In a new study published in the Proceedings of the National Academy of Sciences, ornithologists from the Centre for Ecosystem Science and the Ecology & Evolution Research Centre at the University of New South Wales integrated data from a suite of well-studied species with global citizen science data to estimate species-specific global abundances for 9,700 bird species (92% of all living bird species), and found that there are roughly 50 billion individual birds in the world at present — about six birds for every human on the planet.
The western yellow wagtail (Motacilla flava), a small passerine bird in the family Motacillidae. Image credit: Sci-News.com.
“Humans have spent a great deal of effort counting the members of our own species — all 7.8 billion of us,” said Dr. Will Cornwell, senior author of the study.
“This is the first comprehensive effort to count a suite of other species.”
Dr. Cornwell and his colleagues reached their figures with the help of more than 600,000 citizen scientists who contributed their sightings to the eBird dataset between 2010 and 2019.
The Cornell Lab of Ornithology, who run the eBird.org site, have made the data freely available.
“Large global citizen science databases such as eBird are revolutionizing our ability to study macroecology,” Dr. Cornwell said.
“This type of data simply wasn’t available a decade ago.”
Using these data and detailed case studies where available, the researchers developed an algorithm to estimate the actual global population of each bird species.
This calculation took into account each species’ detectability — that is, how likely it is that a person will have spotted this bird and submitted the sighting to eBird.
Detectability can include factors like their size, color, whether they fly in flocks, and if they live close to cities.
“While this study focuses on birds, our large-scale data integration approach could act as a blueprint for calculating species-specific abundances for other groups of animals,” said first author Dr. Corey Callaghan, who completed the study while he was a postdoctoral researcher at the University of New South Wales and is now based at the German Centre for Integrative Biodiversity Research.
The study dataset included records for almost all (92%) living bird species.
Only four species belonged to what the scientists call the ‘billion club:’ species with an estimated global population of over a billion.
The house sparrow (1.6 billion individuals) heads this exclusive group, which also includes the European starling (1.3 billion), the ring-billed gull (1.2 billion) and the barn swallow (1.1 billion).
“It was surprising that only a few species dominate the total number of individual birds in the world,” Dr. Callaghan said.
“What is it about those species, evolutionarily, that has made them so hyper-successful?”
But while some bird populations are thriving, many others look a lot slimmer: around 12% of bird species included in the study have an estimated global population of less than 5,000.
These include species such as the Chinese crested tern, the noisy scrub-bird, and the invisible rail.
“Quantifying the abundance of a species is a crucial first step in conservation,” Dr. Callaghan said.
“By properly counting what’s out there, we learn what species might be vulnerable and can track how these patterns change over time — in other words, we can better understand our baselines.”
“A range of uncertainty is necessary when making global-level estimates,” said Professor Shinichi Nakagawa, co-author of the study.
“Our findings, while rough in some areas, represent the best-available data we currently have for many species.”
_____
Corey T. Callaghan et al. 2021. Global abundance estimates for 9,700 bird species. PNAS 118 (21): e2023170118; doi: 10.1073/pnas.2023170118
