Basic Information
Common Name: Least Bell's Vireo
Scientific Name: Vireo bellii pusillus
Species Code: VIRBEL
Management Category: SO (significant occurrence at risk of loss)
Occurrence Map
Table of Occurrences

Species Information

MSP Species Background

Goals and Objectives

Goal: Protect, enhance, and restore least Bell's vireo occupied and historically occupied habitat to create resilient, self-sustaining populations that provide for persistence over the long-term (>100 years).

regional NFO 2017, 2018 SO

Management units: 1, 6, 8

In 2017-2019, conduct surveys for least Bell's vireo and habitat assessment to compare population status and recovery in the MSPA before, during, and after SHB/Fusarium complex in infested habitat over time at the Tijuana River Valley and other sites, if funding available, such as Camp Pendelton and San Luis Rey River. Prepare site-specific management recommendations based on survey results and habitat assessments.

Action Statement Action status Projects
SURV-1 Submit monitoring data and management recommendations to MSP web portal In progress
Criteria Deadline year
Least Bell's Vireo Surveys and Reports Completed by 2020 2021
regional NFO 2020 SO

Management units: 1, 2, 3, 4, 5, 6, 8, 10, 11

In 2020, if least Bell's vireo survey results indicate management is needed to reduce impacts from SHB/Fusarium infestation, then develop a management plan that incorporate SHB/Fusarium monitoring and management and prioriotize management recommendations.

Action Statement Action status Projects
PRP-1 Prioritize management actions, focusing on reducing threats and expanding occurrences in areas most likely to remain viable over the long-term in the context of future land development. waiting for precedent action
PRP-2 Develop a management plan for least Bell's vireo that prioritizes management actions for the next five years. waiting for precedent action
PRP-3 Submit management plan to MSP web portal waiting for precedent action
Criteria Deadline year
Management Plan for Least Bell's Vireo Completed by 2021 2021
regional and/or local NFO 2021 SO

Management units: 1, 2, 3, 4, 5, 6, 8, 10, 11

Beginning in 2021, implement the highest priority management actions for least Bell's vireo on Conserved Lands.

Action Statement Action status Projects
IMP-1 Management actions to be determined by the management plan. waiting for precedent action
IMP-2 Submit project data and management actions to MSP web portal waiting for precedent action
Criteria Deadline year
Management actions implemented for Least Bell's Vireo 2021
regional and/or local NFO 2021 SO

Management units: 1, 2, 3, 4, 5, 6, 8, 10, 11

Beginning in 2021, monitor the effectiveness of management actions implemented for least Bell's vireo on Conserved Lands

Action Statement Action status Projects
IMP-1 Submit monitoring data and reports to MSP web portal waiting for precedent action
Criteria Deadline year
Monitoring completed and data and report submitted within 1 year of management actions 2021


