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Study: 4.7 billion birds leave the U.S. each fall

Bird Migration
Radar technology has helped put a more precise number on the birds migrating through the United States each fall and spring. Illustration by Jillian Ditner/Cornell Lab of Ornithology

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Using cloud computing and data from 143 weather radar stations across the continental United States, Cornell Lab of Ornithology researchers recently estimated how many birds migrate through the U.S. and the toll that winter and the birds’ migratory journeys take.

“We’ve discovered that each autumn, an average of 4 billion birds move south from Canada into the U.S. At the same time, another 4.7 billion birds leave the U.S. over the southern border, heading to the tropics,” notes lead author Adriaan Dokter, an Edward W. Rose postdoctoral fellow at the Cornell Lab. “In the spring, 3.5 billion birds cross back into the U.S. from points south, and 2.6 billion birds return to Canada across the northern U.S. border.”

In other words, fewer birds return to their breeding grounds after going through fall migration and spending months on their wintering grounds. But the researchers, writing in Nature, Ecology & Evolution, were surprised to find that the migrants arriving across the U.S. southern border had an average return rate of 76 percent during the 5 years of the study (2013 to 2017) and the birds wintering in the U.S. had only an average return rate of 64 percent.

“Contrary to popular thought, birds wintering in the tropics survive the winter better than birds wintering in the U.S.,” says Andrew Farnsworth, co-author of the study and leader of the Cornell Lab’s aeroecology program. “That’s despite the fact that tropical wintering birds migrate three to four times farther than the birds staying in the U.S.”


To reach these numbers, the researchers developed complicated algorithms to measure differences in biomass picked up by weather radar — in this case, the total mass of organisms in a given area, minus insects and weather. The same technology powers the Lab’s website, which presents real-time migration maps.


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