Blackbirds

Cowbirds are considered to be an important pest in those parts of North America to which the species has expanded its range as a result of the fragmentation of the initially forested landscape by humans. Birds in those regions tend not to be well adapted to cowbird parasitism, and the reproductive success of their populations can be markedly reduced by this relationship. In some cases, cowbirds are sought out and killed by conservation biologists, in order to reduce the negative impact of parasitism on rare and endangered species of birds, such as the Kirtland's warbler (Dendroica kirtlandii) of Michigan.

Some species of blackbirds are highly gregarious, especially during autumn and winter when they aggregate into flocks that can contain millions of birds. Winter flocks of the red-winged blackbird are sometimes regarded as an agricultural nuisance because of damages caused to fields of winter wheat and some other crops. Sometimes, pest-control actions are mounted against these flocks, and millions of these native birds may be killed when they are sprayed with chemicals at their communal roost sites.