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Least Bell's vireo

Compiled by Stillwater Sciences.

least Bell's vireo

Common Name
least Bell’s vireo

Scientific Name
Vireo bellii pusillus

Legal Status
Federal Endangered (1986)
State Endangered (1980)
Other The least Bell's vireo, Vireo bellii pusillus, is one of four subspecies of Bell's vireo recognized in North America (Brown 1993).

Geographic Distribution
The historical distribution of least Bell’s vireo ranged from central northern California through the Sacramento and San Joaquin valleys and Sierra Nevada foothills, and from the southern Coast Ranges (including the Santa Clara River watershed) to Baja California, Mexico (Wilbur 1979 and 1980 as cited in Kus 2002, USWFS 1998). Historical populations were also documented in Owens Valley, Death Valley, and scattered locations in the Mojave Desert (USFWS 1998, Kus 2002).

Today, the breeding range of least Bell’s vireo is limited primarily from Santa Barbara County south to San Diego County (where the majority of remaining populations occur) (Franzreb 1989, as cited in Labinger and Greaves 2001a, Kus 2002). Breeding pairs have also been sighted near Gilroy (Santa Clara County) (Roberson et al. 1997, as cited in Kus 2002) and along the Santa Clara River (Ventura County) (Labinger and Greaves 2001a), Mojave River (San Bernadino County) (Kus and Beack 1998, as cited in Kus 2002), and San Joaquin River (San Joaquin County) (River Partners 2005).

Critical habitat for the species has been designated in Santa Barbara, Ventura, Los Angeles, San Bernardino, Riverside, and San Diego counties (USFWS 1992). Critical habitat patches occur on the Santa Ynez, Santa Clara, Santa Margarita, San Luis Rey, Sweetwater, San Diego, and Tijuana rivers (USFWS 1992).

Local Distribution
A number of historical records document the presence of least Bell’s vireo near and in the Santa Clara River watershed. One museum record confirms the presence of a nesting pair of least Bell’s vireo in Foster Memorial Park on the Ventura River in 1911 (CDFG 2005). Two least Bell’s vireo observations were reported in 1980 and 1988: one 5.6 km (3.5 mi) east of Piru and one at Newhall Ranch (CDFG 2005).

In 1990–1991, three separate records of least Bell’s vireo territorial males and nesting pairs were reported near Saticoy and southwest of Santa Paula (CDFG 2005). Multiple active least Bell’s vireo nests have been observed on the Santa Clara River at the Fillmore Fish Hatchery since 1991 (Labinger and Greaves 2001a, CDFG 2005). Greaves and Labinger (1997) report capturing and banding 266 least Bell’s vireo individuals between 1991 and 1996 in the lower half of the Santa Clara River between I-5 downstream to the Highway 118 bridge near Saticoy.

More recently, Labinger and Greaves (2001a) reported that least Bell’s vireo was the most abundant and widely distributed endangered bird species within the lower Santa Clara River area. Between 1994 and 1999, they found 81 nesting pairs in the lower Santa Clara River, including nine pairs between the McBean Parkway bridge (Santa Clarita, Los Angeles County) and Las Brisas (Ventura County), 25 pairs from the Fillmore Fish Hatchery downstream to the Highway 23 bridge, three pairs from just downstream of the confluence with Sespe Creek to 2 km (1.2 mi) east of Santa Paula, 38 pairs along a 5 km (3.1 mi) segment upstream of the Highway 118 bridge (including the Freeman Diversion), and six pairs between the Highway 118 bridge and the Victoria Avenue bridge (downstream of Highway 101, Ventura) (Labinger and Greaves 2001a). Breeding pairs were found at many of these same locations in 2000 (Labinger and Greaves 2001b). In 2003, there was a record of at least one nesting pair on the Santa Clara River south of the Highway 101 bridge in 2003 (CDFG 2005).

The USFWS has designated critical habitat for the species along the Santa Clara River between Ventura and Los Angeles counties, including “all land within 3,500 feet perpendicularly and generally southward or westward” of State Highway 126 between approximately Piru and Interstate 5 near Castaic Junction (USFWS 1994).

