Wing Angle May Be Key to Bird Flight Origins

Brian Handwerk
for National Geographic News

January 23, 2008

Young birds must master the right wing angles to become good fliers, according to new research that may shed light on the origins of flight itself.

Kenneth Dial of the University of Montana and colleagues have previously documented how birds can run up vertical trees, boulders, or cliffs by angling their wings to create wind force that holds them to the surface.

Even young birds use their small developing wings to perform these incredible feats. Babies master 60-degree slopes the day they hatch, and they tackle steeper challenges during the vulnerable period before they reach level flight.

"It has profound implications that an [undeveloped wing] has a function shortly after hatching for many birds," Dial explained.

Now his team has discovered that the secret of this behavior, and flight itself, begins with learning the optimal wing-to-ground angle needed to generate force.

It appears that both young and adult birds use the same angle for many types of locomotion leading up to level flight.

Tremendous Importance

The research, published in the current issue of the journal Nature, is an important addition to studies of living bird behavior, experts say.

"Birds use [undeveloped wings] to improve their locomotive performance and escape predators," Dial said. "Even if you take out the evolutionary implications, the ecological implications of that are enormous."

Kevin Padian, an expert on the origins of flight at the University of California, Berkeley, said the work is of tremendous importance for flight theory.

"This is really a good insight," said Padian, who was unaffiliated with the research.

"Essentially it says to me that [birds] are always going 'up and forward' in some sense, whether flying or running up trees, and so the angle of attack of the wings is sensibly the same, or nearly so."

Lead author Dial had expected to observe different wing strokes employed for different behaviors in his study of chukars, quail-like ground birds that live in Eurasia. "They really only need one wing stroke, and they use [their wings] the day they hatch in an aerodynamically meaningfully way—even though they can't fly," Dial said.

(Related news: "Dinosaur-Era Bird Could Fly, Brain Study Says" [August 4, 2004].)

Clues to First Flight?

The way in which vulnerable young birds use their wings while transitioning into adult bodies could be a model for how their ancestors developed the ability to fly, Dial said.

"When you step back and look at the fossils they are finding, the long-legged dinosaurs with half a wing, they look very uncannily like today's birds that are going through the juvenile stage," he said.

"When you look at the development of these animals, you may in fact be looking at the strategies that their ancestors employed to get through transitional states [in evolution]."

(See the evolution of fins to wings.)

Researchers have long debated two theories of flight origin: the arboreal theory, which suggests that tree-dwelling bird ancestors leapt, glided, and eventually flew from branch to branch, and the cursorial theory, which holds that forelimb scales became feathers and eventually lifted running reptiles off the ground.

(Related news: "New Birdlike Dino Adds to Debate on Origins of Flight" [October 18, 2005].)

"This really says that flapping baby wings is more likely the explanation for the evolution of flight," Dial said.

"There is no reason you have to start off talking about ground 'up' or tree 'down.' It's, How did animals deal with the environment that they were in?"

Padian of UC-Berkeley agreed.

"The arboreal-cursorial dichotomy is dead and has been for a long time," he said.

"It's a question of how the flight stroke evolved. You can't fly without a flight stroke. Many things live in trees, and even glide, but never truly fly (flap)."

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