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The Flight Of The Hummingbird Decoded

Hummingbird enthusiasts can rattle off all kinds of impressive trivia about the world's smallest feathered friend. They're the only bird able to fly backwards; their tiny hearts, which are the largest in relation to their size among the bird family, beat about a thousand times every minute; and they weigh as much as about three paperclips.

Despite this knowledge, it wasn't until this year that researchers could say for certain how the birds were able to hover. The only other animals that share the ability are insects, and for years, scientists assumed that hummingbirds had more in common with six-legged insects than with other birds.

As it turns out, hummingbirds do have some of the same flight mechanisms as insects, which zip around in much the same way. But now researchers at Oregon State University, the University of Portland, and George Fox University can say with some certainty that they have tapped into some of the secrets of the fast flapping. In a recent article in the journal Nature, researchers made an announcement sure to please trivia-lovers everywhere.

"It was thought that hummingbirds had converged on this flight style of insects because the wing movements were nearly identical to those of insects. It was thought that the way they moved air around was just like insects. It is, to a certain degree, and to a certain degree, it's still like that of the rest of the birds, so it's somewhere in between,” says Doug Warrick of Oregon State University, Corvallis, and co-author of the study. Researchers used to believe that hummingbirds produced equal amounts of lift during both the upstroke and down stroke. What the team in Oregon discovered is that the tiny birds support 75 percent of their weight during the wing's down stroke and 25 percent during the upstroke.

Tracking the movement of a wing that's moving at a top speed of 80 beats per second is tricky, but not impossible. The secret is to look at the air around the bird's wings, something that high-speed cameras haven't been able to do. "The movement of the wings themselves has been well-described for a very long time, but nobody really understood what kind of structure they were producing in the air. You don't know what kind of aerodynamic mechanism they're really using until you can see the air,” explains Warrick.

"Humans suffer from this issue that if we don't see it, it doesn't exist, in a way. We're very visual,” says Bret Tobalske, University of Portland researcher and co-author of the paper. He explains that earlier research was based on looking at the wings and trying to make guesses about the aerodynamics.

To see the wake of the frenzied wings, scientists needed olive oil, a sugar solution, and some pretty high-tech equipment known as digital particle imaging velocimetry, or DPIV. Researchers were able to get the birds to hover in one place by feeding them an artificial nectar solution from a syringe while filling the room with microscopic droplets of olive oil. The droplets are so light that they literally float and capture any movement of the air. A pulsing laser illuminates the droplets for short periods of time, enabling them to be recorded by cameras. "The point of the laser is that it's essentially a flashbulb for the camera that's going to take pictures of the atomized olive oil that we blow into the test section,” says Warrick. The swirling movement of the droplets illustrates how the air around the wings moves.

"By measuring that energy in that wake, we find that there's quite a bit more energy imparted to the air during the downstroke. For a hummingbird that's hovering at a feeder and its body is almost vertical, that means that when it's bringing its wings forward, then it would be downstroke," says Warrick. "Most of their weight support is produced by that portion of the wing cycle–when they're moving their wings forward, just like every other bird.”

Tobalske points out hummingbirds are more of an "extreme athlete” than other birds. He says that a hummingbird the size of a pencil eraser is capable of making a non-stop flight across the Gulf of Mexico. "That's an incredible use of a limited fuel supply.” By studying the musculoskeletal structure of birds, Tobalske says, researchers can gain insights into other muscles.

After years of tracking wing motion with smoke or helium bubbles and manually taking pictures, which would be virtually useless with hummingbirds, DPIV has allowed scientists to have new insights. "This technology has just recently become available and what it does is that the computer replaces the poor biologist who is manually tracking the particles,” says Warrick. "Suddenly you have a tool where you can take much greater samples of much larger portions of the wake in a variety of conditions and really assemble a picture of the wake of the bird.”

The new understandings came with a healthy dose of serendipity. A sales representative had come to Tobalske's lab in Portland to demonstrate the image-capturing technology in Tobalske's wind tunnel. The researchers had been working with hummingbirds, but hadn't planned to use the new technology to study them. "We had the hummingbirds on hand, and not knowing anything else better to try, we ran some air past a ruler and some boring things like that. We thought, ‘Let's just pop a hummingbird in there and see,' and lo and behold, immediately, we started asking, ‘Where's the upstroke?'”

"We put them in there, and fired this thing up, and right away, it was literally like buying a new telescope and you turn it on and point it to the stars and you just see new things right away. It was astonishing,” says Warrick.

"So we said, okay, we'll buy the system,” laughs Tobalske. "The sales rep was giving us a demo, and the demo turned out to be quite exciting.” In fact, it took the scientists some time to convince themselves that the incredible images they were seeing were in fact, real. "It turns out that the upstroke is there, it's just nowhere near as present–it's not doing anywhere near as much as the downstroke,” says Tobalske.

While the research is still ongoing, Tobalske adds that the military has expressed interest in learning more and has plans to develop "bird bots,” which would be miniature autonomous vehicles that could take pictures and be controlled by satellites and operate on small batteries.

The researchers expect to have the new system up and running by next year to continue their work.

Stephanie Cartier is pursuing a master's degree in Technical Communication at the University of Washington.

Images:

Top: Researchers use a feeder to attract hummingbirds to fly to the same spot in the wind tunnel in a study to determine the aerodynamics behind hummingbird flight.

Middle: This image shows the direction of the air currents around a hummingbird's wing.

Bottom: In a rare moment, a hummingbird sits still long enough for a quick picture.

Photos: Bret Tobalske


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