Researchers unlock a secret of bird flight

To human eyes, bird flight seems a complex, almost mystical, aerial dance.

However, it may not be as complicated as it looks. High-speed cameras at The University of Montana Flight Laboratory reveal all birds use a similar wing angle relative to the ground to gain altitude, glide, descend or run up steep surfaces.
UM biology Professor Ken Dial said discovery of this fundamental wing stroke may help explain how birds evolved to take to the skies.

“I think many big findings in biology are fairly straightforward common sense and don’t require a spectacularly convoluted explanation,” he said. “I think that’s where the beauty of this resides — its simplicity and utility are quite striking.”

The new findings have been published as a letter in the international science journal Nature, available online at http://www.nature.com/nature. Dial and two of his students, Brandon Jackson and Paolo Segre, are co-authors. The piece has spawned articles worldwide in outlets as varied as the BBC, National Geographic News and Tehran Times.

Dial has studied bird flight for 25 years, photographing them in wind tunnels to learn how they use their wings to manage the air at different speeds. During this research his lab also studied adolescent birds as they learned to fly.

“We figured the stroke angles were going to be all over the place, but it turned out to just be a single stroke,” he said.

The researchers found that birds use wing strokes confined to a narrow range of less than 20 degrees. This directs aerodynamic forces about 40 degrees above the horizontal, permitting a 180-degree range in the direction of travel.

Dial’s lab already has discovered that birds can use their wings like the spoiler on a race car to run up steep surfaces. He calls this behavior wing-assisted incline running. Those results were published in the journal Science four years ago.

“But once the animal gets up, it has to come down,” Dial said. “When it comes down, it effectively uses the same wing stroke. So this tells the rest of the story. It’s not unlike a helicopter that can’t move its rotor much. But if you change the power and tilt it just a touch, you can come up with all sorts of new maneuvers, and that’s what we are showing.”

Evolutionary biologists have long been divided into two camps about the origins of bird flight. One group believes avian ancestors took wing by climbing and gliding from trees. The other believes early birds ran along the ground, beat their feathered forelimbs and eventually took off.

Dial now offers a rival idea — the ontogenetic-transitional wing hypothesis — which suggests birds evolved incrementally by learning to use their wings to run up steep surfaces. This gave them a survival advantage, and eventually their ancestors became strong enough for true flight. And since the basic angle for wing-assisted incline running and true flight are similar, Dial suggests the transition into the air was easier.

Ground-dwelling partridges called chukars were used in the initial research, and Dial was able to confirm the fundamental wing stoke among 20 other bird species.

Dial has worked at UM for 20 years. As an experimental functional morphologist, he designs experiments to study the function and structure of animals. Besides being a professor in UM’s Division of Biological Sciences and director of the flight lab, he directs the Research Station at Fort Missoula. He also is the former host of “All Bird TV” on Animal Planet.