In its breeding range, Bell's vireo often uses dense shrubbery in riparian areas or otherwise near water. though in western areas
mesquite and
seep-willow (
Baccharis salicifolia) are frequently used. Bell's vireos construct a hanging nest suspended between the ends of low tree branches. however, one California study found that vireo nests in upland habitat are parasitized less than those in river-adjacent riparian habitat.
California subspecies Historically, the least Bell's vireo was a common to locally abundant species in lowland riparian habitat, ranging from coastal southern California through the Sacramento and San Joaquin Valleys as far north as
Red Bluff in
Tehama County. Populations also occurred in the foothill streams of the Sierra Nevada and Coast Ranges, and in
Owens Valley,
Death Valley, and scattered locations in the
Mojave Desert. Least Bell's vireos winter in
Baja California Peninsula. Unlike during the breeding season, they are not limited in winter to willow-dominated riparian areas, but occupy a variety of habitats including mesquite scrub within arroyos, palm groves, and hedgerows bordering agricultural and residential areas. At the time of
endangered species listing by the
U.S. Fish and Wildlife Service in 1986, it had been extirpated from most of its historic range, and numbered just 300 pairs statewide. Populations were confined to eight counties south of
Santa Barbara, with the majority of birds occurring in
San Diego County. In the decade since listing, least Bell's vireo numbers have increased six-fold, and the species is expanding into its historic range. In 1998, the population size was estimated at 2,000 pairs. Nesting least Bell's vireos have recolonized the
Santa Clara River in
Ventura County, where 67 pairs nested in 1998, and the
Mojave River in
San Bernardino County. The northernmost reported sightings in recent years is of a nesting pair of least Bell's vireos near
Gilroy in
Santa Clara County in 1997 and in the upper
Carneros Creek watershed east of
Highway 101 in northern Monterey County in 2001. Roughly half of the current least Bell's vireo population occurs on drainages within
Marine Corps Base Camp Pendleton in San Diego County, particularly in the lower
Santa Margarita River. ==References==