V-flying birds pick efficient flapping pattern

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A bird migrating with others in a V formation turns out to be a master of the well-timed flap, able to match the up-and-down path traced by the wing tip of the bird in front of it. 

Fighter pilots know the benefits of flying in a V formation. Wings create a vortex of whooshing air trailing from their tips, and a rearward plane that keeps one of its wing tips in a vortex from the plane in front gets a boost from the upwash of air and can save fuel. But birds flap their wings, and the vortices spinning off their rising and falling wing tips swoop up and down. For a bird to catch the benefit of another’s wing trail, flap timing has to be right.

Sensitive monitoring devices show that northern bald ibises coordinate their wing beats in the ways that scientists predicted would be efficient, says Steven Portugal of the University of London’s Royal Veterinary College in Hatfield. When flying in a V formation, a bird tends to flap so that one of its wing tips follows the route taken by the nearest wing tip of the bird in front. The wings don’t necessarily rise and fall at exactly the same time. But the follower’s wing tip chases the leader’s along the same path and thus catches the vortex’s helpful uprush of air.

In contrast, when a bird flies directly behind another, the follower tends to flap out of phase with the leader, so its wing tip is down where the leader’s was up. This disconnect helps the follower dodge the substantial downwash of air directly to the rear of a flying bird, Portugal and his colleagues report in the Jan. 16 Nature.

Big birds such as ibises, geese and cranes often fly in V formations. But scientists’ predictions of the efficiency of that formation depend largely on experience with nonflapping wings.

To see whether flapping birds coordinate their wing beats carefully enough to hit the sweet spots in V formations, Portugal and his colleagues worked with Zoo Vienna’s project to train captive-bred Geronticus eremita ibises to migrate from Austria to Italy. The effort to help the critically endangered birds migrate depended on imprinting the newly hatched chicks to treat human volunteers as their mothers.

When migration time came, the human moms rode in the back of a microlight aircraft shouting for the youngsters to follow. Researchers attached tiny backpacks to the birds that contained equipment that noted the location of a bird and its flapping phase. The researchers then recovered the equipment after a stint of real-world flying.

The study builds on other researchers’ work to develop ways to test how birds coordinate their flaps to get a boost from V-flying. “We still don’t know if this actually saves any energy, but the new data are certainly consistent with the theoretical predictions,” says comparative physiologist Michael Dickinson of the University of Washington in Seattle.

“My strong suspicion is that the effect does not scale well,” adds Dickinson, who focuses on flight in much smaller fliers: insects. Even among small birds, he speculates, the wake left by flying may not be very useful. And taking advantage of a flight formation among fast-flapping little birds, he says, “would require superfast reflexes.”

LEARNING ABOUT FLIGHT  Northern bald ibises from a captive breeding program offered researchers a chance to test the birds’ flapping strategies during migration.  S. Portugal et al/Nature 2014; adapted by Ashley Yeager