The brown headed cowbird, Molothrus ater , is a nest parasitic species which is depressing other passerine bird populations, including the least Bell's vireo. The cowbird has expanded its range from the short grass prairie of the high plains into agricultural and suburban landscapes throughout most of North America. The cowbird's preferred habitat is varied and may include prairie, savanna grassland with low or scattered trees, woodland edges, fields, pasture, orchard and residential areas.
Cowbirds are, to a certain extent, dependent upon food provided by humans and livestock operations.
They are opportunistic generalist feeders, foraging primarily on the ground for seeds, arthropods, and animal waste. Cowbirds lay their eggs in the nests of other species. The cowbird's home range includes breeding, feeding, and roosting sites. The size of the home range is highly variable. The cowbird population will thrive as a result of Plans Build-Out, as seen in figures 73 and 74 .