Hummingbirds and nectar bats are the only two vertebrates with the ability to hover in place.
In January of 2016, Stanford graduate student Rivers Ingersoll went to Costa Rica to have a closer look at the phenomenon.
Along with a team of ecologists and engineers, Ingersoll meticulously recorded the flight of over 100 different bats and hummingbirds. The observations made during his three-month trip could someday support the design of new kinds of hovering robots.
Ingersoll collaborated with a long-standing catch-and-release project run by Stanford ecologists in Las Cruces. After birds and bats were caught during the day, Rivers and his team transported them to a contained field station – one that housed an artificial flower for the subjects to feed from.
The engineers used aerodynamic sensors, located on the top and bottom of the test chamber, to measure the birds’ and bats' forces. Scale-like plates detected twists and flutters of hummingbirds weighing as little as 2.4 grams.
The instrumentation, developed in the lab of Ingersoll’s professor David Lentink , measures extremely small changes in vertical force at 10,000 times per second.
By synching the 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 being generated related to the shape of their wings.
“I’d sit and wait for the hummingbird to feed at the flower,” said Ingersoll . “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.”
Ingersoll spoke with Tech Briefs about why it is so important to have an up-close understanding of the hummingbird and nectar bat.
Tech Briefs: What did you find out about how hummingbirds and nectar bats are able to hover?
Rivers Ingersoll: We learned that hummingbirds generate lift on both the downstroke and upstroke to hover more efficiently, while bats rely on sweeping their relatively larger wings over a wide area to hover efficiently.
Tech Briefs: What was most surprising to learn?
Ingersoll: We found that hummingbirds adjust their neck to drink from flowers at different angles without changing their wingbeat frequency or vertical force generation. Across the bat species, we found that nectar bats invert their wings more during the upstroke relative to fruit bats, allowing them to support more of their weight during the upstroke. This is similar to the method used by hummingbirds, suggesting convergent evolution among nectar drinking vertebrates.
Tech Briefs: Why is it important to learn how hovering flight evolved?
Ingersoll: Hovering is a specialized behavior that enables certain species access to the energy-rich resource of nectar in flowers. By learning about how this hovering flight has evolved in hummingbirds and nectar bats, we can fine commonalities that can be applied to our design of flapping winged robots.
Tech Briefs: How did you capture these movements and measurements?
Ingersoll: We used two phantom high speed cameras to capture the movements at 2000 frames per second. Each camera was calibrated, allowing us to recreate the wing motion in 3D. In addition, we designed an Aerodynamic Force Platform to measure the instantaneous vertical force generated by the hummingbirds and bats. This Aerodynamic Force Platform acts like a very sensitive scale, measuring and integrating the pressure generated by the hovering animal.
Tech Briefs: What inspires you as you do this work?
Ingersoll: After spending years designing this new Aerodynamic Force Platform measurement method, I was excited to bring it out to the field to see what I could learn. Each time I trained a new species to hover in the flight chamber, I was able to see for the first time how it generates force over the wingbeat.
Tech Briefs: What’s next for you, now that you have this data?
Ingersoll: We have published our findings in the journal Science Advances , and hope fellow researchers can build off our discoveries to learn more about hovering flight.
What do you think? Share your questions and comments below.
Transcript
00:00:00 [MUSIC PLAYING] If you go to the neotropics, you find this wonder of diversity. There's so many different animals. And one of the special things that evolved is actually the ability to hover. Both hummingbirds and nectar bats evolved to hover in front of flowers to reach the energy-rich resource of nectar. We traveled down to Costa Rica to compare how hummingbirds and nectar bats hover.
00:00:31 A real downside from working in the lab is that you have no access to diversity. It's just not possible to study so many species. Ideally, you want to actually bring your lab to the diversity. In this catch and release field study, the hummingbirds would come to the lab during daytime. And they would just be there for a couple of hours. We collaborated with the Stanford ecology department. So they would catch birds and bats during the day and night.
00:00:57 And then we'd transport them to our field station, where we'd train them to feed from a little artificial flower. And then we'd film them with high-speed cameras as well as measure their forces. It's sort of a scale to weigh the birds, but in midair. The smallest was actually about two grams, so really, really tiny. And as they're flying, they generate pressure waves that are transmitted at the speed of sound
00:01:18 to the bottom and top of this kind of closed flight volume. By using aerodynamic force plates, which are basically really sensitive scales on the bottom and top of this closed volume, we can measure the instantaneous vertical force that these hummingbirds and bats are generating. We were able to compare over 100 different individuals of hummingbirds and bats. We had 17 species of hummingbirds and three species of bats. One of our biggest findings was that all
00:01:41 the hummingbird species seemed to generate very similar vertical force on the downstroke and the upstroke, with a very significant amount on the upstroke. Across 17 species, they all did the same. It was really surprising, actually, to us. And what's really special in hummingbirds is that they're very good at this. They can actually generate lift at both the downstroke and the upstroke.
00:01:59 But the bat species really generate less lift on the upstroke. Interestingly, the nectar bats generate more upstroke force than the fruit bats, as they kind of invert their wings a little more. So hummingbirds are actually aerodynamically much more efficient. But bats make up. What they do is they have a gigantic wing. And they can move that wing much slower.
00:02:19 And that makes them much more energetically efficient. And so if you compare hummingbirds to bats, they're actually equal. We're just interested in how these different species evolve and why they want to generate lift in the upstroke. But secondarily, from a robotic perspective, if we learn things from how these hummingbirds and bats can hover efficiently, then we can apply that as we design our new [INAUDIBLE] vehicles and robots.
00:02:42 [MUSIC PLAYING] For more, please visit us at stanford.edu.
