Santa Clarita Valley History In Pictures

Least Bell's Vireo
Endangered Species

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Common Name: Least Bell's Vireo

Scientific Name: Vireo bellii pusillus

Status: Endangered

Federal Register: 51 FR 16482 (May 2, 1986)

Comments: The least Bell's vireo is found in dense, willow-dominated riparian habitats (near streams and rivers) with lush understory vegetation. Predominately an insectivore, it prefers willows for nesting and foraging, although it will also use the California wild rose and coastal live oak for these purposes. Most nests are located near the edges of thickets, about 3 feet above the ground. The males are protective of their homes and will return to the same nest year after year; therefore they are sensitive to changes in riparian vegetation.

The following is from the USDA Forest Service, for the Cleveland National Forest (similar information specific to the Angeles National Forest is not available):

Status and Distribution: The least Bell's vireo (LBV) was listed as endangered by the USFWS on May 2, 1986. This listing was followed by designation of critical habitat on February 2, 1994. The only critical habitat on NFS lands is near Gibraltar Dam on the Los Padres National Forest. LBV historically occurred from the interior of northern California to northwestern Baja California, Mexico. Populations occurring in the Owens Valley, Death Valley, Sacramento-San Joaquin Valleys and Sierra Nevada foothills, and Tehama County have been completely extirpated (USFWS 1998).

The breeding distribution of LBV is currently restricted to eight counties in California (Kern, San Diego, San Bernardino, Riverside, Ventura, Los Angeles, Santa Barbara, Imperial counties). Historically the vireo was known from Salinas River in Monterey County (USFWS 1998). Currently, the northern limit of breeding populations is the Santa Ynez River in Santa Barbara County (Phil Unitt, Barbara Kus, pers. comm.).

Available census data indicate that the LBV population in southern California increased from an estimated 300 pairs in 1983 to an estimated 1346 pairs in 1996 (USFWS 1998b). Present breeding populations are concentrated in San Diego, Santa Barbara, and Riverside Counties. LBVs winter in southern Baja California, Mexico.

Recovery Plan : Draft Recovery Plan (USFWS 1998). Identified recovery actions are:

  • a. Protect and manage riparian and adjacent upland habitats within the Least Bellís Vireo historical range.
  • b. Conduct research.
  • c. Develop and evaluate least Bellís vireo habitat restoration techniques
  • d. Reintroduce least Bellís vireo to unoccupied habitat in the historic range through translocation.
  • e. Evaluate progress of recovery, effectiveness of management and recovery actions, and revise management plans.
  • f. Provide public information and education.

Suitable Habitat Definition: The least Bell's vireo is a neotropical migrant that breeds in low-elevation riparian habitats, particularly broad cottonwood-willow woodlands and mule fat scrub. It is usually found at less than 600 meters (2000 feet) in elevation (CDFG 1998) although individuals have been reported up to 1300 meters (4200 feet), usually in desert areas. Populations on the coastal slope that are known to be breeding successfully are all below 800 meters (2500 feet). Of 123 occurrences reported in RAREFIND, 87% are at 600 meters (2000 feet) or less and 95% are at 900 meters (3000 feet) or less (CDFG 1998).

Most areas that have vireo populations are in early stages of succession where most woody vegetation is 5-10 years old (Franzreb 1989, Gray and Greaves 1984).

RECON (1990) provided the following habitat parameters: riparian habitat wider than 10 meters (30 feet); shrub cover greater than 10%; tree cover greater than 10%.

USFWS (1998) states that habitat includes the following: dense vegetative cover within 1-2 meters (3-6 feet) of the ground; dense, stratified upper canopy for foraging; usually willow-dominated; there is undisturbed upland habitat adjoining the riparian area, providing an additional area for foraging.

Based on the results of studies completed in the CNF (Wells 1990), the LBV occur in drainages with low to medium shrub cover of arroyo willow (Salix lasiolepis var. lasiolepis) and mulefat (Baccharis glutinosa), with a moderately dense overstory. Vireos also occur to a lesser degree in drainages with low to medium shrub cover with little or no overstory. The vegetative composition of these areas is consistent with LBV nesting habitat parameters mentioned for coastal areas (Salata 1983; Goldwasser 1981). They reported that dense shrub cover with a high degree of understory development is the primary nesting habitat requirement for the LBV.

