Folding the wings may be a bird’s answer to turbulence, according to an Oxford University study in which an eagle carried its own “black box” flight recorder.
Researchers set out to examine how soaring birds such as eagles, vultures, and kites, manage to fly in gusty, turbulent conditions that would keep a light aircraft grounded.
“Soaring flight may appear effortless, but it isn’t a free ride,” said Graham Taylor of the university, co-author of a report on the findings. It “puts an enormous strain on its flight muscles. The nature of rising air masses, such as thermals, is that they create lots of turbulence and buffeting that jolts a bird’s wings and could knock it out of the sky.”
A thermal is a column of rising warm air caused by uneven heating of the Earth’s surface.
The researchers gave a captive steppe eagle (Aquila nipalensis), named “Cossack,” a flight recorder backpack – a 75-gram (2.6-ounce) black box incorporating GPS that also measured acceleration, rotation rate, and airspeed. The investigators then recorded the bird soaring over the Brecon Beacons National Park in Wales.
Analysis of data from 45 flights revealed that in windy conditions the eagle would tuck its wings in response to particularly strong gusts, the scientists found. During these wing-tucks, each lasting about a third of a second, the wings were folded under the bird’s body so that it was effectively “falling,” the researchers explained, adding that the wing tucks may occur up to three times a minute in some conditions.
“We think that, rather like the suspension on a car, birds use this technique to damp the potentially damaging jolting,” Taylor said. “Whilst we won’t see large aircraft adopting collapsible wings, this kind of technique could potentially be used to keep micro air vehicles aloft even in very windy conditions.”
Theories had been suggested to explain why birds perform wing tucks but up until now no one had tested these conclusively, he added. The findings are published in the Journal of the Royal Society Interface.