species, the nature of its cooperative breeding, and its interactions with Screaming Cowbird.