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Brown-headed Cowbird (Molothrus ater) parasitism is thought to be partly influenced by density of the host species, although tests of host density are relatively rare. We examined parasitism rates relative to the density of individual host species and densities of coexisting host species. We monitored 392 nests among coexisting host species and measured their densities among six habitats on Fort Hood Military Reservation, Texas during 1991-1992 to test the hypothesis that coexisting species affect parasitism rates in the endangered Black-capped Vireo (Vireo atricapillus). Black-capped Vireos and White-eyed Vireos (V. griseus) suffered three to four times higher cowbird parasitism than Northern Cardinals (Cardinalis cardinalis) or Painted Buntings (Passerina ciris). After controlling for removal of female cowbirds, which has been conducted on the study site since 1988, parasitism rates in Black-capped Vireos were positively correlated with cumulative host density in general, and Northern Cardinal density in particular. Only density of Northern Cardinals explained a significant amount of variation in parasitism rates in Black-capped Vireos among sites. We suggest that cowbirds may be attracted to conspicuous species, such as cardinals, and that high densities of such species may negatively affect coexisting species by increasing probabilities of being parasitized. Vireo nests were characterized by less nest concealment, greater canopy cover, and more stems than other species. However, nest site and vegetation characteristics did not differ between parasitized and unparasitized nests for any species, suggesting habitat was unimportant to parasitism.

Influence of Alternate Host Densities on Brown-Headed Cowbird Parasitism Rates in Black-Capped Vireos