Birds

Cowbirds are brood parasites, laying their eggs in the nests of other birds and leaving them to the care of foster parents. Unlike parasitic Old World cuckoos, which lay eggs closely resembling those of the host species, cowbirds lay eggs in the nest of more than 200 other species, most smaller than themselves. Some host species eject the unwanted egg, others lay down a new nest linning over it, but most rear the young cowbird as one of their own. The young cowbird grows quickly at the expense of the young of the host, pushing them out of the nest or taking most of the food. It has been suggested that cowbirds became parasitic because they followed roving herds of bison and had no time to stop to nest.

Brown-headed Cowbird

Description	6-8". Male black with glossy brown head; Female plain gray-brown. Both have finch-like bill.
Voice	Squeaky gurgle. Call is check or a rattle.
Habitat	Agricultural land, fields, woodland edges, and suburban areas.
Nesting	4 or5 white eggs, lightly speckeled with brown, laid one at a time in the nests of other songbirds.
Other	Cowbirds are brood parasites, laying their eggs in the nests of other birds and leaving them to the care of foster parents. Unlike parasitic Old World cuckoos, which lay eggs closely resembling those of the host species, cowbirds lay eggs in the nest of more than 200 other species, most smaller than themselves. Some host species eject the unwanted egg, others lay down a new nest linning over it, but most rear the young cowbird as one of their own. The young cowbird grows quickly at the expense of the young of the host, pushing them out of the nest or taking most of the food. It has been suggested that cowbirds became parasitic because they followed roving herds of bison and had no time to stop to nest.
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Brown-headed Cowbird