Brood parasitism by brown-headed cowbirds (Molothrus ater) has negative impacts on a large number of songbird species. Cowbirds are obligate brood parasites, meaning that females lay their eggs in the nests of other species and do not provide care to their offspring. Parasitism by cowbirds often results in reduced reproductive success for the host, sometimes to the exclusion of fledging any of their own young. Clearly parasitism by cowbirds can have a substantial impact on the population dynamics of the host species. Over 200 species of birds are known to be parasitized by cowbirds. Cowbirds breed in shrublands and forests, and especially parasitize host nests located near ecotones, or borders between habitat types. Human land use in general may promote the success of cowbirds; landscapes with forest openings, clearcuts, small tracts of forests, and large amounts of habitat edge have higher parasitism rates than do landscapes with contiguous forest tracts. Cowbirds readily forage in feedlots, overgrazed pastures, and grasslands, and the expansion of agricultural land use over the past century has provided abundant feeding habitat for cowbirds. Large increases in the numbers of cowbirds have been documented and this increase has been implicated as one factor responsible for the decline of a large number of passerines. Compounding their impact is the fact that cowbirds can affect host populations over broad spatial scales. Because they do not protect their young or a nest, they can range large distances in search of suitable feeding areas; researchers have reported maximum daily movements from 7 to 13 km for cowbirds (Rothstein et al., Cook et al., respectively). At Fort Hood, a U.S. Army military installation located in central Texas, cowbirds parasitize the nests of numerous songbird species, including those of the black-capped vireo ( Vireo atricapillus) and the golden-cheeked warbler (Dendroica chrysoparia), two federally endangered species. The black-capped vireo appears to be particularly vulnerable to parasitism. Once her nest is parasitized, a host female often abandons it. The female may then attempt to renest but, when cowbirds are abundant, this nest is also likely to be parasitized.
Negative Impacts of Human Land Use on Dung Beetle Functional Diversity
