The least bell’s vireo (Vireo bellii pusillus) was formerly abundant in the riparian woodlands of California's Central Valley and low elevation riparian streams in southern California and northern Baja, Mexico . It was one of California's most abundant birds in the late 19th and early 20th centuries, but was reduced to just 300 pairs by 1986. The decline was driven by the separate and combined effects of habitat loss and brood parasitism by the exotic brown-headed cowbird (Molothrus ater). It was listed as an endangered species by the state of California in 1980 and the U.S. Fish and Wildlife Service in 1986 . Similar declines occurred in Baja, Mexico, but are not as well documented. The population and habitat extent in Mexico was much smaller historically and currently than in California.
Due to intensive habitat protection and restoration and cowbird control, the California vireo population increased dramatically from 300 estimated pairs in 1986  to 2,500 in 2004 . 38,000 acres of critical habitat were designated in ten areas stretching from Santa Barbara to San Diego County in 1994 . The proposal of critical habitat on U.S. Marine Corp's Camp Pendleton in 1986 spurred the development of a memorandum of understanding between the Army and the U.S. Fish and Wildlife Service to remove exotic plants, trap cowbirds, research and monitor the species, and other conservation measures .
Territorial males in the Prado Basin steadily increased from 19 to at least 600 between 1986 and 2005 [6, 10]. The Santa Ana River Watershed (including the Prado Basin) supported at least 988 territorial males in 2005 . Storm damage limited the 2005 survey effort, so the reported numbers are somewhat less than the actual. The Anza Borego State Park population increased steadily from 32 territorial males in 1986 to 109 in 2005 [3, 4, 5]. The Camp Pendleton population increased from 100 males in 1986 to 827 in 2005, but declined from its 1998 peak of 1,010. A 16 km stretch of the San Luis Rey River increased its population from 24 to 114 between 1986 and 2004, but declined from its 1999 peak of 132 . A population within 5 km of the San Diego River increase from 21 males in 1986 to 33 in 1996 . The Tijuana River population increased from 13 to 142 between 1990 and 1996 .
As the population has increased, the occupied range expanded eastward and northward . In 2005, the vireo appeared on the San Joaquin River National Wildlife Refuge in the Central Valley where it had been absent for 60 years.
Though significant habitat protection and restoration programs have been instituted since 1986, much of the population increase is due to aggressive cowbird trapping funded by mitigation requirements of Endangered Species Act consultations [1, 9]. Several of these populations (e.g. Camp Pendleton, San Luis Rey River, and San Diego River) increased rapidly then plateaued, suggesting that they may now be primarily limited by habitat condition and extent .
The draft 1998 federal recovery plan allows for downlisting to "threatened" status when eleven named river basins have stable or improving populations of at least "several hundred" breeding pairs for five years . Delisting can occur when three additional specific river basins have populations of several hundred breeding pairs each and either all 14 river basin populations are self-sustaining or endowments are established to permanently protect them from cowbirds and exotic plan encroachment. The downlisting criteria would require the existence of approximately 3,300 pairs. The delisting criteria would require about 4,200 pairs.
 USFWS. 1998. Draft Recovery Plan for the Least Bell’s Vireo. U.S. Fish and Wildlife Service, Portland, OR. 139 pp.
 USFWS. 1986. Determination of Endangered Status for the Least Bell’s Vireo. U.S. Fish and Wildlife Service. May 2, 1986 (51 FR 16474).
 Kus, B. 2001. Least Bell’s Vireo Surveys and Nest Monitoring at Anza Borrego Desert State Park in 2000: Final Report. U.S. Geological Survey, Western Ecological Research Center, San Diego Field Station.
 Joregensen, P. 2005. Bell’s vireo population survey and cowbird trapping report for Anza-Borrego Desert State Park (Vireo bellii pusillus) and Picacho State Recreation Area (Vireo bellii pusillus) for the 2004 Breeding Season. California State Parks, Colorado Desert District, Borrego Springs, CA.
 Jorgensen, P. 2005. Personal communication with Paul Jorgenson, California State Parks, Colorado Desert District, November 7, 2005.
 Pike, J., D. Pellegrini, L. Hays, and R. Zembal. 2005. Least Bell's Vireos and Southwestern Willow Flycatchers in the Prado Basin of the Santa Ana River Watershed, CA. Unpublished report prepared by the Orange County Water District and U.S. Fish and Wildlife Service.
 Kus, B. 2006. Least bell's vireo population trend at Camp Pendleton, 1981-2005. Spreadsheet provided by Barbara Kus, USGS Western Ecological Research Center, San Diego, CA, January 12, 2006.
 USFWS. 1994. Designation of critical habitat for least Bell's vireo. U.S. Fish and Wildlife Service, February 2, 1994 (59 FR 4845).
 Kus, B.E. and M.J. Whitfield. 2005. Parasitism, productivity, and population growth: response of least Bell's vireos (Vireo bellii extimus) and southwestern willow flycatchers (Empidonax traillii extimus) to cowbird (Molothrus spp.) control. Ornithological Monographs 57:16-27.
 Hays, L. 2006. Personal communication with Loren Hays, U.S. Fish and Wildlife Service, Carlsbad, CA, January 17, 2006.