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Hummingbirds and bats hover in very different ways

Researchers in the Lentink Lab studied over 100 individual bats and hummingbirds in Costa Rica to see how hovering flight evolved. Their findings will inform their development of flying robots. (Credit: Glenn Bartley via Stanford)

New research digs into how the only two animals that can hover, hummingbirds and certain bats, fly in place.

Each sunrise in Las Cruces, Costa Rica, River Ingersoll’s field team trekked into the jungle to put the finishing touches on nearly invisible nets. A graduate student in the lab of David Lentink, assistant professor of mechanical engineering at Stanford University, Ingersoll needed these delicate nets to catch, study, and release the region’s abundant hummingbirds and bats—the only two vertebrates with the ability to hover in place.

“We’re really interested in how hovering flight evolved,” says Ingersoll. “Nectar bats drink from flowers like hummingbirds do, so we want to see if there’s any similarities or differences between these two different taxa.”

Ingersoll’s nets worked, and he ended up examining more than 100 individual hummingbirds and bats, including 17 hummingbird and three bat species, during his field study. The results appear in Science Advances.

Through a combination of high-speed camera footage and aerodynamic force measurements, he and colleagues found that hummingbirds and bats hover in very different ways. They also found that the hovering of nectar bats—but not fruit bats—share some similarities with hummingbird hovering. This suggests that they evolved a different method to hover compared with other bats in order to drink nectar.

In addition to learning more about bats and hummingbirds, Lentink and others can apply what they learned to engineering problems, such as designing flying robots. Engineers have already created robots they based on hummingbirds and bats but haven’t known which of the natural counterparts of these robots hover most effectively.

Focus on flapping

It is simple to imagine how a flying animal supports itself by flapping downward, but in order to avoid exaggerated bobbing up and down, hovering animals must maintain this support while flapping upward as well. Hummingbirds and bats accomplish this feat by twisting their wings backward on the upstroke, continuously pushing air downward to keep them steadily aloft.

“If you look amongst vertebrates, there are two that can hover in a sustained way,” says Lentink. “Those are hummingbirds and nectar bats. And you’ll find both in the neotropics, like Costa Rica.”

To study these subjects, Ingersoll collaborated with a long-standing bird-banding project Stanford ecologists run in Las Cruces. Borrowing birds and bats from their project, he placed each animal in a flight chamber researchers outfitted with aerodynamic force sensors at the top and bottom of the chamber—equipment Lentink’s lab developed to measure extremely small changes in vertical force at 10,000 times per second.

These plates are so sensitive that they captured the vertical forces that every twist and flutter hummingbirds that weighed as little as 2.4 grams produced.

By synching those force measurements with multiple high-speed cameras recording at 2,000 frames per second, the researchers could isolate any moment of their subjects’ flights to see how the lift they generated related to the shape of their wings.

“I’d sit and wait for the hummingbird to feed at the flower. Once it was feeding, I would trigger the cameras and the force measurements and we’d get four seconds of footage of the hummingbird flapping at the flower,” says Ingersoll.

After their short stint in the flight chamber, Ingersoll returned the birds and bats to release them where the researchers caught them. The whole process took between one and three hours.

Different strokes

The researchers found that the bats and hummingbirds all exerted a similar amount of energy relative to their weight during these flights but that the hummingbirds, fruit bats, and nectar bats all hovered in very different ways.

The hummingbirds hovered in a more aerodynamically efficient way than the bats—the hummingbirds generated more lift relative to drag. In comparing wing shapes, the researchers found this efficiency is likely because the hummingbirds invert their wings more easily. Although the bats struggled with turning over their wings, they exerted a comparable amount of energy because they have bigger wings and larger strokes.

The researchers were surprised to find that nectar bats, which sidle up to flowers like hummingbirds, generated more upward force when the wings were lifting than fruit bats. Looking at their wing shape, the researchers found that nectar bats can twist their wings much more than fruit bats on the upstroke. So nectar bats’ hovering form is like a blend of fruit bats’ and hummingbirds’ hovering.

The researchers plan to build on these findings as part of their work on flapping robots and drones but Lentink also sees potential for more work beyond the lab.

“When Rivers proposed to do this study in Costa Rica, a field study was something I’d never hoped for. Now, he really inspired me,” says Lentink. “There are about 10,000 species of birds and most of them have never been studied. It sounds like too big a study to embark on but that’s what I dream about.”

Additional coauthors are from the University of Bremen. The National Science Foundation and the King Abdulaziz City for Science and Technology Center of Excellence for Aeronautics and Astronautics at Stanford funded the study.

Source: Stanford University