Overall Condition


Overall Trend


Overall Confidence
Metric Condition Trend Confidence
1. Occupied Grid Cells

Number of grid cells with a detection in the San Luis Rey River



Current Status
The current overall condition status of the Least Bell�s Vireo Species Indicator is Good based on the single metric of occupied grid cells surveyed on the San Luis Rey River. This metric is classified as Good condition and Improving trend. Additional metrics will be added as more information becomes available, including expanding Metric 1 to encompass additional riparian systems beyond the San Luis Rey River. As additional data are compiled, other metrics are planned to document conservation of suitable habitat and the success of riparian restoration projects and cowbird trapping programs on vireo population recovery.
Metrics Dashboard
Full metric information for this species is available on our Dashboard.
Metrics Dashboard
Artesian Creek Restoration
Approximately 300 acres along Artesian Creek, a tributary of the San Dieguito River, was restored to coastal sage or native riparian. The majority of restored land was previously used for grazing, with an additional two miles of riparian habitat. The project is located between Camino Del Sur and Del Dios Highway, just south and southwest of Lake Hodges. The restoration has been completed, but annual treatment of invasive species remains including ongoing treatment of eucalyptus, tamarix and palms. The focal invasive species included: mustard, French broom, Scotch broom, Spanish Fleabane, arundo, Austrailian salt bush, Brazilian Pepper, caster bean, lapidium latifolia, garland chrysanthemum, bridal creeper, Italian thistle, fountain grass, dittrichia graveolens, artichoke, eucalyptus, tree tobacco, acacia, palms, pampas grass, pride of Maderia, tamarix, and fennel.
Fairbanks Ranch/Rancho Santa Fe Invasive Removal and Stream Enhancement
The long-term goal of this project is to restore and enhance wetland/riparian habitat along 3 miles, 200 acres, of the San Dieguito River and reduce fire risk to the surrounding community. Key actions include non-native, invasive plant removal, revegetation with native species, volunteer training, community workshops and education of local residents on how to improve habitat and create Fire-Safe landscapes around their homes. A secondary goal of this project is to highlight the importance of diverse partnerships in conserving habitat along the San Dieguito River. For more information, go to:
Least Bell's Vireo Surveys - Tijuana River Valley
In 2017, USGS conducted Least Bell's Vireo surveys at the Tijuana River to document the species' status 2 years after the Kuroshio Shothole Borer/Fusarium Dieback infestation and compare it to historic vireo abundance and distribution.
Lusardi Creek Restoration and Invasive Plant Removal
This project is working to eradicate invasive species and support native plants along Lusardi Creek. Many areas have already been treated and are recruiting natives naturally. Other areas are being planted with natives after invasive plant removal. The current goals include: 1) treat artichoke in several upland areas, 2) continue removing tamarix in the creek, 3) treat any regrowing Peruvian pepper, 4) treat any remaining pampas grass or tree tobacco, 5) work on an overall restoration plan. See the map link below for detailed treatment information.
Monitoring and Documentation of Post-Fire Recovery of Riparian Bird Community
A 2-year study on the effects of fire on the riparian bird community in San Diego County. Tasks included documenting the effects of the 2007 fires on endangered birds, in particularly, the Least Bell's Vireo, and monitoring post-fire recovery of the entire riparian breeding bird community.
SR 94 Wildlife Infrastructure Plan
Proposed road improvements to SR 94 provide an opportunity to mitigate the potential barrier effects of the highway. This project identifies where improvements to existing infrastructure on SR-94 could improve connectivity across the South County preserves, using Best Management Practices from the scientific literature; recommends wildlife movement monitoring to identify where new crossings are needed; and identifies where additional conservation would enhance the integrity of South County linkages. The review prioritizes infrastructure improvements of 35 existing undercrossings inspected by wildlife experts in the field along 14.6 miles of SR-94 where the highway bisects conserved lands. The majority of the recommendations for infrastructure improvement focus on increasing the diameter, and thus the openness ratio (cross-sectional area divided by length), of the undercrossing itself, removing vegetation and debris blocking the undercrossing, restoring habitat in the approach to the undercrossing, and installing fencing to both (1) keep animals off the highway and (2) funnel wildlife to the undercrossings.
