Introduction

Bell’s Vireo is a small, insectivorous, neotropical migrant that breeds in the central and southwestern United States and northern Mexico and winters primarily in central and southern Mexico. The species has no distinctive plumage characteristics and is sexually monomorphic. Within a breeding season, males are territorial and most pairs are monogamous , although some males and females are sequentially polygynous.

Males sing their distinctive song through the spring and summer from dense vegetation characteristic of early successional stages, or any successional stage exhibiting dense understory vegetation—riparian areas, brushy fields, young second-growth forest or woodland, and mesquite brushlands. The literature on breeding for this species is extensive, but there is an almost complete lack of information on density, ecology, and habitat use from its winter range. High rates of brood parasitism by Brown-headed Cowbirds (Molothrus ater) and habitat loss and change have negatively affected nesting vireos in the southwestern United States, causing reduction or extirpation of local populations. The Least Bell’s Vireo in California has been designated an Endangered Species by the U.S. Fish and Wildlife Service and the State of California, but cowbird control and restoration of riparian habitat have recently aided its recovery.