Standing in a northern Everglades marsh in 2015, biologist Corey Callaghan watched as an enormous flock of tree swallows wheeled in the morning sun. As the mass of birds passed overhead, Callaghan and his partner stood in astonishment. Just how many tree swallows were in the flock, he wondered—for that matter, how many birds are in the entire world?
“It was an awesome experience,” Callaghan says. Inspired, he began by counting the birds in the flock he had just witnessed: more than half a million. He came up with that number by taking photographs, counting birds in different segments of the image, and scaling up.
Counting all the world’s birds would be much trickier, for obvious reasons, but years later Callaghan nevertheless set out to be the first to come up with a hard number—or at least a plausible range. In a new paper, he and two other researchers at the University of New South Wales in Sydney, Australia, estimate there are likely between 50 billion and 428 billion birds on Earth.
This range is so broad due to various uncertainties—among them, the difficulty of counting billions of small animals that can fly, the wide and often unclear ranges over which birds roam, and a lack of scientific data in many areas of the world.
The study, published May 17 in the Proceedings of the National Academy of Sciences, uses a unique methodology that combines data gathered by both professional science organizations and citizen scientists, and covers 92 percent of all avian species in the world.
The study is the first attempt to estimate the world population of birds, species by species. For Callaghan, the effort is overdue: “We spend a great deal of time and effort counting humans, but we need to be sure we are keeping tabs on all the biodiversity we share planet earth with.”
Many sparrows, few rarities
According to the paper, the world’s most abundant bird is the familiar house sparrow, with a population of 1.6 billion. Coming in second is the European starling (1.3 billion), followed by ring-billed gulls (1.2 billion), barn swallows (1.1 billion), glaucous gulls (949 million), and alder flycatchers (896 million).
The scientists weren’t surprised to find only a few super-abundant species, and many more rare ones, as is the common pattern in ecology. Overall, the scientists estimate that 1,180 bird species—12 percent of the world’s total—each have a total population below 5,000.
If a species has a total population under 2,500, the International Union for Conservation of Nature would label it an endangered species.
These rarities include everything from the great spotted kiwi (estimated population: 377 individuals) to the Javan hawk eagle (630) and the Seychelles kestrel (under 100). As for the tree swallows that helped pique Callaghan’s curiosity, there are around 24 million of them, as he learned in doing the study.
For comparison, the estimated world population of domestic chickens is somewhere around 25 billion, making them the most abundant bird by far. But this study only looked at wild birds.
It’s unclear how many birds the world has lost in the past few decades, but this study helps provide an estimate to establish a baseline. One 2019 paper calculated that the total population of breeding adult birds in North American has declined by 3 billion since 1970.
The novelty of this study is the way it combines professional and citizen science data, says Lucas DeGroote, a researcher at the Carnegie Museum of Natural History’s Powdermill Avian Research Center.
“It's really ambitious—it's a big undertaking to try and figure out how many birds there are in the world,” DeGroote says. “They thought really deeply about it, and took as many steps as possible to make it as precise as possible.”
The researchers used estimates from three datasets produced by experts around the world for the scientific organizations Partners in Flight, the British Trust for Ornithology, and BirdLife International. They combined those data with observations from eBird, the world’s largest database collected by citizen scientists—in this case, amateur birdwatchers.
The scientists found that in many cases, the density and population estimates gleaned by the professionals and the citizen scientists were relatively similar. They then estimated population sizes for other species, some of which lacked wide-ranging professional data, by inputting information from eBird into their computer model.
The researchers are the first to admit there is a lot of uncertainty inherent in their estimates. But part of the study’s strength is that it quantifies this uncertainty, and provides wide ranges of possible populations for thousands of birds, says Thomas Brooks, chief scientist with the International Union for Conservation of Nature, who wasn’t involved in the paper.
Ken Rosenberg, a conservation scientist at the Cornell Lab of Ornithology, thought the study was “a bold attempt.” But he urges caution in interpreting the data, because there is so much variability and uncertainty in the estimates.
“It's hard to put any stock in the individual numbers for each species,” Rosenberg says, let alone the global estimates: “They sort of threw down the gauntlet [to other researchers]—if you don’t like this number, come up with a better one.”
For Brooks, the paper illustrates just how precious many bird species are, and how close to the brink they could come to the brink if new threats emerge.
“It does tell us about the fragility of nature—we need to keep a close eye on the environment and our impact on it,” Brooks says.
DeGroote concurs. “To do conservation, we have to know how many [of a species] there are and what the trend is. This is a great tool for measuring populations in the future.”