File name Lead Author Year Type
1997 Sensitive Species Survey Results for Pine Creek and Hauser Canyon Wilderness Areas, Descanso Ranger District, Cleveland National Forest, San Diego County, California. Wells, Jeffery M.; Turnbull, Jennifer 1998 report
2008 Species Survey Annual Report Famolaro, Pete 2009 report
2009-2010 Annual Report for Otay Ranch Preserve - Salt Creek and San Ysidro Parcels Bennett, Anna; Dodero, Mark 2011 report
2011 Annual Report for the Otay Ranch Preserve Bennett, Anna; Dodero, Mark; O'Meara, Cailin 2011 report
Annual Report for the Otay Ranch Preserve January 1 - December 31, 2012 O'Meara, Cailin; Bennett, Anna; Dodero, Mark 2013 report
Biannual Report for Otay Ranch Preserve January 1-April 30, 2013 O'Meara, Cailin; Bennett, Anna; Dodero, Mark 2013 report
Biological Monitoring Report for the Tijuana River Valley Regional Park (Monitoring Year 2009) 2010 report
City of Carlsbad Habitat Management Plan Annual Report and Monitoring Summary Year 7, Nov. 2010 - October 2011 2012 report
County of San Diego MSCP Monitoring Summary Report January 1998 - June 2007 County of San Diego 2007 report
Distribution and Breeding Activities of the Least Bell's Vireo and Southwestern Willow Flycatcher at the San Luis Rey River, San Diego County, California Rourke, James W.; Kus, Barbara 2007 report
Distribution, Abundance, and Breeding Activities of the Least Bell's Vireo at Marine Corps Base Ca Rourke, James W.; Kus, Barbara 2008 report
Distribution, Abundance, and Breeding Activities of the Least Bell's Vireo along the San Diego River, California Wellik, Mike J.; Lynn, Suellen; Kus, Barbara 2009 report
Distribution, Abundance, and Breeding Activities of the Least Bell's Vireo along the San Diego River, California Lynn, Suellen; Wellik, Mike J.; Kus, Barbara 2009 report
Distribution, Abundance, and Breeding Activities of the Least Bell's Vireo at Marine Corps Base Camp Kus, Barbara; Rourke, James W. 2006 report
Encinas Creek Habitat Conservation Area (Formerly known as the North County Habitat Bank) Annual Work Plan October 2010 - September 2011 2010 report
Factors Influencing the Incidence of Cowbird Parasitism of Least Bell's Vireos Kus, Barbara; Sharp, Bryan L. 2006 fact sheet
Figure 3 Superior Ready Mix Mission Gorge Quarry Site LBVI Survey Areas other
FINAL Baseline Biodiversity Survey for the San Luis Rey River Park 2011 report
Habitat Management Plan for the Rancho La Costa Habitat Conservation Area 2005 report
Least Bell's Vireo Response to Kuroshio Shot Hole Borer/Fusarium Dieback at the Tijuana River, California - 2017 Data Summary Howell, Scarlett L.; Kus, Barbara 2018 report
Least Bell's Vireo Survey Guidelines 2001 protocol
Least Bell's Vireo Surveys and Nest Monitoring at Anza Borrego Desert State Park in 2000 Kus, Barbara; Wells, Jeffery M. 2001 report
Least Bell's Vireo Surveys at Selected Drainages in San Diego County, California Kus, Barbara; Lynn, Suellen 2008 report
Least Bell's Vireo Surveys on the San Luis Rey River, College Boulevard in Oceanside to Interstate 15 in Fallbrook, San Diego County, California Kus, Barbara; Lynn, Suellen 2008 report
Least Bell's Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Pr Ferree, Kimberly; Kus, Barbara 2008 report
Least Bell's Vireos and Southwestern Willow Flycatchers at the San Luis Rey River Flood Control Proj Kus, Barbara; Ferree, Kimberly 2008 report
Modeling Least Bell’s Vireo Habitat Suitability in Current and Historic Ranges in California Preston, Kris; Kus, Barbara; Perkins, Emily 2021 report
Modeling Least Bell’s Vireo Habitat Suitability in Current and Historic Ranges in California Preston, Kris; Kus, Barbara; Perkins, Emily 2021 report
Pilgrim Creek Restoration Project: Bird Community and Vegetation Structure Kus, Barbara; Peterson, Bonnie L.; Beck, Peter P. 2009 report
Quarry Creek Preserve Management Plan 2011 report
Recording - July 2020 SDMMP Management and Monitoring Coordination Meeting 2020 other
Recording - July 2020 SDMMP Management and Monitoring Coordination Meeting Preston, Kris; Gillespie, Breahna 2020 recording
State of the Regional Preserve System in Western San Diego County Preston, Kris; Perkins, Emily; Brown, Chris; McCutcheon, Sarah; Bernabe, Annabelle; Luciani, Emilie; Kus, Barbara; Wynn, Susan 2022 report
Surveys for the Least Bell's Vireo and Southwestern Willow Flycatcher at the San Luis Rey River Kus, Barbara; Peterson, Bonnie L.; Wellik, Mike J. 2002 report
Ten Years of Riparian Bird Monitoring Along the Otay River Clark, Kevin 2020 powerpoint presentation

