Loss of reproductive productivity in hosts of the Brown-headed Cowbird (Molothrus ater), a nest parasite, may limit the success of conservation and recovery efforts that focus on restoring avian habitat. We investigated the relationships between parasitism frequency of 2 vireos—the endangered Black-capped Vireo (Vireo atricapillus) and the White-eyed Vireo (Vireo griseus)—and characteristics of the neighboring avian assemblage. Our objective was to identify assemblage characteristics that are useful for predicting parasitism frequency We hypothesized that parasitism frequency of the vireos would decrease with increasing alternate host abundance and increase with increasing abundance of cowbirds. All Black-capped Vireo nests (n = 20) and 81% of White-eyed Vireo nests (n = 27) were parasitized. Parasitism frequency of White-eyed Vireos was positively correlated with the number of cowbirds detected (r = 0.84, df = 4, P = 0.018). Parasitism frequency of White-eyed Vireos was negatively correlated with abundance of the Black-and-white Warbler (Mniotilta varia; r = 0.891, df = 4, P = 0.009) but was not correlated with abundance of other cowbird hosts. The relationship between Black-and-white Warbler abundance and parasitism frequency may be due to chance, given the small sample size and narrow range of parasitism frequencies, but the relationship may also reflect an interaction (requiring further investigation) between parasitism and the vegetation characteristics associated with Black-and-white Warblers. Our results imply that reducing cowbird abundance will reduce parasitism frequency of Black-capped Vireos. When the number of cowbirds in an area predicts parasitism, land managers would benefit from identifying a target cowbird abundance based on the levels of parasitism the host population can tolerate while still meeting productivity and population objectives for the species.