Birds Use Wings as Running Aid,
Provide Insight into Origin of Flight

Many species of ground birds flap their wings vigorously as they run uphill (or up a tree) for cover. A casual observer might think the birds are too addled—or perhaps bird brained—to get up and fly to safety. But recent research by one biologist published in the 17 January 2003 issue of Science has found that the wing beating isn't just a big flap about nothing; it actually helps the bird get uphill in a hurry. What's more, understanding the aerodynamics involved may form a crucial piece of the paleontological puzzle of just how flying evolved.

Dr. Kenneth P. Dial, a professor of biology at University of Montana, Missoula, used chukar partridges (Alectoris chukar) to study "wing-assisted incline running" (WAIR). Dial found that even hatchling chukars can get up inclines of up to 45 degrees, without flapping. But once the birds added wing movement, their scaling abilities improved dramatically: hatchlings could climb a 50-degree slope; four-day old chicks could climb a 60-degree slope; and 20-day-old chicks could scale a 95-degree vertical surface. Adult chukars, moreover, use WAIR to scoot up a 105-degree overhang—a feat rock climbers may very well envy.

Dial analyzed the role of wing-beating by both suturing accelerometers to adult chukars and videotaping their running styles. What he found was that one portion of the wing's beat gives the bird a little lift to run up the hill. Equally important, another portion of the wing's beat presses the bird's feet into the ground, which provides increased traction. Dial, himself a pilot, also removed the flight feathers from some juvenile chukars and trimmed the flight feathers on others. Those with no flight feathers couldn't get up anything steeper than 60 degrees. Those with trimmed feathers did better, but still couldn't ascend as steep a slope as the control birds. "That's consistent with the idea that the wings actively aid the legs in getting up steep surfaces, much like spoilers on a race car help with traction," he says.

But why would birds flap their wings and run when they're in danger, rather than taking to what seem like the more friendly skies? It's all a matter of energy economics, Dial says. The wings on galliform birds (that's the order that includes game birds and domestic fowl) have fast-twitch (anaerobic) muscles that are good for explosive movements, but tend to fatigue quickly. The legs have fast-twitch oxidative (aerobic) muscles that are better for extended energy output. "It seems that using the wings and the legs together is more efficient and powerful than simply flying into the air," Dial says.

Dial's findings could fill the gaping hole in paleontologist's understanding of the evolution of feathered wings and avian flight. Paleontologists now believe that birds evolved from small, non-flying theropod dinosaurs. And fossil remains show that some of those dinosaurs had feathers. But until now, no one was exactly sure what those feathers were for—whether, for instance, these creatures were in the midst of losing their wings or whether the feathers evolved for insulation or, perhaps, display. Dial's work suggests that these "proto-wings" were an intermediate step between running and flying and provided the same running aid that wings do for today's chukars.

To bolster his argument, Dial notes that the two-legged chukars use their wings during running the same orientation that four-legged animals use their front legs. That is, the wings move in a "head to tail" movement, Dial says, "not a belly to back movement, as you see in a flying bird. So it makes sense that such movements could also help bi-pedal dinosaurs with long, running legs, especially creatures like the Caudipteryx," a turkey-sized, fast-running, feathery-tailed dinosaur that many believe is the closest-known ancestor of birds today. Dial hypothesizes that such a proto-wing would generate aerodynamic forces that may also have helped early birds make controlled descents from their perching points—something chukar chicks still do to this day.

—Susan Davis

17 January 2003

See also videos of chukar partridges exhibiting WAIR behavior (courtesy of Kenneth Dial):

• Chukar partridge chick (AVI format; 2.4 MB)
• Chukar partridge running up a tree; camera at 90° angle (AVI format; 1.2 MB)
• Chukar partridge running up bale of hay; camera at 90° angle (AVI format; 1.9 MB)

See also related press coverage:

• ABC News, "Study: Frantic Flapping May Have Led to Flight"
• Daily Telegraph, "Bird clue to flight of dinosaur"
• The Guardian, "Running bird gives rise to flight theory"
• Nature, "Flapping a forerunner to flight?"
• Newsday, "Birds Study Reveals Info on Dinosaurs"
• New York Times, "Chicks Offer Insight Into Origin of Flight"
• Reuters, "Study: Frantic Flapping May Have Led to Flight"
• Science Daily, "New Study Suggests Missing Link That Explains How Dinosaurs Learned To Fly"
• Scientific American, "Origin of Bird Flight Explained"
• Washington Post, "Biologist Finds Clues to Birds' Flying Start"