Migratory birds coordinate their wing flaps with much more finesse than previously thought, so as to reap the best energy savings from flying in formation, suggests a new study.
In 2011, as part of a reintroduction programme, captive-bred ibises following an ultralight aircraft to their wintering grounds arranged themselves in the shape of a V. Data loggers on their backs captured every position and wing flap, yielding the most compelling experimental evidence yet that birds exploit the aerodynamics of the familiar formation to conserve energy.
Theoretical models had previously shown that the V formation, seen in other migratory birds such as geese, could enable trailing birds to save energy. But the models also indicated that the birds' coordination would have to be exceptionally precise to make a difference, and many scientists had doubted that the animals could achieve such a feat during flight, says ecophysiologist Steven Portugal at the Royal Veterinary College in Hatfield, UK.
To take maximum advantage of the V’s aerodynamics, each bird would have to position its wing in the upward-moving part of the vortex of air swirling off the end of the wingtip of the bird in front. But that vortex moves up and down because the bird in front is flapping. So the bird behind must not only put itself in the right place, but must also flap at just the right time — which changes depending on the distance between the birds — to keep riding the upwash. Faced with this complexity, scientists posited alternative reasons for the formation, suggesting that it might protect the birds against predators or let a flock put better navigators up front.
Thea Cunningham gets the scoop from Steven Portugal about his team's findings.
Tracking the V
For the new study, which is published today in Nature1, Portugal and his colleagues wanted to put the aerodynamics hypothesis to the test. They used specialized data loggers they had developed over several years, which record Global Positioning System (GPS) data five times per second in sync with an accelerometer for counting wingflaps.
But one problem remained: how to access a flock of free-flying birds that were tame enough to catch repeatedly? The answer came from a conservation project in Austria. There, a team led by biologist Johannes Fritz, recently awarded major funding by the European Commission, was reintroducing the northern bald ibis (Geronticus eremita), which had been extinct in Europe for about 400 years. Since 2003, the conservationists had trained the birds to follow human foster parents, who led them from breeding areas in Austria and Germany to wintering grounds in the Italian region of Tuscany, using an ultralight aircraft. At Fritz’s invitation, in August 2011 Portugal fitted 14 young ibises in Salzburg with his data loggers.
Portugal collected data for three flight days of the 36-day paraplane-led migration. From that, he selected a problem-free seven-minute segment to analyse. To his surprise, the analysis showed that the birds’ formation fitted the theoretical predictions of aerodynamics. “They’re placing themselves in the best place and flapping at the best time,” he says.
But that did not seem to be all that was going on. Portugal and his team also reported that the birds frequently shifted into seemingly less-optimal positions, such as directly behind the bird in front, adjusting their flapping to avoid downwash. It is not clear why they would leave the energy-saving V position, says behavioural ecologist Martin Wikelski of the Max Planck Institute for Ornithology in Radolfzell, Germany. The answer may come with better GPS technology that can measure the birds’ vertical positions as well as their horizontal ones, says biologist Ty Hedrick of the University of North Carolina at Chapel Hill.
Whatever their reasons, the birds clearly have a better ability to sense and respond to airflow than researchers gave them credit for. "In general, we know much less about the sensory system than the motor system in animal flight,” says biologist Bret Tobalske of the University of Montana in Missoula. One possibility, suggests Portugal, is that they use special feathers.
“They are just so aware of where each other are and what the other bird is doing,” Portugal says. “And that’s what I find really impressive.”
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