• Summary
• Text account
• Data table and detailed info
• Distribution map
• Reference and further resources

Justification of Red List category
This species has an extremely large range, and hence does not approach the threshold for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km² combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population trend appears to be increasing, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely large, and hence does not approach the threshold for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.

Population justification
The global population is inferred to number 5,700,000 mature individuals (Partners in Flight 2019). The California subspecies now numbers approximately 3,000 territorial males (Kus et al. 2010).

Trend justification
The species has been increasing at an average rate of 1.1% per year between 1970 and 2017 (Partners in Flight 2019). Assuming that the increase continues at the same rate, this would equate to an increase of 12% over the past ten years. The population in North America is increasing rapidly at a rate of 29% over the past ten years (Pardieck et al. 2018).

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 increased at an average rate of 37% annually between 1966 and 2017. 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).

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). 

Habitat loss and modifications through agricultural spread, grazing, sand and gravel extraction, flood control structures and practices, urban development and spread of invasive vegetation pose threats to the species. Secondarily, high rates of brood parasitism by Brown-headed Cowbirds (Molothrus ater), as a result of habitat structural change, might 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). However, over the last 50 years, the population has increased in North America at a rate of 2.6% annually (T. Meehan in litt. 2018).

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). 

Text account compilers
Martin, R., Hermes, C.

Contributors
Benstead, P., Bird, J., Butcher, G., Harding, M., Kus, B.E., McCreedy, C., Meehan, T., Rosenberg, K., Sharpe, C.J., Wells, J. & Westrip, J.R.S.

---

Recommended citation
BirdLife International (2024) Species factsheet: Bell's Vireo Vireo bellii. Downloaded from https://datazone.birdlife.org/species/factsheet/bells-vireo-vireo-bellii on 15/12/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from https://datazone.birdlife.org/species/search on 15/12/2024.