Population Trends
Least Bell’s vireo is reported to have been common to abundant in its historical range before undergoing a sharp decline in abundance and range during the first half of the 20th century (USFWS 1998, Labinger and Greaves 2001a, Kus 2002). USFWS (1998) report that:

By 1986, the population [of least Bell’s vireo] had declined to an estimated 300 pairs, with the majority occurring in San Diego County. Restoration efforts and brown-headed cowbird control have allowed populations to increase in recent years. In 1998, the population size was estimated at 2,000.

Based on population monitoring and compiled data covering 1991 to 2000, Labinger and Greaves (2001a and 2001b) characterize the Santa Clara River population as relatively stable to slightly increasing and with increasing distribution along the river corridor. Between 1994 and 1999, Labinger and Greaves (2001a) documented 124 least Bell’s vireo territories and a doubling of the vireo population from 25 to 57 pairs in their original cumulative study area of 29 km (18 mi). In 1999, after expanding their study area to include an additional 14 km (9 mi), Labinger and Greaves (2001a) documented a total of 163 territories and 80 pairs. In 2000, they the documented 81 pairs in their expanded study area (Labinger and Greaves 2001b). Count data from 18 points in the lower 50 km (31 m) of the river reveal that the mean relative abundance of least Bell’s vireo steadily increased from 0.28 in 1994 to 1.25 in 1998, before decreasing to 0.75 in 1999 (Labinger and Greaves 2001a). In an upper reach of the river that had been affected by an oil spill, the mean relative abundance of least Bell’s vireo decreased from 0.1 in 1994 to 0.03 in 1995 and then steadily returned to 0.1 in 1998 and 1999 (Labinger and Greaves 2001a).

Life History and Timing
The draft recovery plan for least Bell’s vireo (USFWS 1998) describes the species as:

…a sub-tropical migrant, traveling some 2,000 miles annually between breeding and wintering grounds. Preliminary results of studies of color-banded birds indicate that least Bell’s vireos have a life span ranging to seven years.

Least Bell’s vireos generally arrive in California from mid- to late-March for a breeding season that typically ends in late September (USFWS 1986, Kus 2002). During this period they are known to breed almost exclusively within riparian habitats (USFWS 1998). Least Bell’s vireos have been documented to return to the same breeding site year after year (Greaves 1989).

According to the U.S. Fish and Wildlife Service (USFWS) (1998):

Males establish and defend territories through counter-singing, chase and sometimes physical combat with neighboring males. Territory size ranges from .20 ha to 3.03 ha (0.5 to 7.5 acres). … Newman (1992) investigated the relationship between territory size, vegetation characteristics, and reproductive success for populations of vireos at the San Diego and Sweetwater Rivers, but found no significant factors which could account for the variability in territory size observed at his sites.

Nests are built by both breeding pair members within a few days of pair formation, and generally take between four and five days to complete (Kus 2002). Typically three to four eggs are laid beginning a day or two after nest completion (Kus 2002). Both male and female share in egg incubation, although females incubate more than males during the day and appear to be the exclusive incubator at night (USFWS 1998 Kus 2002). Incubation lasts about 14 days, and nestlings fledge 10–12 days after hatching (USFWS 1998, Kus 2002).

The fledgling stage is described in USFWS (1998):

Least Bell’s vireo may attempt as many as five nests in a breeding season (B. Kus, pers. comm.), although most fledge young from only one or two nests. … Adults continue to care for the young for at least two weeks after fledging when territorial boundaries may be relaxed as family groups range over larger areas. Fledglings generally remain in the territory or its vicinity for most of the season, although the behavior of older fledglings produced early in the year has not been well studied.

Habitat Requirements and Associated Vegetation

Territory Habitat
Least Bell’s vireos primarily occupy riparian habitats along open water or dry parts of intermittent streams, generally below 460 m (1,500 ft) in elevation (USFWS 1986; Small 1994, as cited in Dudek and Associates 2005, Kus 2002). They are generally associated with the following vegetation types: southern willow scrub; cottonwood forest; mule fat scrub; sycamore alluvial woodland; coast live oak riparian forest; arroyo willow riparian forest; wild blackberry; and mesquite in desert localities (Kus 2002).

