Birds must fly in all manner of windy weather. At first glance, this may not seem like such an impressive feat; after all, flying is what birds are born to do. But if you consider that they remain suspended in the air while being continually buffeted by sudden gusts of wind, which do not send them tumbling, the trick appears more remarkable. Humans caught on a ship in inclement weather have a hard time remaining upright even with handrails to grab onto, so it is quite striking how birds deal with wind in such a graceful style. Stability also ensures safe landings and successful prey capture, and provides flying animals with a steady view of the world. Spurred on to learn more, a team of researchers from the Royal Veterinary College and the University of Bristol, UK, set out to study how birds remain stable when flying through unexpected gusts of wind.
The team trained a barn owl to glide through a 17 m laboratory corridor equipped with high-speed cameras that tracked the bird's movement. To deliver a perturbing gust to the flying bird, they put a fan at the end of the corridor placed beneath the flight path to deliver an unexpected blast to the underside of the bird. The owl experienced three increasing gusts of 3.1, 4.5 and 5.2 m s–1 – comparable to gentle, yet destabilizing, breezes – as it flew through the corridor. Then they measured how the owl coped with the gusts by tracking its wing movements, in addition to comparing the movement of the flying bird's torso with the movement predicted from computer simulations, to calculate how well the gusts were tackled.
Employing a near-immediate response when gliding into the unexpected gust, the bird pivoted its wings around its shoulders, forming a V-shape when viewed from the front, with its head and torso located at the bottom, while its wings formed each ascending limb of the V. This initial manoeuvre deflected 32% of the lift generated by the gust within 80 ms of its impact, ensuring that the bird's torso and head remained on an unperturbed course. The team found that the bird's quick response was aided by the build and shape of the wing: the mass distribution along its length ensures that the wing's pivoting motion imparts no secondary upward or downward movement to the torso, keeping the head firmly in place during the unexpected impact. Then, after the initial wing rotation, the bird tipped the front edge of the wing downward, which decreased the lift force generated by the gust, reducing the impact of the gust by an additional 48%, thereby allowing the bird to continue smoothly on its way.
Analysing the manoeuvre, the team found that the speed of the response was the key to maintaining stability, suggesting that the bird's central nervous system may not be important for tackling the initial impact of a gust. This means that passive mechanisms, such as the shape and weight of the bird wing, absorb wind gusts, much like a suspension system in a car ensures a smooth ride through rough terrain. This novel discovery is also exciting news for engineers trying to build small aircraft. Understanding how shape and form endow flying stability may greatly simplify the task of flying through unpredictable weather conditions without needing to depend on complex sensors to gauge stability. Taking a hint from the barn owl: flapping wings can make for unflappably steady flight.