The Brown-headed cowbird is a brood parasite of the vireo. A brood parasite lays eggs in other birds' nests. The hatchlings of the cowbird hatch earlier, and are larger and more aggressive than vireo hatchlings. Thus, the parent vireo spends most of it's time feeding the parasite and less food is available for its own offspring. When cowbirds are present, vireo nests rarely produce fledged young.   Brown-headed Cowbirds were originally associated with Bison on the Great Plains, feeding on insects associated with the grazing animals. With the introduction of wide-spread agriculture in the United States, cowbirds came to associate themselves with grazing cattle. This has allowed the numbers of cowbirds to explode, and the range of cowbird habitat to expand greatly. The destruction and fragmentation of riparian habitat, and the movement of agriculture into close proximity with riparian areas has allowed the cowbird to access riparian and woodland species that were previously unparasitized by cowbirds.