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.
Rosenberg et al. (2016) estimated the population at 5.9 million mature individuals. The California subspecies now numbers approximately 3,000 territorial males (Kus et al. 2010).
This species is thought to be increasing in North America. Taken from long-term trends (Christmas Bird Count data for 1966-2017), data suggest an annual increase of 2.61% (0.47% annual decline to 6.88% annual increase) in this species. This would equate to an increase of 37.31% (5.67% decrease to 126.57% increase) over three generations (T. Meehan in litt. 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).
Text account compilers
Hermes, C., Benstead, P., Bird, J., Harding, M., Martin, R., Sharpe, C.J., Westrip, J.
Rosenberg, K., Butcher, G., Meehan, T., McCreedy, C., Wells, J., Kus, B.E.
BirdLife International (2018) Species factsheet: Vireo bellii. Downloaded from http://www.birdlife.org on 18/12/2018. Recommended citation for factsheets for more than one species: BirdLife International (2018) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 18/12/2018.