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How A New Theory Of Bird Evolution Came About

March 3, 2009
University of Montana
A major new theory for the evolution of flight is changing textbooks around the world. It involves wing-assisted incline running and a fundamental bird wing angle. One of the scientists who led the discover describes his research. Using high-speed cameras, he analyzed how birds change the angle of their wings as they gain altitude, glide, descend or run up steep surfaces.

Ken Dial at The University of Montana has unveiled a major new theory for the evolution of flight that is changing textbooks around the world. It involves wing-assisted incline running and a fundamental bird wing angle.

When Ken Dial made one of the sweetest, most surprising discoveries of his career, he swore like a sailor.

It happened like this: The University of Montana researcher and two grad students were working in the University’s Flight Laboratory, a high-tech avian research facility at Fort Missoula. Once a U.S. Cavalry stable, the building now sports a modern interior with offices, aviaries, holding cages, a surgical suite, a wind tunnel, electrical gear and lots of different bird species.

Using high-speed cameras, the three documented how birds change the angle of their wings as they gain altitude, glide, descend or run up steep surfaces. Dial, a self-described experimental functional morphologist, has long been interested in how birds are put together – muscles, nerves and bones – and how what goes on inside them affects their behavior. Decades ago he already had made X-ray movies of birds in flight, and now – at 1,000 frames per second – he was trying to understand, down to the most minute detail, the mechanics of how they take to the air.

One of his grad students, Paolo Segre, was putting the birds through their paces and recording the results. He somewhat sheepishly reported to Dial that the avians – in this case chukar partridges – weren’t really changing their wing angle as they flew higher, descended or flapped their wings to help them run up steep surfaces.

“You’re full of soot,” Dial responded. (Actually he didn’t quite say that. Anyone who has worked in the lab with Dial long enough knows his language gets a bit more salty.) “Go back and do it again. This can’t be right. The physics doesn’t make sense.”

But when the chastened Segre tried again, this time with the help of the other student, Brandon Jackson, results were similar. Soon Dial was involved, and all scratched their heads as they watched videos of the birds perform in ultra-slow motion. Something wasn’t right …

“Then all three of us went, ‘Holy ----! They really aren’t changing their wing angle!’” Dial said. “Then we stopped to think about how that could be. And we realized we had to rethink what we were imagining the birds were doing.”

What the birds were doing was keeping their wing strokes confined to a narrow range of less than 20 degrees for a wide range of behaviors. This similar wing flap directs aerodynamic forces about 40 degrees above the Earth’s surface, permitting a 180-degree range in the direction of travel.

Dial and his crew – like the rest of the scientific community – had always assumed that birds were doing something extremely complex with their wing angles as they flew. What they discovered was something simpler and basic hidden in behaviors everyone has seen countless times. It was a fundamental wing angle.

“I had it wrong,” Dial said. “It turns out they weren’t changing hardly anything at all.” He holds out his hand flat, angled slightly above the horizon. “The wing is doing this the whole time, and the body is slinging around it like a gymnast on the rings. The wings always produce a force that is similarly orientated against gravity. The body slings around so much that it looks like the wings change position. But they don’t.”

Dial said bird wings produce lift and thrust forces at the constant angle. If they encounter a rock, cliff or any other textured substrate within the path of the forces of the wing stroke, birds use this force to hold themselves against the substrate while the legs do the work to lift the animal up the obstacle. If there is no obstacle, the same wing stroke functions for flight.

The findings became a big deal when they were published last January in Nature.

Bird flight – an almost magical aerial dance to the human eye – had just become a lot simpler. And Dial thought the basic nature and utility of the finding might help explain how birds evolved to take to the skies in the first place.

The son of an aeronautical engineer, Dial grew up blocks from the runways of Los Angeles International Airport. He became fascinated by airplanes and the idea of flight, but his youthful experiences in the outdoors – especially as a Boy Scout – gave him a stronger attraction to the biological world.

“I never lost my little-kid attitude of ‘Daddy, how does the world work? What’s under this rock? How did this thing come to be?’” Dial said. “I became greatly interested in vertebrate design and evolution – how things with backbones are put together. Everything from a fish to a bat to a whale is related, and how could that be? How could they move and have similar muscles and bones and some be the size of an eraser and some the size of a building? That is a terribly exciting thing to think about.”

