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Drafting With Ease: Land Birds Rely On Wind, Uplift To Save Energy For Long Ocean Crossings

Researchers also raised questions about the birds’ ability to make long-haul sea crossings if global warming affects the uplifts.

GPS data reveals how large land birds can fly over the open ocean for many miles without taking a break: They rely on wind and uplift to reduce energy — “even adjusting their migratory routes to benefit from the best atmospheric conditions,” researchers say.

“For centuries, bird-watchers assumed that large land birds only managed short sea crossings of less than 100 kilometers (62 miles) and completely avoided flying over the open ocean,” a new study states. “However, recent advances in GPS tracking technology have overturned that assumption.”

Researchers at the Max Planck Institute of Animal Behavior and the University of Konstanz in Germany used GPS trackers attached to 65 birds of five species and analyzed 112 sea crossings over nine years.

Until recently, experts assumed that large birds such as falcons and buzzards avoided crossing open seas that spanned more than roughly 60 miles because flapping their wings uses so much energy they couldn’t sustain a long nonstop flight.

Instead, the study found that birds crossing hundreds of miles of open water exploited wind and uplift to reduce their energy output during flight.

Unlike seabirds, the land birds in the study are not able to rest and hunt in the water. As a result, they have to be able to make the crossing in one nonstop flight.

A Peregrine falcon with a GPS tracker on its back on the Arctic island of Kolguev. The device allows scientists to remotely monitor the location and behavior of the birds and their flight patterns. (Olga Kulikova/Zenger)

Researchers also raised questions about the birds’ ability to make these long-haul sea crossings if global warming affects the uplifts, which can be affected by changes in atmospheric temperature.

“Our findings show that many land birds are dependent on atmospheric support to complete their migrations over the open sea, indicating their vulnerability to any changes to the Earth’s atmospheric circulation patterns,” said Elham Nourani of the University of Konstanz, the study’s lead author.

“Until recently, uplift was assumed to be weak or absent over the sea surface. We show that is not the case. Instead, we find that migratory birds adjust their flight routes to benefit from the best wind and uplift conditions when they fly over the sea. This helps them sustain flight for hundreds of kilometers.”

Land birds are capable of flying hundreds of miles over the open sea. Suitable uplift means less drag, making sea crossings less energetically demanding, and strong uplift allows the birds to soar, researchers say. (Elham Nourani/Zenger)

The study found, for example, that the oriental honey buzzard flies 434 miles over the East China Sea during its annual migration from Japan to Southeast Asia in an 18-hour nonstop flight.

“By making use of uplift, these birds can soar up to 1 kilometer (0.62 miles) above the sea surface,” Nourani said.

Collaborative studies like this “are important to unravel general patterns about how migratory birds depend on the weather patterns,” Nourani said. “This enables future studies to make robust predictions about how these birds will be impacted by climate change.”

The study, “How land birds cross the open ocean,” was published in Proceedings of the Royal Society B Biological Sciences on Sept. 8.

Edited by Judith Isacoff and Matthew B. Hall

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