Scientists have estimated that there are 50 billion individual birds in the world, or about six birds for every human, in an analysis that represented 92% of known species and could help to refine conservation efforts.
Compiling amateur observations and expert estimates from around the globe, the researchers found that there are many more rare bird species than common ones. The team reported the findings Monday in Proceedings of the National Academy of Sciences.
"Mother Nature just loves rare species," said Corey Callaghan, a postdoctoral research fellow in global change ecology at the German Centre for Integrative Biodiversity Research and first author of the paper. "This is not a terribly surprising pattern, but to see it so strongly at the global scale is what's really novel here."
Being able to quantify how abundance differs among birds in different families and locations is important for preservation efforts and can help scientists understand how these avian communities evolved. However, most knowledge of bird populations is drawn from local-scale observations. One 1997 study put the world's overall bird population at between 200 billion and 400 billion but didn't provide estimates for different species.
In contrast, Callaghan and his team wanted to investigate the abundance of different types of birds around the world. They collected reports of 9,700 species from eBird, an online database that includes more than 800 million observations from volunteer birdwatchers. These reports were submitted from 2010-19.
For 724 of the species, the researchers were able to find population estimates from experts at organizations such as the British Trust for Ornithology. Callaghan and his colleagues compared the expert data with the birdwatchers' observations for those species to "train" computer models to estimate the populations of the rest of the species.
While acknowledging that their estimate is an imperfect one, the researchers arrived at a total of about 50 billion birds on the planet. This figure represented the midpoint of a range of estimates, the team noted in the paper, while the average estimate was 428 billion individual birds in the world. Callaghan said that when working with such a skewed distribution, the midpoint is a better measure of central tendency.
"It was nice to see that we're in the same ballpark as the previous estimates," Callaghan said.
The 8% of species for which eBird did not provide birdwatcher observations are generally very rare and probably wouldn't have shifted the estimates much, he added.
In line with what scientists have suspected, there were relatively few species with large populations. About 12% of species the researchers examined had populations of fewer than 5,000 individuals.
The most abundant species was the house sparrow, with an estimated population of 1.6 billion, followed by the European starling, ring-billed gull, barn swallow, glaucous gull, alder flycatcher, black-legged kittiwake, horned lark, sooty tern and Savannah sparrow.
There were an impressive 28 billion perching birds, which makes sense, given that this group encompasses more than half of all known bird species. By contrast, the least abundant orders were mesites, which include three species of ground-dwelling birds endemic to Madagascar, and kiwis, a group of five species found only in New Zealand.
The researchers also found that omnivores and birds that dined on insects and other invertebrates were more abundant than scavengers and nectar-sipping birds.
Intriguingly, the proportions of rare and common species were pretty similar across the bird family tree.
"In a group of thrushes, you're just as likely to have really, really common birds and really, really rare species as you are in pigeons," Callaghan said.
He and his colleagues aren't sure exactly why that is. The pattern may be related to how new species often form, with hardy generalists first arriving in an area, and highly specialized species evolving over time.
"There are species that live on one mountaintop in the Philippines or one island in Indonesia," Callaghan said. "Because they have such a small range, they occur in low density."
Still, some species could also be rare because of deforestation, climate change and other human-related disturbances. Population estimates like the one Callaghan and his team created could help researchers determine which species should be prioritized for conservation.
In this study, countries with extensive track records of bird monitoring, such as the United States and the United Kingdom, were overrepresented in the experts reports. And eBird observations from many areas were sparse, including large parts of Africa and Siberia.
"Right now, we have pretty high uncertainty in our estimates," Callaghan said. "I wouldn't say that what we produced and published is conservation-ready; it's one step in that direction."
Still, he says, future estimates — for birds as well as other kinds of animals — will benefit from the increasingly plentiful data collected by amateur naturalists.
"Those observations are growing everywhere in the world, and they're just going to get better and better," Callaghan said. "You're talking about hundreds of millions of observations that ecologists did not have 10 years ago."
The study, "Global abundance estimates for 9,700 bird species," published May 17 in Proceedings of the National Academy of Sciences, was authored by Corey T. Callaghan, German Centre for Integrative Biodiversity Research; and Shinichi Nakagawa and William K. Cornwell, UNSW Sydney.