There Are More Than 50 Billion Birds in the World

Citizen scientist data was key in new study data.

Three house sparrows perching on fence
House sparrows are part of the 'billion club'. 10kPhotography / Getty Images

Everyone better stock up on bird feed.

There are around 50 billion birds on the planet, according to a new study from Australian researchers. That works out to about six birds for every person on Earth.

Australian researchers tallied the feathered fliers with help from citizen scientists and detailed algorithms.

“We spend a great deal of time and effort counting humans (i.e., human censuses) - but we need to be sure we are keeping tabs on all the biodiversity we share planet earth with,” lead author Corey Callaghan, who completed the research while he was a postdoctoral researcher at University of New South Wales (UNSW) Sidney, tells Treehugger. “Of course, this is difficult, and expensive. We show the potential for using global citizen science datasets to achieve this goal!!” 

Researchers started with best-available estimates for about 700 species. They integrated that information with citizen science data from eBird, an online database that includes nearly 1 billion observations around the world.

“Through this statistical integration, we were able to predict the expected density for the species that we don't have good 'best-available estimates' for,” Callaghan says.

“It took a while, in part because our best-available estimates were biased towards North America and Europe. And we went through many iterations to try and find the best approach," Callaghan adds. "But our goal was to make sure we estimated the uncertainty surrounding each estimate as well, which also took some careful thought. We relied heavily on citizen science data and citizen science observers to extrapolate our estimates to many parts of the world.”

The calculations tried to account for what’s known as each species’ "detectability." That’s the likelihood that each bird species is actually detected and the sighting is submitted.

“So in simplest terms, if there is an Ostrich 5 meters from you, there is a high likelihood you will 'detect' it. But in contrast, a small songbird in bush might not be detected,” Callaghan explains. “We tried to account for some of this in our methods by including traits such as body size and bird color (e.g., brightness of a bird).”

The results were published in the journal Proceedings of the National Academy of Sciences.

The Billion Club of Birds

Researchers found that only four bird species belonged to what they dubbed “the billion club”: species with an estimated population of more than a billion members. That includes the house sparrow (1.6 billion), the European starling (1.3 billion), the ring-billed gull (1.2 billion), and the barn swallow (1.1 billion).

“The question of ‘why’ these birds are the most abundant is still up for debate. In part, they have large ranges, and for the European Starling and House Sparrow, they have been introduced in many parts of the world and are super successful invaders,” Callaghan says. “So it probably has something to do with a generalist life history and a wide niche breadth. But this is the focus of much research.”

The study data includes records for almost all bird species (92%) that currently exist. Researchers say that it’s unlikely that the remaining 8% would have much of an impact on the final numbers.

Those species are mostly potentially extinct or presumed extinct species, as well as “sensitive species” which are facing threats, and sometimes their locations are not made available to researchers, and species in some areas where there just wasn’t enough data from eBird.

“This is the first stab at something of this magnitude, and admittedly there is some uncertainty involved in the process. So we are likely ‘off’ on some species, but probably pretty close on other species. But our overall estimate and finding that there are many common species is probably relatively accurate,” Callaghan says. 

“But hopefully, as more data continue to be collected, the process can be repeated and updated to better understand the absolute abundance of birds in the world," Callaghan adds. "So I really hope (and think) that going forward, citizen science data will play an important role in biodiversity monitoring at local, to regional, to global scales. We just need to understand how best to use all these data, and that is what this study tries to do.” 

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  1. Callaghan, Corey T., et al. "Global Abundance Estimates for 9,700 Bird Species." Proceedings of the National Academy of Sciences, vol. 118, no. 21, 2021, p. e2023170118, doi:10.1073/pnas.2023170118