INFLUENCE OF ALTERNATE HOST DENSITIES ON BROWN-HEADED COWBIRD PARASITISM RATES IN BLACK-CAPPED VIREOS

Brown-headed Cowbird (Molothrus ater) parasitism is thought to be part ly influenced by density of the host species, although tests of host d ensity are relatively rare. We examined parasitism rates relative to t he density of individual host species and densities of coexisting host species. We monitored 392 nests among coexisting host species and mea sured their densities among six habitats on Fort Hood Military Reserva tion, Texas during 1991-1992 to test the hypothesis that coexisting sp ecies affect parasitism rates in the endangered Black-capped Vireo (Vi reo atricapillus). Black-capped Vireos and White-eyed Vireos (V. grise us) suffered three to four times higher cowbird parasitism than Northe rn Cardinals (Cardinalis cardinalis) or Painted Buntings (Passerina ci ris). After controlling for removal of female cowbirds, which has been conducted on the study site since 1988, parasitism rates in Black-cap ped 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 para sitism rates in Black-capped Vireos among sites. We suggest that cowbi rds may be attracted to conspicuous species, such as cardinals, and th at high densities of such species may negatively affect coexisting spe cies by increasing probabilities of being parasitized. Vireo nests wer e characterized by less nest concealment, greater canopy cover, and mo re stems than other species. However nest site and vegetation characte ristics did not differ between parasitized and unparasitized nests for any species, suggesting habitat was unimportant to parasitism.