Dial was a mammal guy in college, and his doctorate was in mammalian ecology, life history biology and evolution. In the ’80s he came to believe his university education was imbalanced – that he was trained primarily in field biology and statistics and needed to know more about form, function, anatomy and physiology. This resulted in a postdoctoral fellowship at Harvard University and time spent at a medical school and a museum of comparative zoology.

“Over those three years I got my head screwed on about the internal components of vertebrates,” he said. “And I came to realize there are tons of work on bird behavior and bird ecology, relatively speaking, but next to nothing on how animals move, how they are put together and what goes on inside to allow them to behave.”

While completing his education, Dial pursued another passion: aviation. He earned his pilot’s license in 1981. He also holds instrument, commercial and multi-engine licenses, is rated for turboprop and jets, and can fly as an airline transport pilot. Over the years he has amassed more than 2,000 flight hours.

Why would a college professor rise to that level of flying proficiency?

“I have no idea,” Dial said. “I was training when I was doing my doctorate and taking my post-doc in Boston. Don’t bring this up around my wife, Karen (of 34 years); she watched me go through all these gyrations training as both a pilot and biologist. When I find something I’m interested in, I become insatiable about things.”

Dial came to UM in 1988 and soon also took flight as a world-class ornithologist. He’s had 24 years of consecutive National Science Foundation funding, and his research program has garnered millions in grants. He has written more than 60 scientific papers and has been published six times in the prestigious journals Nature and Science.

Dial has made a career out of slowing down time to reveal what’s hidden right before our eyes.

Case in point: In 2003 he used high-speed cameras to study how chukar partridges go from chicks scurrying around on the ground to full-fledged flight. How do they do it? What are the mechanics of their development? What good is a half-grown wing that birds can’t fly with yet?

The vast majority of the world’s birds are songbirds. Dial said these are highly evolved, “derived” fliers because they employ sophisticated parental care. They locate a habitat and build a complex nest. The parents tag team taking care of the chicks. The chicks sit in the nest for two to six weeks, depending on the species, and grow to adult size. Then they eventually stretch their wings and fly away.

Not so with chukars and similar ground dwellers such as quail, turkeys and chickens. They can walk the day they hatch to help avoid predators, but they develop the ability to fly incrementally. They have to fend for themselves. Dial wanted to videotape the process these more primitive birds go through.

An experiment was designed where chukar chicks were placed on an elevated perch such as bales of hay. One was removed from its siblings and videotaped as it scrambled and flapped back to the group on a daily basis. In another test, the birds were placed on a table, and one was separated on another table. On day one, the tables were 2 inches apart. In a week they were 3 yards. By the end the bird was flying 100 yards between tables. The point of these experiments was to document how the creatures developed flying performance both vertically and horizontally, from hatchling to adult.

Dial was away at Harvard doing research when a breakthrough came. He had employed two high school students – his son, Terry, and family friend Ross Randall – to exercise and videotape the birds. One day when Dial called to ask how the birds were doing, his son said, “Lousy! They aren’t flying anymore. They are cheating!”


Terry informed his dad that during the elevated-perch experiment, the flapping birds appeared to be running straight up instead of flying. They were cheating by using their clawed feet in combination with their half-grown wings.

It was an epiphany. Slow-motion cameras slowed down reality enough to reveal the birds used their wings like the spoiler on a race car to stick their feet to the surface they were ascending. With their developing wings, the birds could run up nearly vertical surfaces such as hay bales, trees, rocks and cliffs. (Some species can run up inverted surfaces that are more than 90 degrees.) People have seen chickens doing it since the dawn of history, but everyone assumed they were flying. The videos revealed that, no, they were flap-running.

Dial dubbed the behavior WAIR – wing-assisted incline running. He said WAIR gives ground-dwelling avians a survival advantage. His lab has tested it in at least 20 bird species, and even duck-like ones use WAIR. (Notable exceptions are large flightless birds such as ostriches and emus. Other than for heat regulation and mating rituals, the use of their wings remains an enigma.)

The WAIR findings spawned an article in Science that garnered worldwide attention. It popped the eyes of many in the ornithological world.

What use is half a wing?

