Birds flying south. How many will return? | 9/23/2018 | EarthSky Voices
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In a new study, migratory birds arriving across the U.S. southern border each spring had an average return rate of 76 percent. Birds wintering in the U.S. had an even lower return rate – 64 percent – back into Canada.

A new study used cloud computing and data from 143 weather radar stations to estimate how many birds migrate through the United States.

Adriaan - Dokter - Fellow - Cornell - Lab

Adriaan Dokter, a postdoctoral fellow at the Cornell Lab of Ornithology, is the lead author of the study, published September 17, 2018, in the peer-reviewed journal Nature Ecology & Evolution. Dokter said in a statement from Cornell:

We’ve discovered that each autumn, an average of four 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.

Spring - Birds - Cross - US - Points

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.

Researchers knew that fewer birds return to their breeding grounds after going through fall migration and spending months on their wintering grounds. But the actual numbers surprised them. They found that migrant birds arriving across the U.S. southern border had an average return rate of 76 percent during the five years of the study (2013 to 2017).

Birds - US - Return - Rate - Canada

Birds wintering in the U.S. had an even lower return rate to Canada, only an average of 64 percent.

The study used weather radar to track the comings and goings of migratory birds.

Andrew - Farnsworth - Study - Co-author - Leader

Andrew Farnsworth is a study co-author and leader of the Cornell Lab’s aeroecology program. Farnsworth said:

Contrary to popular thought, birds wintering in the tropics survive the winter better than birds wintering in the U.S. That’s despite the fact that tropical wintering birds migrate three to four times farther than the birds staying in the U.S.

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