HABITAT SELECTION BY SYMPATRIC BROOD PARASITES IN SOUTHEASTERN ARIZONA: THE INFLUENCE OF LANDSCAPE, VEGETATION, AND SPECIES RICHNESS

Abstract

In the southwestern United States and northern Mexico, obligate brood parasitic bronzed cowbirds (Molothrus aeneus) and brown-headed cowbirds (M. ater) are sympatric and parasitize some of the same host species. Host species selection is driven, in part, by cowbird breeding habitat preferences, yet little is known about bronzed cowbird habitat selection, and no study has quantified bronzed cowbird and brown-headed cowbird habitat selection in sympatry. During 1998 and 1999, I compared the abundance and distribution of brown-headed cowbirds and bronzed cowbirds and the hosts they parasitize across 151 point-count locations in 10 major habitat types in the Huachuca Mountains and San Pedro River valley of southeastern Arizona. These sympatric brood parasites overlapped extensively in 4 of 10 habitats surveyed (pine-oak forest, urban, montane riparian, and foothill riparian). Passerine species richness was a better predictor of brown-headed cowbird and bronzed cowbird presence than host abundance. When combined with distance to foraging area and habitat type, passerine species richness was the best predictor of cowbird presence in logistic regression models.