When Charles Darwin unleashed his revolutionary theory of evolution in the mid-1800s, one of the first questions doubters nailed him with went something like this: You have the four limbs of a reptile and then a beautiful flying bird. What are the intermediary steps? Darwin, what use is half a wing?

There wasn’t much the esteemed naturalist could say back then. And during the next 150 years, scientists largely divided themselves into two bickering theoretical camps regarding the evolution of flight.

The arborealists, who are generally ornithologists, think bird ancestors first took wing by climbing trees or cliffs and then gliding down from them. Certain lizards and flying squirrels exhibit this behavior. In the opposing camp are the cursorialists – usually paleontologists who note the similarities between dinosaur and bird fossils – who claim early birds ran along the ground, beat their feathered forelimbs and eventually took off.

Dial said a lot of “silver-backed biologists” have spent their careers writing untold volumes of work defending the aboreal and cursorial positions. He said they can be rigidly dogmatic in their beliefs.

Now Dial and his crew have discovered in the laboratory that half a wing indeed can be useful. He has entered the evolution-of-flight fray by offering a third rival idea – the ontogenetic transitional wing hypothesis, or OTWH. (Ontogenetic means the development of something.) This theory suggests that birds evolved incrementally by using their half-developed wings to run up steep surfaces (WAIR) and gained a survival advantage. Then they flapped their proto-wings to return to the ground safely. And, by the way, it’s no great leap to cross between these behaviors because they are linked by a fundamental, constant wing angle.

“We think our theory is a convergence of thought that’s a more complex marriage of the arboreal or cursorial camps,” Dial said. “We have taken the beautiful sage elements from each one, and I feel we integrated them perfectly to say you never needed to go strictly from the ground up or tree down.”

“The eons-long evolution of flight is revealed to us in the development of baby birds,” Dial said. “Our thesis came out from the demonstration of what living animals actually do. And now we have fossils that we never imagined being discovered in China, South America and Africa that look exactly like we expected – dinosaurs with feathers; dinosaurs with half a wing.”

He said the evolution of flight in birds was a messy affair that likely happened during a span covering tens of millions of years. The first bird-like fliers began appearing 150 million years ago.

But just imagine this: There was a time before flying birds, a time when reptilian things on two legs screeched and flapped their suddenly useful forelimbs to escape predation, reach safer habitats and rise above the competition to survive and reproduce. According to Dial’s theory, there was a period before true bird flight when, for proto-birds at least, WAIR and its related behaviors were the only show in town.

For a time, half a wing was good enough, and it became the stepping stone nature needed to populate the skies.

Many people are interested in Dial’s work for its possible applications beyond biology. His dogged pursuit to understand every detail of flight in nature may someday contribute to the worlds of aerodynamics and aeronautics.

“We talk about getting microstructures onto aircraft wings,” he said. “You could have the full structure of the wing, and on top of it you could have these very light tabs like feathers that change the airflow in a very sophisticated, computerized way. You could get greater lift, much greater endurance, more control and probably a lighter structure.”

His research also could lead to aircraft wings that change shape to accommodate slow and fast flight, and do it all with one structure. Dial said an airplane is two structures – an engine and a wing – while a helicopter is more like a bird, because the spinning rotor is both the wing and propeller.

“Birds and helicopters have a lot in common in the sense that their wing and propeller give them both lift and thrust,” Dial said. “So there have been discussions of how to change the shape of the propeller dynamically.”

Dial said the government also has approached him about the possible defense applications of his research, such as creating spy robots that can climb walls or fly. But that’s a path he’s chosen not to tread.

“Perhaps selfishly, I’m still too much like that juvenile little boy asking, ‘How does the world work?’” he said. “I’m still too engaged in trying to figure out how all this spectacular tapestry of life fits together.”

Story Source:

Materials provided by University of Montana. Note: Content may be edited for style and length.

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University of Montana. "How A New Theory Of Bird Evolution Came About." ScienceDaily. ScienceDaily, 3 March 2009. <>.
University of Montana. (2009, March 3). How A New Theory Of Bird Evolution Came About. ScienceDaily. Retrieved February 28, 2024 from
University of Montana. "How A New Theory Of Bird Evolution Came About." ScienceDaily. (accessed February 28, 2024).

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