The Brown-headed Cowbird, North America's best known brood parasite, lays its eggs in the nests of many different species. Originally limited to the short-grass plains of the US and Canada, the Brown-headed Cowbird has expanded its distribution as European settlement opened forests and homogenized the environment into the agricultural and suburban landscapes of today. Cowbird expansion has exposed naive populations and new species to brood parasitism, and the pressure on such host populations can sometimes be substantial. Female cowbirds wander widely and show overlap in their breeding ranges. Cowbirds are difficult to study since breeding activities are distributed among many host nests. With a wide distribution in the US and Canada, the species' breeding range extens south to northern Baja California, Guerrero, Michoacán, Guanajuato, San Luis Postosí, and northern Tamaulipas; its wintering distribution extens further south to southern Baja California, Oaxaca, and central Veracaruz.