Need for Speed May Have Led Birds to Take Flight

Using high-speed cameras and chukar partridges fitted with accelerometers, a Montana researcher may have solved a mystery that has puzzled biologists for more than a century: how birds first learned to fly.

The problem is simple. Full-fledged wings could not have evolved in one step, and rudimentary wings would seem to provide no benefit that would promote further evolution. Behavioral ecologist Kenneth P. Dial, however, may have found that benefit.

Dial reports today in the journal Science that flapping elementary wings gives birds more traction when running up slopes, allowing them to escape predators by clambering up trees and other obstacles that they would otherwise be unable to negotiate. Forcing the birds’ feet to the ground like the rear wing of a race car, flapping allows the birds to climb overhangs as great as 105 degrees -- almost upside down.

“It turns out the proto-wings, precursors to wings birds have today, acted ... to keep the animals sure-footed even while climbing up nearly vertical surfaces,” Dial said. From that point, it would have been a relatively short step to the development of full-fledged wings, he noted.

Most researchers now agree that birds evolved from small dinosaurs. Discoveries over the last few years have shown that many small dinosaurs possessed feathers, presumably evolved to help keep them warm.

Researchers studying the evolution of flight have generally split into two camps: the arborealists, who think the first fliers jumped from trees, and the cursorialists, who argue that flapping proto-wings helped the small dinosaurs run faster, until they finally achieved speeds sufficient for flight.

Dial’s findings are “more of a compromise,” bridging the chasm between the two camps, said paleontologist Luis Chiappe of the Natural History Museum of Los Angeles County. The proto-wings would help them climb to perches from which they could leap, he noted.

Dial’s approach was triggered by his now-15-year-old son, Terry, who told his father that he had seen chukar partridges -- relatively clumsy birds in the same family as turkeys and quail -- run straight up bales of hay. Dial found that others had noted these brief bursts of climbing activity in the wild, but that no one had actively investigated them.

Working with Terry and another high-school student, Ross Randall, Dial began studying the ungainly animals. Unlike most common birds, the chukar partridges are hatched fully feathered and capable of leaving their nests to forage and escape predators. They are not yet able to fly, however.

Dial worked with hatchlings and adults with their feathers intact and with other birds whose feathers he trimmed. He attached tiny accelerometers to the birds to measure how much downward force they could generate and filmed their climbs with high-speed cameras.

He reports today that hatchlings are able to climb 50-degree slopes, 4-day-olds can negotiate 60-degree slopes, and 20-day-olds can accomplish vertical ascents. Adults can negotiate a 105-degree slope.

Dial also found that, while running, chukars flap their wings in a manner different from other birds. Although they are using the same motion, instead of generating a force forward and down, as is necessary for flight, the birds are generating it forward and up, to anchor them more firmly to the surface.

“Those are the two most important things about Ken’s work,” said paleobiologist Kevin Padian of UC Berkeley. “He’s showing us that these little birds run up trees. Nobody ever knew that before. Also, he found they are flapping their wings in a way different from everybody else. Those are blow-the-door-off discoveries.”

Further confirmation of the importance of the upward force came when Dial clipped the feathers of both hatchlings and adults to reduce the amount of force they could generate. On average, the clipped birds could only negotiate slopes about 20 degrees shallower than their fully fledged compatriots.

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Short movies showing the birds climbing can be viewed at www.nsf.gov/od/lpa/news/03/pr0308_images.htm.