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Flapping a forerunner to flight?

Ancestral bird might have moved like chicken.
17 January 2003

KENDALL POWELL

Partridges flap to increase traction.
© Science / Illust. by R. Petty
movie

Chickens have always seemed like farmyard failures at flight. But their flap-and-run technique may represent just the leap needed for theories on the origin of flight to get off the ground.

Lab treadmill tests show that partridges and other ground-based birds such as chickens, turkeys, quail and grouse flap their small wings while running to improve traction on steep inclines and vertical surfaces1.

The finding, by Kenneth Dial of the University of Montana in Missoula, provides a new model for how flying dinosaurs and ancestors to birds may have evolved.

"Their wings are being used not to take them to the heavens - that's where our heads have been - but to stick them to the ground," Dial says. A modified flap-running may have helped the first flyers climb trees or cliffs to take refuge from predators and give them a more controlled descent back down, he proposes.

Fully grown partridges can reach vertical heights of more than five metres without a running start, and can even navigate overhangs 10 degrees beyond vertical. Baby birds with immature wings perform almost as well as adults.

The birds cannot perform their wall-clinging trick as well when Dial trims or plucks their wing feathers. And all birds, fully feathered or not, have a hard time climbing a smooth surface.

By placing accelerometers on the birds' backs and taking high-speed videos, Dial determined that the birds' wings produce a force that pushes the animals towards an inclined surface. In the video, he saw birds swivel their wing position to match the steepness of an incline.

In other words, the flapping wings act like the spoiler on the back of a racing car, which aerodynamically pushes the car towards the track for better traction. Dial proposes that this new-found function for wings may mimic an intermediate step in the evolution of flight that biologists had overlooked.

No one can assume that Dial is seeing an ancestral state in his birds, says palaeobiologist Kevin Padian of the University of California, Berkeley, but the work shows an ability that ancestral birds may have used. "Until you have a fully functional wing, you can't fly," he says. "But a precursor wing would give an aerodynamic advantage that could help you run up a tree trunk."

Winging it

Many biologists believe that birds sprung from a type of dinosaur known as theropods. These had similarities with ground birds: they had strong hindlimbs, they ran quickly along the ground, and some had feathered forelimbs. The winged dinosaur Archaeopteryx had a shoulder joint that allowed its wings to swivel in several directions, instead of just flapping perpendicular to the body as in flight.

The question of whether flight evolved from the ground up, from trees down, or from some combination of the two is an ongoing debate. Richard Prum, ornithologist at the University of Kansas Natural History Museum in Lawrence, says: "The main question now is which components of avian flight evolved in the context of terrestrial locomotion and which evolved in the context of aerial flight?"

References
  1. Dial, K. P. Wing-assisted incline running and the evolution of flight. Science, 299, 402 - 404, (2003). |Homepage|


© Nature News Service / Macmillan Magazines Ltd 2003

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