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Brown-headed Cowbirds are well known as "brood parasites", sneaking their eggs into the nests of birds of other species to be reared by adoptive "host" parents (which are often much smaller than them). During the breeding season, Brown-headed Cowbirds tend to engage in courtship and nest parasitism mainly in the morning and to feed in the afternoon. A single female may travel more than 6 km through woodlands to lay as many as several dozen eggs in a breeding season. Providing bird food in spring around the edges of large woodlands unintentionally facilitates brood parasitism by cowbirds, which can greatly reproduce the reproductive output of their hosts.
In the breeding season, males display by fluffing up their body feathers, partly speading their wings and tail, and bowing deeply while singing. Groups of males sometimes perch together, singing and displaying. The male's song is a squeaky gurgle.
Brown-headed Cowbird eggs are whitish with brown and gray spots concentrated at the larger end and are often easily recognized because they look different and larger than the other eggs in a host nest. A female may lay nearly an egg per day for several weeks, up to 40 in a breeding season (rarely as many as 70 or more). A female will often remove a host egg before adding one of her own. Brown-headed Cowbird eggs have been found in the nests of more than 220 species and over 140 of these are known to have successfully reared young cowbirds. Fed by their host parents, cowbird nestlings develop rapidly, usually leaving the nest after about 10 or 11 days.
The diet of the Brown-headed Cowbird consists mainly of seeds and insects (seeds account for around half the diet during the breeding season, but more than 90% in the winter). Centuries ago, these birds probably followed bison herds on the Great Plains, as they often follow cattle and horses across North America today, feeding on insects flushed from the grass. This dramatic range expansion over the past century or two has negatively impacted a number of parasatized songbird species, some quite seriously.
(Kaufman 1996; AOU 1998; Dunn and Alderfer 2011)