Current Breeding Season Status of the Least Bell’s Vireo in California’s Central Valley and Surrounding Foothills

Abstract

The Least Bell’s Vireo (Vireo bellii pusillus) is federally, and state listed as Endangered in the U.S. and California and has been recommended for downlisting to Threatened by the U.S. Fish and Wildlife Service. This taxon, once abundant in the Central Valley of California, was extirpated from this region by the middle of the 20^th century. Since its listing in the 1980s, the subspecies has been increasing at many southern California locations. Beginning in the early part of the 21^st century Least Bell’s Vireos began appearing at sites in the Central Valley and surrounding foothills in the breeding season and these observations have continued. To update the status and distribution of the Least Bell’s Vireo in its historical Central Valley center of abundance, I reviewed all available reports of breeding season observations of this taxon from 2004 through 2024. These reports indicate that establishment of a breeding population has not occurred but suggest that re-establishment of a breeding population within this region may be feasible. Suggested specific actions could facilitate this re-establishment.