Bell’s Vireo (Vireo bellii) is a migrant passerine, breeding across central and south-western U.S.A., and through northern Mexico; while it overwinters in southern Baja California (Mexico), along the western coast of Mexico and into Guatemala, El Salvador and Honduras, and rarely into Nicaragua (see Brewer 2018). It occurs in arid areas, particularly inhabiting dense shrub and scrubby vegetation as well as second-growth forest (see Brewer 2018).
The species has experienced a degree of habitat loss and degradation as a result of many different drivers, including agricultural and urban expansion, flood control measures, and sand and gravel extraction. Habitat modification has also allowed for the spread of Brown-headed Cowbirds (Molothrus ater), which has led to reductions in the breeding population of south-western U.S.A. (Kus et al. 2010, C. McCreedy in litt. 2016). Invasive species have also impacted the species, with invasive vegetation impacting the species’s habitat and the Polyphagus Shot Hole Borer indirectly affecting habitat suitability as this invasive weevil species farms fungi which can cause the death of host trees (Leathers 2015, B. Kus in litt. 2016). These threats are thought to have been the driver of potentially rapid declines, and as such the species is currently listed as Near Threatened under criteria A2bc+3bc+4bc (see BirdLife International 2018).
Following the publication of Partners in Flight (PiF) Landbird Conservation Plan (Rosenberg et al. 2016) and The State of North America’s Birds 2016 (North American Bird Conservation Initiative 2016) we have reviewed the new information held in these publications, particularly regarding population trends. This has allowed us to reassess the species outlined in these publications against IUCN Red List Categories and Criteria. As the PiF data are long-term trends (1970-2014), where possible we have also used data from the North American Breeding Bird Survey (Sauer et al. 2017) to assess more recent trends over the period relevant to the Red List. Having completed this review, Bell’s Vireo appears to warrant a change in Red List status. Therefore, we present here our reassessment against all criteria for the species.
Criterion A – Rosenberg et al. (2016) suggest that between 1970 and 2014 the species underwent an increase of 38%. Short term data from Sauer et al. (2017) show a significant annual increase of 2.63% (1.15-4.29%) between 2005 and 2015, which would equate to a 37.6% increase (1.51-67.6%) over three generations (12.3 years). The species’s trend may have even been stable since 1980 (Sauer et al. 2008 per Kus et al. 2010). The U.S. contains c.76% of the population, and so the Mexican population would have to have undergone extremely rapid declines over the same period for the species to at least approach the threshold for Vulnerable, and this is not considered to be likely. Therefore, the species does not warrant listing under this criterion.
Criterion B – The species’s range is far too large to warrant listing under this criterion (Extent of Occurrence [breeding/resident] = 4,930,000km2; Extent of Occurrence [non-breeding] = 1,380,000km2).
Criterion C – Rosenberg et al. (2016) estimate the global population size to be 5,900,000 mature individuals. This is far too large to warrant listing under this criterion.
Criterion D – The species’s population size and range are far too large to warrant listing under this criterion.
Criterion E – To the best of our knowledge there has been no quantitative analysis of extinction risk conducted for this species. Therefore, it cannot be assessed against this criterion.
Therefore, Bell’s Vireo does not approach the threshold for Vulnerable under any criteria and as such would warrant listing as Least Concern. We welcome any comments or further information but please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of the species’ Red List status. Therefore, please make sure your comments are about the proposed listing.
BirdLife International. 2018. Species factsheet: Vireo bellii. Downloaded from http://www.birdlife.org on 13/03/2018.
Brewer, D. 2018. Bell’s Vireo (Vireo bellii). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/61250 on 12 March 2018).
Kus, B.; Hopp, S. L.; Johnson, R. R.; Brown, B. T. 2010. Bell’s Vireo (Vireo bellii), version 2.0. In: Rodewald, P. G. (ed.). The Birds of North America. Cornell Lab of Ornithology, Ithaca, New York, U.S.A. https://doi.org/10.2173/bna.35.
Leathers, J. 2015. EUWALLACEA SP. NR. FORNICATUS: POLYPHAGOUS SHOT HOLE BORER (PSHB). Available at: http://blogs.cdfa.ca.gov/Section3162/?tag=polyphagous-shot-hole-borer. (Accessed: 03/02/2017).
North American Bird Conservation Initiative. 2016. The State of North America’s Birds 2016. Environment and Climate Change Canada, Ottawa, Ontario.
Rosenberg, K. V., Kennedy, J. A., Dettmers, R., Ford, R. P., Reynolds, D., Alexander, J. D., Beardmore, C. J., Blancher, P. J., Bogart, R. E., Butcher, G. S., Camfield, A. F., Couturier, A., Demarest, D. W., Easton, W. E., Giocomo, J. J., Keller, R. H., Mini, A. E., Panjabi, A. O., Pashley, D. N., Rich, T. D., Ruth, J. M., Stabins, H., Stanton, J. and Will., T. 2016. Partners in Flight Landbird Conservation Plan: 2016 Revision for Canada and Continental United States. Partners in Flight Science Committee.
Sauer, J. R.; Hines J. E.; Fallon, J. 2008. The North American Breeding Bird Survey results and analysis 1966-2007. Version 5.15.2008a. USGS Patuxent Wildl. Res. Center.
Sauer, J. R.; Niven, D. K. ; Hines, J. E.; Ziolkowski, Jr, D. J.; Pardieck, K. L.; Fallon, J. E.; Link, W. A. 2017. The North American Breeding Bird Survey, Results and Analysis 1966 – 2015. Version 2.07.2017 USGS Patuxent Wildlife Research Center, Laurel, MD.