The Least Bell’s Vireo (Vireo bellii pusillus) has been recorded as a breeding species in California and in Baja California, Mexico, south to about latitude 30°N (AOU 1957). Recent field work has shown this vireo to be rare or absent from about two-thirds of its former breeding range in California (Goldwasser et al. 1980), so it seemed desirable to make a preliminary survey of its status in Mexico.
A review of literature and museum records, along with consultations with ornithologists, resulted in the identification of 14 Mexican locations where Least Bell’s Vireos had been seen or collected during the breeding season (April–July), and that probably represented breeding localities (see Figure 1 and Appendix).
During 19–23 June 1980, Keith Axelson and I visited eight of those localities to see if vireos were present. We also checked for vireos in potential habitat en route. Because habitat loss and degradation and brood parasitism by Brown-headed Cowbirds (Molothrus ater) are implicated in the decline of the Bell’s Vireo in California (Goldwasser et al. 1980), we made a preliminary assessment of these factors.