Nest building by males begins in March with females arriving in late March to early April. Nests are generally located in the fork of a forb, shrub, or tree within one meter of the ground surface. These areas generally have an open mid-story with an overstory consisting of willow, cottonwood, sycamore or oak. Canopy cover is generally greater than 50% with occasional small openings.

LBVs often forage on willows for insects, usually within the riparian habitat. Occasional feeding in both oak woodland and adjacent chaparral on the Santa Margarita River has been detected (Salata, 1983). Foraging can occur up to 30m (100 feet) from the edge of the riparian vegetation (Kus and Miner 1987).

Population Levels & Trends on National Forest Service lands: Cleveland NF: The current LBV population in the CNF is 7 pairs as surveyed in 1999 (Wells and Turnbull 2000). This is about 0.5% of the total known population. Most of the existing and potential habitat for the LBV in the CNF is not expected to support a large number of breeding pairs because the habitat is at or near the upper elevation limit for this species. The habitat is typically located in long, narrow drainages where riparian vegetation is sparsely distributed. Currently the LBV occurs in lower Cottonwood Creek on the Descanso District and in Santa Ysabel Creek and along the San Luis Rey River on the Palomar District . Historic or potential habitat is present in lower Pine Valley Creek (Descanso District) and along the San Diego River (Palomar District), as reported by Jones (1985). In recent years the Cottonwood Creek population has been stable while the Pine Creek population has disappeared, and the Santa Ysabel Creek population has consisted of 0-1 pair.

Threats: Although the cowbird has been blamed for the vireo's decline in California (Tate 1981), cowbird parasitism may be symptomatic of the more crucial problem of habitat loss and degradation (Gray and Greaves 1984). For example, in degraded habitats which usually support only a small number of vireos, cowbirds are often responsible for serious reductions in vireo productivity. Land use patterns surrounding the riparian zone are also important. If only a small amount of cowbird foraging habitat is available, or no habitat is present, there should be little or no problem with nest parasitism (Laymon,1987). Development and loss of habitat are the major threats on private lands. On National Forest lands grazing may be indirectly affecting some populations, and in some areas high levels of recreation use may occur. Along major transportation corridors there is a potential risk from hazardous waste spills.

Protection of Occurrences / Degree of Risk to Occurrences on NFS lands: Cleveland National Forest: Grazing has been excluded from all occupied habitat. Restoration of several miles of riparian habitat has been completed to improve vireo habitat. Currently Forest populations are fairly well-protected. Habitat along Pine Valley Creek could be affected in the event of hazardous waste spills along the I-8 corridor at the Pine Creek bridge.

Degree to which NFS lands can contribute to recovery: The low-elevation riparian habitats preferred by Bell's vireos are uncommon on Forest lands. Consequently, Bell's vireo numbers are low on national forest system lands and make up a small percentage (2 per cent) of the total regional population. Due to the limited known and potential habitat present on NFS lands, these lands may contribute minimally to the recovery of this species.

Conservation Considerations: Consider monitoring of larger populations and additional surveys of potential habitat. Within the assessment area, habitat degradation and nest parasitism by brown-headed cowbirds are the biggest threats to least Bell's vireos (USFWS 1998). Habitat loss and cowbird parasitism are occurring primarily in urbanizing areas and do not appear to be a major factor on public lands.

On the Santa Margarita River within the Camp Pendleton Marine Corps Base, cowbird parasitism of least Bell's vireo nests was at 47 percent in 1983 when the base began an intensive cowbird trapping program. With annual trapping, by 1990 the brood parasitism rate had dropped to less than 1 percent. The Santa Margarita River Bell's vireo population rose from 60 pairs in 1983 to 319 pairs in 1993. Since the initiation of annual cowbird removal programs, similarly dramatic population increases have occurred on the Tijuana and lower San Luis Rey rivers and in the Prado Basin (USFWS 1998).

Bell's vireo populations on National Forest System lands have not been nearly as successful in spite of past cowbird control projects. The Santa Ynez River population has dropped from fifty-five breeding pairs in 1980 to fewer than thirty pairs in 1994 (Greaves 1997). The population along Pine Valley Creek has dropped from five pairs in 1994 to zero in 1997,1998, and 1999, although in neighboring Cottonwood Creek the population has consistently been five to eight pairs (Wells and Turnbull 2000). Factors other than cowbird parasitism appear to be limiting Forest populations.

