According to a recent study*, there are about 50 billion living birds in the world, six times as many as there are people. It was not easy to come up with that figure but the diagram below gives a graphic representation of how the researchers did it. It’s not easy to understand, but basically, using eBird data, accurate censuses of birds in a given region, comparing relative abundances of birds, and extrapolating from there, the authors came up with 50 billion (a previous study in 1997 estimated between 200 and 400 billion).
A number of bird species in this study have small populations: 1180 species, 12% of the approximately 10,000 species of birds in the world, have population estimates of fewer than 5000 individuals. The Great Spotted Kiwi, for example, numbers 377 individuals. Relatively few species are very abundant. “The top 10 most abundant birds in the world, and their approximate global population estimates, are House Sparrow (1.6 billion), European Starling (1.3 billion), Ring-billed Gull (1.2 billion), Barn Swallow (1.1 billion), Glaucous Gull (949 million), Alder Flycatcher (896 million), Black-legged Kittiwake (815 million), Horned Lark (771 million), Sooty Tern (711 million), and Savannah Sparrow (599 million).”
There are of course, problems with such a study. Seabirds, for example, are colonial nesters and often breed in immense flocks on remote islands and rarely seen by birders at that time. Conversely, shorebirds might be seen in large flocks during their migratory stops. Some birds have large ranges, some very narrow. And eBird data is not homogeneous or consistent throughout the world.
The study, whether very or vaguely accurate, points out some interesting fundamental questions. For example, what are the population dynamics of a particular species in space and time? How is a species’ abundance related to its life history? How is abundance influenced by anthropogenic habitat changes? Which species, genera, families, or orders are most worthy of protection?
Many scientific studies begin with the collection of data on which to base future observations. Sort of like a control group. To know if bird populations are actually decreasing or increasing, we have to know what the history of their population fluctuations are and why their population sizes fluctuate. This study attempts to do that.
What most surprised me about this study is the list of the 10 most abundant birds. Barn Swallow I can believe because I see them everywhere and they have a global distribution. Same with Starlings and House Sparrows, although I’ve spotted fewer of them in recent years. Gulls, kittiwakes, and tern population numbers I accept because I know they have large colonies in inaccessible areas of the world. But the Alder Flycatcher, found only in North America, nesting farther north than any other flycatcher of the family Tyrannidae, has a population of 896 million? Hard to believe. And the Horned Lark, a bird I see a lot less than I used to, has a population that numbers 771 million? Really?
*Corey T. Callaghan, Shinichi Nakagawa, William K. Cornwell. Global abundance estimates for 9,700 bird species. Proceedings of the National Academy of Sciences, 2021; 118 (21): e2023170118 DOI: 10.1073/pnas.2023170118