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Battling Birds Pull Extreme Aerial Maneuvers

Hotshot jet pilots are no match for cliff swallows. The acrobatic migratory birds rocket over bridges and skim over lakes, careening around each other at accelerations that would knock an Air Force ace out cold. By tracking these contests with high-speed cameras, a new study gives the first, in-depth peek into avian aerodynamics in the wild. The findings may even provide insight into how to design better micro air vehicles—tiny drones.

“This technology will be brilliantly useful,” says biomechanics expert Jim Usherwood of the United Kingdom’s Royal Veterinary College in Hatfield, who was not involved in the study. High-resolution field studies like this have never been done before for birds, Usherwood continues.

For cliff swallows (Petrochelidon pyrrhonota), the trouble starts when they return from wintering in South America to their summer homes in North America. After arrival, they seek out their old mud nests—usually located under concrete bridges and freeways—and start rebuilding their homes. But rather than hunt down a fresh supply of mud, some swallows prefer stealing supplies from their more industrious neighbors. Others take things further and will even lay an egg or two in their neighbor’s nest before taking off.

Aerial squabbles ensue if the intruders are caught in the act, and a new study takes advantage of these fights to learn how birds execute high-speed maneuvers. The team placed three cameras along a North Carolina lake crossed by a highway bridge that houses several cliff swallow nests and waited for the battles to commence.

The cameras captured some impressive maneuvers, the researchers report online today in The Journal of Experimental Biology. For instance, a pursued bird would occasionally speed toward a concrete pylon. Right at the last moment, it would peel away in a seeming attempt to make the other bird crash. Actual collisions weren’t observed, but this evasion strategy was extra hard for the pursuer because it wouldn’t chase from directly behind. “There's a little downwash air region behind each bird that would make flying difficult for the one in pursuit,” says one of the study’s authors, biologist Tyson Hedrick of the University of North Carolina, Chapel Hill. “Similar to geese in a ‘flying V,’ the following bird stays close, but clear of downwash.” The lead swallow would bank in the direction away from the chaser 65% of the time, making the follower exert more energy to keep up or forcing a bailout.

The team was also surprised to learn that most of the time, chasers copied the motion of fleeing intruders, wingbeat for wingbeat (see video). This rapid processing of visual information—seeing the leader’s wings, shooting those messages into the brain and then down into the muscles—all happened within 81 milliseconds, according to the team. This rivals the reflexes of some of the fastest fliers, such as dragonflies, which conduct similar moves in 20 milliseconds. Given that cliff swallows are voracious insectivores, this talent may help them catch food.

Swallows also pull very hard turns to escape a foe, with one extreme case reaching 7.8 g. Fighter pilots usually pass out at about 5 or 6 g, which is why these experiments have garnered interest, and partial funding, from the Office of Naval Research. The Navy may use the findings to build better guidance systems for micro air vehicles. However, the swallows’ biomechanics are complex, and for now the team is simply trying to glean a few tricks. “Swallows probably have a better onboard pilot than anything we're going to build anytime soon,” Hedrick says.