This article is from the Birds FAQ, by Lanny Chambers with numerous contributions by others.
Many cowbird species, such as Brown-headed Cowbird (Molothrus ater)
and Bronzed Cowbird (Molothrus aeneus), are brood parasites. That is,
female birds lay their eggs in nests of birds of other species; the
cowbird chicks hatch first and outcompete the other chicks for food
and parental attention.
This behavior is an evolutionary adaptation. Birds are not moral
agents, so we cannot describe brood parasitism as immoral. Nevertheless,
many birders cannot help but find it repugnant, particularly when treated
to the spectacle of a cowbird chick being frantically fed by parents
smaller than the chick itself. This revulsion no doubt contributes
to cowbirds' bad press.
However, cowbirds have been helped along by human activities. They
prefer as a habitat open lands, such as prairies, and the edges of
woodlands, and humans have created limitless areas of cleared space
and limitless lines of edges over the past century through development
and roadbuilding. Cowbirds have thus spread widely, and they are now
too successful for the survival of many other bird species. Thus they
are trapped systematically by authorized persons in areas where they
threaten endangered species, and some prominent ornithologists are
calling for mass harvests of cowbirds on their winter roosts.
Because they are native species, cowbirds in North America ARE protected
under the Migratory Bird Treaty Act.