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Research solves bird-flight mysteries
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Research solves bird-flight mysteries
The mind wanders on long bike rides. Bret Tobalske found his own roaming as he pedaled to work in 1989, while pursuing his UM master’s degree at Coram Experimental Forest near Hungry Horse. As the trees rolled past, he watched woodpeckers and their unusual heavy-flying style. It consisted of flapping bursts followed by short periods where the birds tucked their wings and coasted through the air — much like an Olympic ski jumper straining for that extra inch.
Why do they do that? Tobalske thought. Are they resting mid-flight?
Seemingly simple questions and a wandering mind can take a person a long way. He started studying this wing-tucking behavior used by many flying birds — called bounding — which led to other questions regarding the mechanics of bird flight. It also led him to earn a doctorate at UM, a Fulbright fellowship in France, postdoctoral work at Harvard and faculty positions at Allegheny College and the University of Portland.
Now he’s the new director of UM’s cutting-edge and recently renovated Flight Laboratory and Field Research Station at Fort Missoula. He took the reins from renowned bird researcher Ken Dial, who wanted more time to concentrate on writing and research. Dial was one of Tobalske’s mentors during his graduate student days at UM.
“His research program is just on fire,” Dial says of Tobalske. “He’s more productive than I will ever be, and we are lucky to have him. I hope that together we can become an awesome force internationally regarding bird flight.”
Tobalske says bounding is an excellent strategy for small, fast birds because air drag goes up exponentially with increasing flight speed. Occasionally closing their wings gets them out of the airstream and reduces drag. It also gives birds a brief rest, which is important because flying is one of the most energy-intensive ways for an animal to move. Tobalske also has learned that birds actually produce lift with only their bodies and tails.
“It’s called body lift, and it’s a contribution of just the cigar shape of the body and the tail itself,” he says. “So during their bounding leap, they can support about 15 to 20 percent of their body weight with just their shape.”
Studying the relationship between form and function in birds and why they choose to fly at different speeds has been an overarching theme of Tobalske’s work for nearly two decades. In an early breakthrough while studying birds using wind tunnels and other techniques, he found the animals aren’t constrained to use their muscles in a fixed way when they fly, which cut against the grain of scientific thought at the time.
Tobalske also started thinking about how air is essentially a non-dense fluid, and that the power generated internally by birds as they flap their wings should have interesting effects on the air “fluid flow.”
“As the animal pushes down on the air, the air pushes back on the animal and it stays in the air,” he says. “You have equal but opposite forces, as described by Newton’s Third Law.”
In order to visualize these invisible aerodynamic forces, Tobalske became an early expert at using particle image velocimetry on flying birds. During this process, a chamber is filled with a fine mist of olive oil. It looks like a smoky bar but smells like a pizzeria. A laser is then shot against a bird as it flies. Computers and high-speed cameras that shoot 1,000 frames per second then record the mini-tornadoes of oil particles formed by the bird’s wings and body.
|Art-science integration: This piece was produced by Tobalske and artist Fernanda D’Agostino.
The process produces digital images in which tiny swirls of arrows reveal the speed and direction of forces moving around the bird. The pictures are so interesting that in 2004 Tobalske was approached by artist Fernanda D’Agostino, a UM alum who now lives in Portland, to colorize his scientific work. The results have been exhibited in China and at technology conferences in France and Spain.
“It’s been a real integration of art and science and something I never imagined I would get into,” Tobalske says.
He brought his $150,000 PIV system with him when he became director of UM’s Flight Laboratory last August. He believes it’s the first time this technology has been available on campus.
Tobalske has worked with a wide variety of birds over the years, and not all of them are willing to fly on demand inside a mist-filled box while being shot by lasers. Tobalske admits to many fruitless hours trying to get stubborn pigeons to fly in such a situation. So he and his partners turned to one of nature’s supreme fliers — the hummingbird.
“Of the 9,000 species of birds, hummingbirds are the best,” he says. “Their default setting is flying. Other birds want to sit and perch, but we once had this female hummingbird that set the record by flying for 90 minutes straight — and that’s at 40 wing beats per second.”
Though hummingbirds generally weigh only as much as three paperclips, they can be highly aggressive and territorial with one another, especially the males. Tobalske has research video of a male viciously dive bombing one of its fellows and chasing it off. He generally studies Rufous hummingbirds, which are found in Montana. He says the birds migrate and can zip around at 25 mph.
Hummingbirds are interesting to Tobalske because they can hover and fly much like insects. They use a figure-eight sweeping motion when they fly, much like a human treads water. For years scientists assumed that the upstroke and downstroke of hummingbird wings support the birds equally as they flew, as they do with dragonflies and bees.
“But what we observed is that while hummingbirds converge on the bee style of flight, they retain a little bit of the bird component, where the upstroke does less than the downstroke does,” Tobalske says. “With most birds, there is evidence that the upstroke is inactive — that it is just a recovery stroke that sheds a bit of drag. But somewhere along the way, hummingbirds acquired the ability to support a little bit of their weight with the upstroke.”
In another major observation, Tobalske learned that hummingbirds flip their wings over at the end of each wing stroke, using a technique called pronating and supinating. So hummingbird lift comes from both sweeping their wings and then spinning them.
“This is the first time this has ever been shown in a live animal,” he says. “Insects do flip their wings similar to hummingbirds. But insects, lacking an internal skeleton, can’t use the muscles and pectoral girdle and wings to actively alter the twist and curvature of the wing like a hummingbird does.”
Tobalske’s partners in this research are Doug Warrick of Oregon State University and Don Powers of George Fox University. During the past three years, they have used a movable feeding apparatus filled with sugar water to turn hummingbirds around 180 degrees as they fly while their wake is illuminated by lasers in the misty PIV chamber.
“We can move the birds back and forth, side to side, whatever we want,” Tobalske says. “So our next step will be to actually study maneuvering.”
Part of the reason he studies bird flight is from pure fascination, but he hopes understanding how feathered creatures move about and migrate will ultimately help wildlife managers with their ecology and conservation efforts.
Waving goodbye at the end of his interview, Tobalske says, “We humans are so visual that for us it doesn’t exist until you see it. But when you wave to somebody, you have created a whole invisible vortex trail in the air, just as a bird does. It’s just that you aren’t using it to support your weight.”
— By Cary Shimek
|(Above) A Calliope hummingbird buzzes its wings at UM’s Flight Laboratory.
|An image produced by particle image velocimetry reveals the speed and direction of forces moving around a flying hummingbird.
|Bret Tobalske, director of UM’s Flight Lab and Field Research Station