Brown-headed Cowbird Parasitism

(Cowbird Photo http://www.columbia.edu/itc/cerc/danoff-burg/invasion_bio/inv_spp_summ/Molothrus_ater.htm)

Brown-headed Cowbirds (Molothrus ater tend to live in grasslands, so historically they didn't share habitiat with the Golden-cheeked Warbler. However, because of habitat fragmentation, cowbirds now put heavy pressure on Golden-cheeked Warblers.

(eggs Photo http://fsc.fernbank.edu/Birding/birdID/cowbird.htm)
Brown-headed cowbirds) are brood parasites. They lay their eggs in the nests of other birds. Cowbirds are obligate brood parasites which mean that they cannot build nests or raise their own young.

(Baby cowbirds http://www.berkeley.edu/news/media/releases/2004/08/05_cowbird.shtml)

All this means that the Warbler ends up raising baby cowbirds instead of baby warblers.

(Cowbird Trap http://www.griffithwildlife.com/cowbird.html__http://www.audubon.org/bird/research/)
To protect Golden-cheeked Warblers and other threatened species, managers will sometimes put up cowbird traps on the golden cheeked warblers breeding grounds. They capture and then euthanize the cowbirds. At Fort Hood military base, trapping cowbirds has produced a huge improvement in the Golden-cheeked Warbler's reproduction.

sournces:
http://www.columbia.edu/itc/cerc/danoff-burg/invasion_bio/inv_spp_summ/Molothrus_ater.htm

http://fsc.fernbank.edu/Birding/birdID/cowbird.htm

http://www.berkeley.edu/news/media/releases/2004/08/05_cowbird.shtml

http://www.griffithwildlife.com/cowbird.html

http://www.audubon.org/bird/research/