Most vireo territories contain both dense vegetative cover within 1–2 meters of the ground, the preferred habitat for nesting, and a dense, stratified overstory canopy, the preferred habitat for foraging (Goldwasser 1981, USFWS 1998, Labinger and Greaves 2001a). In the Santa Clara River watershed, Labinger and Greaves (2001) documented least Bell’s vireo territories in early successional cottonwood/willow forest, southwestern willow woodland, and mulefat scrub. While vegetative structure was found to be more important in territory selection than the presence of particular plant species, willow trees and shrubs were found to be the most common plant species in the vicinity of vireo territories and the preferred species for nest placement (Labinger and Greaves 2001a).

Least Bell’s vireos have been observed to maintain territories that include upland habitats adjacent to riparian areas, such as coastal sage scrub (USFWS 1998). Upland habitats have also been documented for foraging and for nesting when early spring floods inundate riparian areas (Kus and Miner 1989, USFWS 1998). It has also been hypothesized that berry-producing upland vegetation, such as laurel sumac (Malosma laurina) and elderberry (Sambucus mexicana), may supplement the vireo diet in marginal habitats (Kus and Miner 1989).

Nesting Habitat
Least Bell’s vireo primarily nests in small remnant segments of vegetation typically dominated by willows (Salix spp.) and mulefat (Baccharis salicifolia) but may also use a variety of shrubs, trees, and vines (Olsen and Gray 1989). Nests are typically built within one meter (3.3 ft) of the ground in the fork of understory vegetation (Franzreb 1989, as cited in Kus 2002). Cover surrounding nests is moderately open midstory with an overstory of willow, cottonwood (Populus sp.), sycamore (Platanus sp.), or oak (Quercus sp.). Crown cover is usually more than 50 percent and contains occasional small openings. On the Santa Clara River, Labinger and Greaves (2001a) found that the dominant plant species used for nest support (in 57 percent of observed nests) were willows (Salix lasiolepis, S. exigua, S. laevigata, and S. lasiandra), followed by mulefat (28 percent of nests). The remaining 11 percent of nests observed were scattered throughout a variety of tree, shrub, and forb species, including poison oak (Toxicodendron diversilobum), white alder (Alnus rhombifolia), mugwort (Artemisia douglasiana), and Fremont cottonwood (Populus fremontii). Vireo’s were also found nesting in two invasive, non-native plants: four percent of nests observed were found in giant reed (Arundo donax) stands and two of the 426 nests observed were found in tamarisk (Tamarix sp.) plants (Labinger and Greaves 2001a).

Foraging Habitat
Kuss (2002) indicates that the vireo typically forages in riparian and adjoining upland habitat. Grinnell and Miller (1944) indicated that foraging occurs at all levels of the canopy, but appears to be concentrated in the lower to mid-strata, particularly when pairs have active nests. Salata (1983) found that 69 percent of 131 foraging observations were within 4 meters (12 feet) of the ground. Miner (1989) found a similar peak in foraging activity in vegetation between 3–6 meters (9–18 feet) in height. Moreover, she determined that the distribution of vireo foraging time across all heights was not simply a function of the availability of vegetation at those heights, but rather represented an actual preference for the 3–6 meter zone (Miner 1989).

Ecological Interactions
Least Bell's vireos are insectivores, preying on a wide variety of insect types including bugs, beetles, grasshoppers, moths, and particularly caterpillars (Chapin 1925; Bent 1950). It is likely that vireos do not require water for drinking (Kus 2002). They obtain prey primarily by foliage gleaning (picking prey from leaf or bark substrates), and hovering (removing prey from vegetation surfaces while fluttering in the air). Both Salata (1983) and Miner (1989) observed vireos occasionally capturing prey by hawking (pursuit and capture of flying prey), and Miner (1989) noted a behavior she called "clinging," which she described as hovering, but with the feet in contact with the vegetation.

The invasion of exotic plant species into riparian habitats increases habitat fragmentation and can decrease suitable vireo nesting habitat in some cases. Invasive non-native plants found in current least Bell's vireo habitat include castor bean (Ricinus communis), cocklebur (Xanthium strumarium), tamarisk, and giant reed (USFWS 1998). Giant reed is of prime concern due to its ability to disperse throughout an entire drainage and its rapid growth rate, which allows it to outcompete and restrict the growth of other native riparian plants (Kus 2002). When natural riparian vegetation types, such as the structurally diverse native riparian scrub and mature forest communities required by the vireo, are replaced by thick stands of giant reed, bird species abundance and other native wildlife have been found to decline (Bell 1994, Bell 1997, Herrera and Dudley 2003, Kisner 2004). Labinger and Greaves (2001a) observed over the course of their study (1994–1999) that while dense thickets of giant reed supported very low bird diversity:

… a low to moderate mixture of giant reed with native willow woodland supported high bird diversity in some areas [such as near the Freeman Diversion]…. In such areas giant reed was also used for nesting, as noted by at least 17 nests of least Bell’s vireo, one nest of southwestern willow flycatcher, and several other species such as Anna’s hummingbird, bushtit, and common yellowthroat.