Current Distribution Rangewide

The historic breeding range was centered in the San Joaquin, Sacramento and Central Valleys and extended south to Baja California. Despite a large increase in overall population numbers over the last few decades, most nesting occurs from Santa Barbara County southward and from northwestern Baja California south to at least Cataviña, while the vireo is rare in the northern part of its historic range. Southern California supports 99% of the total population, with 54% in San Diego County and 30% in Riverside County [1]. The range during the nonbreeding season includes the Cape region of Baja California, with stragglers in southern California [2; cited from 3].

Known Populations in San Diego County

Occurrences found in Anza Borrego Desert State Park, San Felipe Valley Wildlife Area, Bureau of Land Management, Barrett Reservoir Open Space, Cleveland National Forest, Tijuana River Valley Regional Park, Mission Trails Regional Park, El Capitan Reservoir Open Space, City of Escondido Open Space, San Luis Rey River Park, Kit Carson Park, Escondido Creek Preserve, Hodges Reservoir Open Space, Encinitas Creek, San Dieguito River Park, Santa Margarita River Park, 4-S Ranch Specific Plan Habitat Management, Los Jilgueros Preserve, Rosemary's Mountain, Otay Lakes Cornerstone Lands, Rancho Jamul Ecological Reserve, San Diego National Wildlife Refuge, Otay Ranch Preserve, Flood Control Channel Southern Wildlife Preserve, SD River Corp. City of Oceanside Open Space, Rainbow Water District, San Diego Revier Ecological Reserve, Rincon San Luiseno Band of Mission Indians, Groves Open Space, San Diego Metropolitan Transit Development, Faubus Farms, Pilgrim Creek Ecological Reserve, City of Carlsbad Municipal Golf Course, Wilmont and Morro Hills, Santa Margarita Preserve, Torrey Pines State NAtural Reserve, Border Field State Park, Hodges Reservoir Open Space, Sweetwater Reservoir Open Space, Buena Vista Creek Ecological Reserve, Los Penasquitos Canyon Preserve, Lilca Ranch, Mount Miguel Open Space, McGinty Mountain Ecological Reserve, City of Chula Vista Central City Preserve, Tijuana Slough Natuinal Wildlife Refuge, Otay Valley Regional Park, San Vicente Reservoir Cornerstone Lands, Pamo Valley, Kit Carson Park (Refer to MOM for more sites).

List Status


Habitat Affinities

Dependent upon riparian habitat during breeding season. Prefers willow-dominated woodland or scrub that typically exists along streams and rivers. Other habitat types used include Baccharis scrub, mixed oak/willow woodland, mesquite woodland, and elderberry scrub. Habitat characteristics that appear to be essential include dense cover from 1-2 meters in height for nesting and foraging, and a stratified canopy providing both foraging habitat and song perches for territorial advertisement [4]. Not limited in winter to willow-dominated riparian areas. Occupy a variety of habitats including mesquite scrub within arroyos, palm groves, and hedgerows bordering agricultural and residential areas [5,6]. During migration, uses coastal scrub, riparian, and other woodland habitats [7; cited from 5]. Elevational extremes of -54 m in Death Valley to 1260 m in Bishop, Inyo County [8; cited from 5].

Taxonomy and Genetics

No changes to taxonomic classification or accepted nomenclature have been published or proposed since listing [1]. Originally thought to be one of four subspecies of Bell's Vireo recognized by the American Ornithologist's Union [AOU 1957; cited from 5], recent research suggests they are not one species with four subspecies, but two species, each with two subspecies. Results support distinctiveness of the Least Bell’s Vireo as a taxon and its recognition as state and federally endangered [9].

Seasonal Activity

Typically arrive on breeding grounds in southern California in mid-March through May, with majority of birds arriving during the latter half of April. In San Diego County, majority of population breeds along San Luis Rey and Santa Margarita rivers [10]. Nesting lasts from early April through July, but adults and juveniles remain on breeding grounds into late September/early October before migrating to wintering grounds in southern Baja California, Mexico [4,11]. Occasionally occur in California in the winter [5].

Life History/Reproduction

Males vocally conspicuous and sing throughout breeding season from exposed perches. Females arrive approximately 1-2 weeks after males, are more secretive, and often seen early in the season traveling through habitat with the male [11]. Typically breed in their first year as adults; breed in dense, low, early successional vegetation where they lay eggs at one-day intervals with three to four eggs per clutch, incubate for about 14 days, and fledge young about 10–12 days later [4,5,12]. Both sexes feed and brood nestlings. Fledged young may be cared for by both parents, or, if the pair renests, primarily by the male [5]. Monogamous, though may switch mates between successive nesting attempts within the same season and between years (serial monogamy) [5;6]. Will attempt to renest following unsuccessful nests and will occasionally raise two broods in the same season [4]. Have an open-cup nest placed in the horizontal fork of a tree or shrub branch and bound at the rim. Nests typically constructed of soft plant strips and shreds, leaf fragments, small pieces of bark, spider webs, and other material. Usually lined with soft substances such as plant down or hair [13; cited from 5].

Diet and Foraging

Primarily insectivorous; takes a wide variety of prey species, including caterpillars, beetles, bugs, moths, grasshoppers [14 cited from 4;13 and 15 cited from 5] and small spiders in the breeding season [12]. Obtain prey primarily by foliage gleaning and hovering, with occasional use of hawking (pursuit and capture of flying prey) and clinging (hovering but with the feet in contact with the vegetation) [5]. Individuals may forage in woodlands or scrub habitat near nesting habitat, concentrated in lower to mid-canopies, especially when actively nesting [12,14; cited from 6]. Also forage in upland vegetation adjacent to riparian corridors particularly late in the season [16,17,18; cited from 6].


