Host and nest selection by brown-headed cowbirds within a riparian area in central Arizona

Abstract

Management strategies are needed to reduce the rate at which brown-headed cowbirds (Molothrus ater) parasitize their hosts. I investigated whether vegetation management could be used to reduce parasitism by seeking differences in nest-site microhabitats of hosts in a riparian area of central Arizona. During 1993 and 1994, I quantified vegetation characteristics in 0.04 ha plots centered on 128 nests of 4 commonly parasitized species and 4 infrequently parasitized species. I compared characteristics between (1) parasitized and unparasitized nests of common hosts, and (2) nests of common and infrequent hosts. Factors associated with outcome of parasitism were vegetation volume at nest, size of nest substrate, distance from nest to visual obstruction below nest, and presence of large trees near the nest. Whether nests belonged to common hosts or infrequent hosts was best predicted by nest height. My results indicate riparian areas should be managed for large trees and numerous shrubs when the goal is to reduce parasitism.