Habitat degradation can occur when the structure or composition of riparian vegetation is altered. Dense shrub cover within 1 to 2 meters (3 to 6 feet) of the ground is important for Bell's vireos and this cover can be significantly reduced by overgrazing, off-road vehicle activity, concentrated recreation use, channel clearing, and large discharges of water from upstream reservoirs. Invasions of non-native plants, particularly arundo and tamarisk, are also crowding out native plants and degrading habitats for the Bell's vireo in a number of locations. Two well protected Forest populations, those at Gibraltar Reservoir and at Pine Valley Creek, have declined. It is possible that the lack of disturbance in these systems has allowed maturation of the riparian vegetation to the point where it no longer has the structure that is attractive to vireos. In general vireos are associated with early to mid-successional riparian areas.

Literature Cited or Consulted:

  • California Department of Fish and Game. 1998. California Natural Diversity Database. CDFG, Sacramento CA.
  • Goldwasser, S. 1981. Habitat requirements of the least Bell's vireo. California Department of Fish and Game, Job IV-38.1, Final Report. 16p.
  • Gray, M.V. and J. Greaves. 1984. The riparian forest as habitat for the least Bellís vireo. Pp. 605-611 in R. Warner and K. hendrix,eds. California Riparian Systems:ecology, conservation and productive management. Univ. Calif Press, Davis.
  • Greaves, J. 1989. Maintaining site integrity for breeding least Bells vireos. Pp. 293-298 in USDA Forest Service General Technical Report PSW-110. 544 pp.
  • Greaves, 1993. Bellís vireo and cowbird management, Gibraltar Reservoir area 1993. Prepared for US Forest Service, California Dept of Fish and Game, and US Fish and Wildlife Service.
  • Greaves, J. and Z. Labinger. 1997. Site tenacity and dispersal of least Bellís vireos. In Proceedings of the Wildlife Society Conference, Western Section, Feb. 5-8 1997.
  • Jones, Barry. 1985. Status of the least Bell's vireo on the Cleveland National Forest, Descanso and Palomar Ranger Districts. Sweetwater Environmental Biologists, Spring Valley, Calif. 24p.
  • Kus, B.E., and K.L. Miner. 1987. Foraging behavior of the least Bell's vireo: use of riparian and non-riparian habitats. San Diego State University, San Diego, CA. 22 pp. Unpubl. Rep.
  • Kus, B. Personal communication. Director, San Diego Field Station, US Geological Survey, Biological Resources Division. San Diego, CA
  • Laymon, S.A. 1987. Brown-headed Cowbirds in California: historical perspectives and management opportunities in riparian habitats. Western Birds 18:63-70.
  • RECON. 1990. Draft comprehensive species management plan for Least Bell's Vireo. Prepared for SANDAG. Unpublished report. 244 pp plus appendices.
  • Salata, L. 1983. Status of the least Bellís vireo on Camp Pendleton, California. Report on Research done in 1983. Unpubl. Report, USFWS, Laguna Niguel, CA.
  • Unitt, P. 1984. Birds of San Diego County. San Diego Natural History Museum, San Diego, Calif. 276 pp.
  • Unitt, P. Personal communication. Curator of Birds and Mammals, San Diego Natural History Museum.
  • USFWS. 1986. Endangered and threatened wildlife and plants: Determination of endangered status for the Least Bellís Vireo.
  • USFWS. 1998. Draft recovery plan for the Least Bellís Vireo. USFWS, Portland, OR. 139 pp.
  • Wells, Jeff. 1990. Population status of the least Bell's vireo on the Cleveland National Forest, Descanso Ranger District. U.S. Forest Service, Cleveland National Forest. Unpubl. Rep. 19p.
  • Wells, J. and J. Turnbull. 1998. 1997 Sensitive species survey results for Pine Creek and Hauser Canyon Wilderness Areas, Cleveland National Forest. Unpublished report to Cleveland National Forest, 28 pp.
  • Wells, J. and J. Turnbull. 2000. 1999 Sensitive species survey results for Pine Creek and Hauser Canyon Wilderness Areas, Cleveland National Forest. Unpublished report to Cleveland National Forest, 28 pp.

Survey Considerations: There is not currently an official FWS protocol. However FWS has produced survey guidelines recommending 8 surveys spaced one week or more apart between April and June. Patch size: Per pair, estimated at 0.8 ha (RECON 1990).

Photos: California Environmental Protection Agency (top); U.S. Forest Service (bottom)

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