Soaring birds tuck in wings to cope with turbulence

Collapsible wings help soaring birds such as eagles, vultures and kites to fly in 'gusty' turbulent flight conditions that would keep a light aircraft grounded, suggests a new research.

"Our evidence suggests that wing-tucking (collapsing the wings) is a direct response to a substantial loss of lift that occurs when a bird flies through a pocket of atmospheric turbulence," said study author Graham Taylor of Oxford University in Britain.

For the study, the researchers gave a captive steppe eagle (Aquila nipalensis), called 'Cossack', its own flight recorder backpack - a 75g black box incorporating GPS that also measured acceleration, rotation rate, and airspeed - and recorded it soaring over the Brecon Beacons in Wales.

An analysis of data from 45 flights revealed that in windy conditions the eagle would collapse its wings in response to particularly strong gusts rather than hold them out stiffly as an aircraft would.

During these 'wing tucks', the bird's wings were briefly (for around 0.35 seconds) folded beneath its body so that it was effectively 'falling'.

"Soaring may enable a bird to travel long distances but it also 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," Taylor noted.

"We think that, rather like the suspension on a car, birds use this technique (wing-tucking) to damp the potentially damaging jolting caused by turbulence," Taylor added.

This kind of technique could potentially be used to keep micro air vehicles aloft even in very windy conditions, he pointed out.

The findings appeared in the Journal of the Royal Society Interface.