The Brown-headed Cowbird, North America's best known brood parasite, lays its eggs in the nests of many different species. Originally these "Buffalo Birds" were limited to short-grass plains, where they followed herds of North American Bison (Bison bison) and fed on the insects stirred up by their movement. The Brown-headed Cowbird has since dispersed widely as European settlement in North America opened forests and homogenized the environment into the agricultural and suburban landscapes of today. The expansion of the Brown headed Cowbird has exposed new species and naive populations to brood parasitism, and the pressure on such host populations can be substantial. During the breeding season, female Brown-headed Cowbirds wander widely, overlap the home ranges of other females, and may lay 40 eggs per season.
The Brown-headed Cowbird is difficult to study because its breeding activities are distributed among many host nests. Herbert Friedmann's monograph (Friedmann 1929) provides a beginning to the vast and sometimes conflicting literature on the Brown-headed Cowbird (see also Friedmann 1963, Friedmann et al. 1977, Friedmann and Kiff 1985). Life history studies of host species also give information on cowbird biology and host–cowbird interactions; see especially studies of Prairie Warbler (Setophaga discolor) (Nolan and Thompson 1978), Kirtland's Warbler (Setophaga kirtlandii) (Mayfield 1960, Walkinshaw 1983), Ovenbird (Seiurus aurocapilla) (Hann 1937) and Song Sparrow (Melospiza melodia) (Nice 1937). Besides the Kirtland's Warbler, other endangered or threatened species are also "good" hosts for the Brown-headed Cowbird, so management concerns require a full understanding of this species' biology.