This North American species is found in western and southern Canada, throughout the United States and in Mexico. The northern populations are migratory, moving south to Mexico in winter.
Males tend to be larger than females in this species. The males are 19-22 cm long, have a wingspan of 36 cm and weigh 42-50 g. The females are 16-20 cm long, have a wingspan of 28-32 cm and weigh 38-45 g.
Brown-headed Cowbirds occur in grasslands with low and scattered trees as well as woodland edges, brushy thickets, prairies, fields, pastures, orchards, and residential areas. They generally avoid forests.
They feed mostly on seeds from grasses and weeds, with some crop grains. Grasshopper and beetles are also taken, often been caught as cows and horses stir them into movement. The females also eat snails and even the eggs of other birds in order to supply the extraordinary calcium demand of laying so many eggs.
Brown-headed cowbirds are brood-parasites, so the females lay their eggs in the nests of other birds. Over 140 host species of the brown-headed cowbird have been described, from birds as small as kinglets to as large as meadowlarks. Common hosts include the yellow warbler (Dendroica petechia), song and chipping sparrows (Melospiza melodia and Spizella passerina), Red-eyed Vireo (Vireo olivaceus), Eastern and spotted towhees (Pipilo erythrophthalmus and P. maculatus), and red-winged blackbird (Agelaius phoeniceus). A female can lay up to 36 eggs in a season, usually 1-7 per nest. The eggs hatch after 10-12 days of incubation by their hosts. Cowbird chicks tend to grow faster than their nestmates, allowing them to get more attention and food from their foster parents, and will fledge 8-13 days after hatching.
IUCN status – LC (Least Concern)
Originally a bison-following bird of the Great Plains, the brown-headed cowbird spread eastward in the 1800s as forests were cleared. This species greatly benefited from the human caused changes to the landscapes of North America and its population is now 56 million strong and believed to be mostly stable or slightly increasing. Its habit of nest parasitism can threaten species with small populations, such as the endangered Kirtland’s warbler (Dendroica kirtlandii) and black-capped vireo (Vireo atricapillus).