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 ; 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 with the data he had available and, during the next 150 years, scientists were largely divided into two 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 think early birds ran along the ground, beat their feathered forelimbs and eventually took off.

University of Montana biology Professor Ken Dial and his lab say 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 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.”

He uses the terms 'theory' and 'hypothesis' interchangeably so what that says is up to you but OTWH is based on more than speculation and says he has demonstrated how half-developed wings can be useful. He speaks of putting baby chukars in a chamber filled with a fine mist of olive oil. “It looks like a smoky bar, but it smells like an Italian restaurant,” Dial said. As the bird flap-runs up a steep surface, a laser is shot against the wings while cameras record and computers measure the mini-tornadoes of oil particles spawned by the aerodynamic forces of the moving appendages. This produces images where tiny swirls of arrows reveal the speed and direction of these forces. Dial said the whole process is called digital particle imagery velosometry.

The method

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 s**t,” Dial says he responded. “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 s**t! 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 had always believed 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. Dial and his two grad students co-wrote the piece, which spawned articles worldwide, from National Geographic News to the Tehran Times.

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.

Baby Birds And Evolution Of Flight

“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, he says, and it became the stepping stone nature needed to populate the skies.

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 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 Nature and Science.

When he wasn’t studying birds, flying or fly-fishing, Dial found time to host 26 episodes of the Discovery Channel’s “All Bird TV” during its two-season run in the late ’90s. With rough language firmly in check (on camera anyway), he introduced viewers to interesting feathered friends from Alaska to Costa Rica.

He also has the word “FLIGHT” emblazoned on his car license plate.