Removal of Host Nestlings and Fecal Sacs by Brown-headed Cowbirds

Using video to study nest predation at Fort Hood, Texas, we documented female Brown-headed Cowbirds (Molothrus ater) removing nestlings at 7 of 133 (5.3%) Black-capped Vireo (Vireo atricapillus) and 1 of 11 (9.1%) Golden-cheeked Warbler (Dendroica chrysoparia) nests. It has been suggested that female cowbirds depredate nests they find late in the host's nesting cycle to stimulate renesting by hosts, thereby enhancing future opportunities for parasitism. At Fort Hood, the function of nestling removal by cowbirds is unclear, because cowbirds caused nest failure during only two of eight visits. At three nests, we also observed cowbirds ingesting or removing fecal sacs produced by host nestlings. Fecal sac ingestion by cowbirds, though apparently infrequent, may provide a nutritional benefit.