"Long before it's in the papers"
January 27, 2015


Birds beat turbulence by folding wings, study finds

Oct. 16, 2014
Courtesy of Oxford University
and World Science staff

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. 

Cossack the Steppe eagle with his flight re­cord­er. (Cred­it: Graham Taylor)

“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.

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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, are able 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), called “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.