Comparing the relative sizes of bird species has long seemed an impossible task—too many species simply lack reliable counts. A recent influx of citizen science data, however, allowed researchers to make global abundance estimates for 9,700 species, about 92 percent of all birds on Earth. Biologists Corey T. Callaghan, Shinichi Nakagawa and William K. Cornwell, all at the University of New South Wales in Australia, combined scientific data for 724 well-studied species with counts from the app eBird, where people around the world can submit bird sightings. The researchers used an algorithm to extrapolate estimates for all species in their sample. The results, published recently in the Proceedings of the National Academy of Sciences USA, confirm a common pattern among animals: across the globe there are many species with small populations isolated in niche habitats and relatively few species that have managed to expand over a wide territory and grow their population into the hundreds of millions or billions. Eventually the findings could help with conservation efforts. “The next step is, Which species are rare because that's just the way Mother Nature made them, and which species are rare because we [humans] screwed up?” Callaghan asks. This project did not try to answer these questions, but it is a “necessary first step” toward doing so, he says.

Graphic groups bird species by population size and shows circles scaled according to the number of species in each group.
Credit: Jen Christiansen (graphic), Liz Wahid (birds); Source: “Global Abundance Estimates for 9,700 Bird Species,” by Corey T. Callaghan, Shinichi Nakagawa and William K. Cornwell, in PNAS, Vol. 118, May 25, 2021 (data)