Vireo bellii 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Aves Passeriformes Vireonidae

Scientific Name: Vireo bellii Audubon, 1844
Common Name(s):
English Bell's Vireo
Taxonomic Source(s): del Hoyo, J., Collar, N.J., Christie, D.A., Elliott, A., Fishpool, L.D.C., Boesman, P. and Kirwan, G.M. 2016. HBW and BirdLife International Illustrated Checklist of the Birds of the World. Volume 2: Passerines. Lynx Edicions and BirdLife International, Barcelona, Spain and Cambridge, UK.

Assessment Information [top]

Red List Category & Criteria: Near Threatened ver 3.1
Year Published: 2017
Date Assessed: 2016-10-01
Assessor(s): BirdLife International
Reviewer(s): Butchart, S. & Symes, A.
Contributor(s): Butcher, G., Rosenberg, K., Wells, J., McCreedy, C. & Kus, B.E.
Facilitator/Compiler(s): Benstead, P., Bird, J., Harding, M., Sharpe, C.J., Martin, R
This species has undergone moderately rapid declines across its breeding range, hence is considered to be Near Threatened.

Previously published Red List assessments:

Geographic Range [top]

Range Description:Vireo bellii is a migrant, breeding in central and south-western USA and northern Mexico and wintering from south Baja California along the west coast of Central America, through Mexico, El Salvador, Guatemala, Nicaragua and Honduras (Brown 1993). Across the U.S. the population declined at an average rate of 2.7% annually between 1966 and 2007, but recent trend estimates indicate that the population may have stabilised (Kus et al. 2010, Sauer et al. 2014). Declines have occurred in many U.S. states, with Breeding Bird Survey (BBS) data recording significant declines in Arizona, Oklahoma and the central BBS area (Kus et al. 2010). In California, the population of the subspecies V. belli pusillus has recovered strongly following intensive conservation action, from around 300 territorial males in 1986 to 3,000 in 2006 (Kus et al. 2010).
Countries occurrence:
El Salvador; Guatemala; Honduras; Mexico; Nicaragua; United States
Additional data:
Continuing decline in area of occupancy (AOO):Unknown
Extreme fluctuations in area of occupancy (AOO):NoEstimated extent of occurrence (EOO) - km2:1380000
Continuing decline in extent of occurrence (EOO):UnknownExtreme fluctuations in extent of occurrence (EOO):No
Continuing decline in number of locations:Unknown
Extreme fluctuations in the number of locations:No
Upper elevation limit (metres):2000
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:Rosenberg et al. (2016) estimated the population at 4.5 million individuals. The California subspecies now numbers approximately 3,000 territorial males (Kus et al. 2010).

Trend Justification:  This species has undergone a large and statistically significant decrease over the last 40 years in North America (66.1% decline over 40 years, equating to a 23.7% decline per decade; data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven 2007).
Current Population Trend:Decreasing
Additional data:
Continuing decline of mature individuals:Unknown
Extreme fluctuations:NoPopulation severely fragmented:No
Continuing decline in subpopulations:Unknown
Extreme fluctuations in subpopulations:NoAll individuals in one subpopulation:No

Habitat and Ecology [top]

Habitat and Ecology:It frequents dense, low, shrubby vegetation in its breeding range where it has been extensively studied (Brown 1993). It feeds almost exclusively on invertebrates, and forages typically between three and six metres above ground level (Kus et al. 2010). 

Continuing decline in area, extent and/or quality of habitat:Unknown
Generation Length (years):4.1
Movement patterns:Full Migrant

Threats [top]

Major Threat(s): Habitat loss and modifications through agricultural spread, grazing, sand and gravel extraction, flood control structures and practices, urban development and spread of invasive vegetation have caused declines and continue to threaten the species. Secondarily, high rates of brood parasitism by Brown-headed Cowbirds (Molothrus ater), as a result of habitat structural change, have caused reductions in breeding populations in south-west USA (Kus et al. 2010, C. McCreedy in litt. 2016). A recently emerged threat is the Polyphagus Shot Hole Borer, an invasive south-east Asian weevil that farms several species of fungi to provision its larvae, one of which frequently goes on to kill the host tree, with the potential for reducing habitat suitability for Bell's Vireo (B. Kus in litt. 2016, Leathers 2015). The species was recorded amongst the many found to suffer mortality from stationary structures during migration, and an estimate of the annual mortality represented in excess of 0.5% of the total population size (Longcore et al. 2013). The actual impact on population trends is unclear, but this level of mortality may be sufficient to affect local trends.

Conservation Actions [top]

Conservation Actions: Conservation Actions Underway
Trapping cowbirds can significantly reduce brood parasitism (Kus 1999, Kus and Whitfield 2005, Kosciuch and Sandercock 2008), and is a standard management tool in California (Kus et al. 2010). Habitat restoration, largely through removal of invasive plant species and planting cuttings and nursery stock of native riparian species, along with regulatory protection of habitat has stabilised and possibly reversed the population decline in California (Kus et al. 2010). Stopping grazing along the San Pedro River in Arizona resulted in recovery of native vegetation and abundance of Bell's Vireo more than doubled (Krueper et al. 2003). 

Conservation Actions Proposed
Protect key habitats for the species, remove invasive plants and restore native vegetation within the species range (Kus et al. 2010). Increase the density and complexity of scrub patches and create supplementary, dense, habitat patches (Kus et al. 2010). Continue to monitor population trends through the BBS and ascertain current and projected rates of decline. Determine relative contributions of habitat condition and parasitism to regional declines. Improve winter distribution knowledge and investigate connectivity between wintering areas and breeding subpopulations (Kus et al. 2010). 

Amended [top]

Amended reason: New information added to Conservation Actions, Geographic Range, Population and Threats text, with updated references.

Citation: BirdLife International. 2017. Vireo bellii (amended version of 2016 assessment). The IUCN Red List of Threatened Species 2017: e.T22705156A110799620. . Downloaded on 21 April 2018.
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