4.7 billion birds leave U.S. to winter in the tropics each fall
Scientists at Cornell University used a combination of cloud computing
and radar stations to tally the number of birds that leave and return to
the United States each year during fall and spring migrations.
Researchers also traced the movement of birds that winter inside the
U.S.
From 2013 to 2017, an average of 4 billion birds moved south from Canada
into the U.S. each fall, while another 4.7 billion birds crossed the
southern border, headed for the tropics. In the spring, 3.5 billion
birds flew back across the southern border. An average of 2.6 billion
returned to Canada.
The new data allowed scientists to calculate an annual return rate for
each subset of migrators. During the five-year study, 76 percent of
birds wintering in the tropics returned to the United States. Birds
wintering in the U.S. had a return rate of only 64 percent.
"Contrary to popular thought, birds wintering in the tropics survive the
winter better than birds wintering in the U.S.," Andrew Farnsworth,
leader of the aeroecology program at the Cornell Lab of Ornithology,
said in a news release. "That's despite the fact that tropical wintering
birds migrate three to four times farther than the birds staying in the
U.S."
Researchers at Cornell designed sophisticated models to analyze radar
images and differentiate between different biomasses. The algorithms
showed sparrows, American robins and dark-eyed juncos most frequently
crossed the northern border, breeding in Canada and wintering in the
U.S. Warblers, orioles and tanagers most frequently crossed the southern
border.
Sparrows, robins and juncos fly shorter migration distances, whereas warblers, orioles and tanagers migrate longer distances.
Differences in return rate suggest birds face a greater number of hazards in the U.S. during the winter.
"All birds need suitable habitats with enough resources to get them
through the winter," said Ken Rosenberg, conservation scientist at the
Cornell Lab. "Birds wintering in the U.S. may have more habitat
disturbances and more buildings to crash into, and they might not be
adapted for that."
Research suggests birds wintering in the U.S. have better reproductive
success, offsetting the increased mortality rate. Birds wintering in the
tropics produce fewer offspring, but more adults survive the winter to
reproduce again the following spring.
The new study, published Monday in the journal Nature Ecology and
Evolution, suggests the advantages of the tropics could disappear
without improved conservation efforts.
"Longer distance migrants seem to be gambling on having high survival in
the tropics, and they're therefore more sensitive to what happens to
their wintering grounds," said Adriaan Dokter, a postdoctoral fellow at
the Cornell Lab. "Even a small decrease in survival due to changes in
their tropical habitats might cause a precipitous decline."
Last week, Cornell scientists detailed their ability to predict bird
migrations up to seven days in advance by analyzing radar images.
Previous studies suggest bird migrations are occurring earlier and
earlier as the planet warms, a phenomena that could cause problems for
birds reliant on seasonal food resources.
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