The ecological interaction of most concern to least Bell’s vireo populations is brood parasitism by brown-headed cowbirds (Molothrus ater). Dudek and Associates (2000) provide a description of brood parasitism:

Cowbirds lay their eggs in the nests of other songbirds. The cowbird often removes a number of the host’s eggs and replaces them with an equal number of cowbird eggs. Cowbird eggs require a relatively short incubation period, thus the young cowbird hatches earlier than the host’s eggs. The effects of parasitism include reducing nest success rate and egg-to-fledgling rate and delaying successful fledgling. A common response to parasitism is abandonment of the nest. The success rate of re-nesting is often reduced and there may be inadequate time to prepare to migration. In California, parasitism rates range from 50 percent to 80 percent, considered to be a high parasitism rate.

USFWS (1998) describe least Bell’s vireo as a common host species that readily accepts cowbird eggs. In the Santa Clara River watershed, Labinger and Greaves (2001a) documented cowbird parasitism of vireo nests and suggest that such parasitism may be limiting productivity of host species. Labinger and Greaves’ (2001a) study “…did not find a significant correlation between cowbird abundance and vireo productivity. Parasitism rates of subpopulations of least Bell’s vireos were never more than 20 percent and typically less than 10 percent.” Removal of cowbird eggs and chicks from vireo nests during nest monitoring has been cited to enhance vireo productivity by up to 44 percent in some studies (USFWS 1998).

Sensitivity to Anthropogenic Watershed Disturbances
Least Bell’s vireo is sensitive to the direct loss and degradation of habitat and increased rates of cowbird parasitism that result from or are exacerbated by urbanization and other development within and near riparian areas. Urbanization and agriculture, including runoff from agricultural fields and roadways, livestock grazing, water diversion projects, traffic noise, feral pets, and recreational use of habitat can result in the direct loss of vireo habitat, and degrade and fragment habitat to the extent that it is no longer usable or increases the vulnerability of the population. Least Bell's vireo often nest near recreational open spaces or trails. Nest failure and abandonment can be caused by human disturbance such as trampling of nests or nest sites or clearing of vegetation (USFWS 1998).

Habitat fragmentation is thought to be one of the primary factors responsible for vireo population decline, has been attributed to development within riparian areas and the establishment and spread of non-native plant species. Habitat fragmentation results in smaller populations spread out among remaining suitable patches. These smaller, more isolated populations then become more vulnerable to habitat destruction (through flooding or development, for example), disease, low production years, and parasitism (USFWS 1998, Labinger and Greaves 2001a).

The abundance of brown-headed cowbirds is believed to increase in areas with development near riparian areas (USFWS 1998). Brood parasitism by brown-headed cowbirds is the other primary factor, in addition to habitat fragmentation, responsible for the decline of least Bell’s vireo (Kus 2002, Labinger and Greaves 2001a).

Key Uncertainties
The following key uncertainties and information gaps regarding least Bell’s vireo were identified by Kus (2002):
  • Whether any reproductive parameters are density-dependent.
  • Whether dispersal is density-dependent.
  • The effect of different cowbird control regimes on vireo parasitism rates and reproductive success.
  • The use of restored habitat by vireos.
  • The status of wintering habitat and identification of current or potential threats.
  • Identification of predators and establishing means of control.
  • Identification of additional and potential least Bell's vireo breeding habitat within its historical range.
Bell, G. P. 1994. Biology and growth habits of giant reed (Arundo donax). In N. E. Jackson, P. Frandsen, and S. Douthit, editors. Arundo donax Workshop Proceedings. November 19, 1993, Ontario, CA.

