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Bird Species Abundances, from Biggest to Smallest

A new study found large species are rare, but those with fewer individuals are abundant

Chart shows global abundance estimates for four bird species.

Jen Christiansen (graphic), Liz Wahid (birds)


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Comparing the relative sizes of bird species has long seemed an impossible task—too many species simply lack reliable counts. A recent influx of citizen science data, however, allowed researchers to make global abundance estimates for 9,700 species, about 92 percent of all birds on Earth. Biologists Corey T. Callaghan, Shinichi Nakagawa and William K. Cornwell, all at the University of New South Wales in Australia, combined scientific data for 724 well-studied species with counts from the app eBird, where people around the world can submit bird sightings. The researchers used an algorithm to extrapolate estimates for all species in their sample. The results, published recently in the Proceedings of the National Academy of Sciences USA, confirm a common pattern among animals: across the globe there are many species with small populations isolated in niche habitats and relatively few species that have managed to expand over a wide territory and grow their population into the hundreds of millions or billions. Eventually the findings could help with conservation efforts. “The next step is, Which species are rare because that's just the way Mother Nature made them, and which species are rare because we [humans] screwed up?” Callaghan asks. This project did not try to answer these questions, but it is a “necessary first step” toward doing so, he says.

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Credit: Jen Christiansen (graphic), Liz Wahid (birds); Source: “Global Abundance Estimates for 9,700 Bird Species,” by Corey T. Callaghan, Shinichi Nakagawa and William K. Cornwell, in PNAS, Vol. 118, May 25, 2021 (data)

Clara Moskowitz is a senior editor at Scientific American, where she covers astronomy, space, physics and mathematics. She has been at Scientific American for a decade; previously she worked at Space.com. Moskowitz has reported live from rocket launches, space shuttle liftoffs and landings, suborbital spaceflight training, mountaintop observatories, and more. She has a bachelor's degree in astronomy and physics from Wesleyan University and a graduate degree in science communication from the University of California, Santa Cruz.

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Jen Christiansen is author of the book Building Science Graphics: An Illustrated Guide to Communicating Science through Diagrams and Visualizations (CRC Press) and senior graphics editor at Scientific American, where she art directs and produces illustrated explanatory diagrams and data visualizations. In 1996 she began her publishing career in New York City at Scientific American. Subsequently she moved to Washington, D.C., to join the staff of National Geographic (first as an assistant art director–researcher hybrid and then as a designer), spent four years as a freelance science communicator and returned to Scientific American in 2007. Christiansen presents and writes on topics ranging from reconciling her love for art and science to her quest to learn more about the pulsar chart on the cover of Joy Division's album Unknown Pleasures. She holds a graduate certificate in science communication from the University of California, Santa Cruz, and a B.A. in geology and studio art from Smith College. Follow Christiansen on X (formerly Twitter) @ChristiansenJen

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Liz Wahid is a science illustrator with a focus on wildlife conservation and ornithology. You can find her Web site at OrnithArt.com.

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Scientific American Magazine Vol 325 Issue 2This article was originally published with the title “Counting Birds” in Scientific American Magazine Vol. 325 No. 2 (), p. 84
doi:10.1038/scientificamerican0821-84