Fort Hood Project

Cowbird Research and Monitoring

Bronzed cowbird  © The Nature Conservancy

Brown-headed cowbirds are nest parasites, which is to say they construct no nests of their own, but instead lay eggs in the nests of other bird species.  The cowbird young hatch sooner than those of the host species, and either actively expel the eggs or nestlings of the host species, or out-compete them for food.  Cowbirds have greatly expanded their numbers and range over the past century in response to landscape changes related to cattle grazing and urbanization.  Before North America was settled by European immigrants, cowbirds followed the great bison herds, feeding in the short grass left behind by those animals’ grazing.  Now, however, the bison are gone, replaced by cattle which do not migrate, and by lawns, parks, and golf courses, all of which are excellent cowbird habitat.  Many songbirds which adapted over thousands of years to periodic cowbird parasitism are threatened by its constant presence now. 

Brown-headed cowbirds  © The Nature Conservancy

On Fort Hood, brown-headed cowbirds are a threat to both endangered bird species, but pose the most serious threat to the black-capped vireo.  Conservancy managers trap and shoot cowbirds on Fort Hood in order to restore the ecological balance on which the vireos, warblers, and other songbirds depend.  Parasitism rates on black-capped vireo nests, monitored by Conservancy scientists for the past 12 years, have declined from over 90% to about 5% as a result of cowbird trapping and removal.  We believe that warbler and vireo population increases over the past decade are largely a result of cowbird control.

Recent cowbird-related research projects have included an assessment of the range expansion of bronzed cowbirds (also nest parasites), impact of cowbird control on black-capped vireo nest success and population growth, and a comparative assessment of the efficiency and effectiveness of shooting versus trapping in controlling cowbirds.