On average, 20% of first-time breeders dispersed away from natal drainages, with a higher proportion of males (22%) than females (13%) dispersing [Kus unpubl. data;5]. Site fidelity is high where they may place nests in the same shrub used in the previous year [18, Kus unpubl. data; cited from 5].


Threatened by loss of riparian habitat due to increased urbanization, agricultural practices, and invasive plant species. Nests also parasitized by the Brown-headed Cowbird (Molothrus aster) [1,5,6,19].

Literature Sources

[1] U.S. Fish and Wildlife Service. 2006. Least Bell’s vireo (Vireo bellii pusillus) 5-year review: Summary and evaluation. Prepared by the Carlsbad Fish and Wildlife Office, Carlsbad, California.

[2] Ehrlich, P. R., D. S. Dobkin, and D. Wheye. 1992. Birds in jeopardy: the imperiled and extinct birds of the United States and Canada including Hawaii and Puerto Rico.

[3] NatureServe. 2015. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia. Available from: Accessed: October 12, 2016.

[4] Wells, J. M., and B. E. Kus. 2001. Least Bell's Vireo surveys and nest monitoring at Anza Borrego Desert State Park in 2000.

[5] Kus, B. E. 2002. Least Bell's Vireo (Vireo bellii pusillus). In The Riparian Bird Conservation Plan: a strategy for reversing the decline of riparian-associated birds in California. California Partners in Flight. Accessed October 5, 2016.

[6] DUDEK. 2014. Draft Least Bell's Vireo Account; Draft DRECP and EIR/EIS – Appendix Q, Baseline Biology Report; Desert Renewable Energy Conservation Plan and Environmental impact Report/Environmental Impact Statement

[7] Brown, B. T. 1993. Bell's Vireo: Vireo Bellii. American Ornithologists' Union.

[8] Grinnell, J. and A. Miller. 1944. The distribution of the birds of California. Pacific Coast Avifauna Nr. 27. Museum of Vertebrate Zoology. University of California, Berkeley.

[9] Klicka, L. B., B. E. Kus, and K. J. Burns. 2016. Conservation genomics reveals multiple evolutionary units within Bell’s Vireo (Vireo bellii). Conservation Genetics 17 (2): 455-471.

[10] Blundell, M. A. and B. E. Kus. 2011. First record of interspecific breeding of Least Bell's Vireo and White-eyed Vireo. The Wilson Journal of Ornithology 123 (3): 628-631.

[11] RECON Environmental, Inc. 2008. Least Bell's Vireos and Southwestern Willow Flycatchers at the San Luis Rey River Flood Control Project Area in San Diego County, California: Breeding Activities and Habitat Use. Prepared by the US Geological Survey Western Ecological Research Center, San Diego, California.

[12] Kus, B. E., S. L. Hopp, R. R. Johnson and B. T. Brown. 2010. Bell's Vireo (Vireo bellii), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology. Retrieved from the Birds of North America Online: bna/species/035. Accessed July 2011."

[13] Bent, A. C. 1950. Life histories of North American wagtails, shrikes, vireos, and their allies. Vol. 197. Courier Corporation.

[14] US Fish and Wildlife Service. 1998. Draft Recovery Plan for Least Bell’s Vireo. US Fish and Wildlife Service. Portland, Oregon. Prepared by the Portland Fish and Wildlife Office, Portland, Oregon.

[15] Chapin, E. A. 1925. Food habits of the vireos.

[16] Kus, B. E. and K. L. Miner. 1989. Use of Non-Riparian Habitats by Least Bell's Vireos.

[17] Gray, M. V. and J. M. Greaves. 1984. Riparian forest as habitat for the least Bell's vireo. California Riparian Systems: Ecology, Conservation and Productive Management, R. Warner and K. Hendrix, eds., Univ. California Press, Davis, California.

[18] Salata, L. R. 1983. Status of the Least Bell's Vireo on Camp Pendleton, California: research done in 1983. Final Report, U.S. Fish and Wildlife Service, Laguna Niguel.

[19] Patten, M. A. "LEAST BELL'S VIREO.