Bell, G. P. 1997. Ecology and management of Arundo donax, and approaches to riparian habitat restoration in Southern California. In J. H. Brock, M. Wade, P. Pysek, and D. Green, editors. Plant Invasions: Studies from North America and Europe. Blackhuys Publishers, Leiden, The Netherlands.
Bent, A. C. 1950. Life histories of North American wagtails, shrikes, vireos, and their allies. U. S. Nat. Mus. Bull. 197.

Brown, B.T. 1993. Bell’s vireo. In A. Poole, P. Stettenheim, and F. Gill, editors. The Birds of North America, No. 35. Academy of National Sciences, Washington DC.

CDFG (California Department of Fish and Game). 2005. California Natural Diversity Database. Electronic database. Sacramento, California. Accessed April 2005:

Chapin, E. A. 1925. Food habits of the vireos. USDA Bull. 1355.

Dudek and Associates. 2000. Least Bell’s vireo: western Riverside County MSHCP species accounts. Dudek and Associates, Encinitas, California. Franzreb, K.E. 1989. Ecology and conservation of the endangered least Bell's vireo. U.S. Fish and Wildlife Service, Biol. Rep. 89(1). 17 pp. Goldwasser, S. 1981. Habitat requirements of the least Bell's vireo. Calif. Dept. of Fish and Game Final Report., Job IV-38.1. Greaves, J., and Z. Labinger. 1997. Site tenacity and dispersal of least Bell’s vireos. Transactions of the Western Section of the Wildlife Society 33:18-33.

Grinnell, J., and A. Miller. 1944. The distribution of the birds of California. Pacific Coast Avifauna No. 26. Herrera, A. M., and T. L. Dudley. 2003. Reduction of riparian arthropod abundance and diversity as a consequence of giant reed (Arundo donax) invasion. Biological Invasions 5:167-177.

Kisner, D. A. 2004. The effect of giant reed (Arundo donax) on the Southern California riparian bird community. Graduate thesis. San Diego State University, San Diego, California.

Kus, B. 2002. Least Bell's vireo (Vireo bellii pusillus). In California Partners in Flight. The riparian bird conservation plan: a strategy for reversing the decline of riparian-associated birds in California. Available at:

Kus and Miner 1989. Use of non-riparian habitats by least Bell’s vireos. USDA Forest Service Gen. Tech. Rep. PSW-110.

Labinger, Z., and J. Greaves. 2001a. Summary report of avian studies (1994-1999) following the ARCO/Four Corners January 17, 1994 oil spill on the Santa Clara River, California. Report prepared for U.S. Fish and Wildlife Service, Ventura, California.

Labinger, Z., and J. Greaves. 2001b. Results of 2000 avian surveys and least Bell’s vireo monitoring: restoration phase of the ARCO/Four Corners January 17, 1994 oil spill on the Santa Clara River, California. Report prepared for U.S. Fish and Wildlife Service, Ventura, California.

Miner, K. L. 1989. Foraging ecology of the least Bell' vireo, Vireo bellii pusillus. Unpublished Master's thesis. San Diego State University, San Diego, California.

Newman, J. 1992. Relationships between territory size, habitat structure and reproductive success in the least Bell's vireo, Vireo bellii pusillus. Unpublished Master's thesis. San Diego State University, San Diego, California.

Olsen, T.E. and M.V. Gray. 1989. Characteristics of least Bell’s vireo nest sites along the Santa Ynez River. USDA Forest Service Gen. Tech. Rep. PSW-110.

River Partners. 2005. Least Bell’s vireo returns to the Central Valley. River Partners Journal 1(1): 1-3. River Partners, Chico, California.

Salata, L. 1983. Status of the least Bell's vireo on Camp Pendleton, California: report on research done in 1983. Unpubl. Rept., U. S. Fish and Wildlife Service, Laguna Niguel, California.

USFWS (U.S. Fish and Wildlife Service). 1992. 50 CFR Part 17 endangered and threatened wildlife and plants: designation of critical habitat for the least Bell’s vireo (Vireo bellii pusillus). Final Rule.

USFWS (U.S. Fish and Wildlife Service). 1994. Designation of critical habitat for the least Bell's vireo. Federal Register 59:4845–4867.

USFWS (U.S. Fish and Wildlife Service). 1998. Draft recovery plan for least Bell’s vireo. U.S. Fish and Wildlife Service, Portland, Oregon.

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