To promote the survival of the Least Bell's Vireo (Vireo bellii pusillus), the California Department of Transportation (Caltrans) began a program of trapping Brown-headed Cowbirds (Molothrus ater) on the Sweetwater River of San Diego County in 1986. The trapping was part of the mitigation required for a U.S. Army Corps of Engineers 404 permit to compensate for the removal of 1 acre of vireo habitat for construction of a bridge. The U.S. Fish and Wildlife Service (USFWS) provided details of the trapping scheme through an interagency cooperation agreement.
This paper addresses the present USFWS management method for reducing brood parasitism of vireos through trapping of cowbirds. Such trapping should manage both the Least Bell's Vireo and Brown-headed Cowbird effectively, but we contend that the present program does not fully address the behavior or ecology of the latter species.