Before the colonial settlement of North America, cowbirds followed the bison to feed on insects stirred up by their hooves. They once parasitized as few as 50 host species that nested in wooded areas on the margins of the Great Plains. Many of these had evolved defenses such as ejecting the cowbird eggs from the nest or abandoning the nest to start a second brood. But the spread of farming created vast new expanses of suitable habitat, and the nest parasites were quick to exploit it. They are now found from coast to coast and victimize at least 200 other species, and the newer victims lack innate defensive behavior.
Birds nesting at the forest edge are at special risk.
In the 1960s, before control measures were begun, cowbirds laid their eggs in 70 percent of the nests of the Kirtland's warbler, a vulnerable species with a small population, narrow habitat requirements, and limited breeding and wintering ranges. Only cowbird removal has prevented the warbler's extinction. The least Bell's vireo and black-capped vireo have been hit nearly as hard. Some songbirds that show rapid declines in BBS data, such as the prairie warbler and wood thrush, also suffer high rates of cowbird parasitization. Cowbird parasitization and loss of nesting and wintering grounds constitute a potent triple whammy for these host species.