- We studied brown-headed cowbird (Molothrus ater) parasitism on the Arizona Bell's vireo (Vireo bellii arizonae) and the yellow-breasted chat (Icteria virens) to determine if removing adult and nestling cowbirds and addling cowbird eggs resulted in increased host nesting success. Our investigation took place in Arizona along the lower Colorado River at Bill Williams River National Wildlife Refuge. At 3 plots adult cowbirds were removed by trapping (Treatment A); at 3 plots adult cowbirds were removed and cowbird eggs were addled (Treatment B); and at 3 plots no cowbird control was implemented (Reference plots). A significantly greater proportion of vireo nests were parasitized on Reference plots than either treatment type across the study. The proportion of successful vireo nests was not significantly greater across Reference and Treatment plots during 1997, but was significantly greater in 1998. The proportion of successful and unsuccessful nests did not significantly vary across treatment types for the chat in either year. There was a lower proportion of singly and multiply parasitized nests on Treatment plots compared to Reference plots during 1998, but not during 1997. Predation was the leading cause of nest failure for Bell's vireos and yellow-breasted chats in 1997 and 1998.