Hummingbirds are the fighter pilots of the avian world, diving and weaving at speeds of up to 55 kilometers per hour—then turning on a dime to hover midair, wings frantically beating, as they refuel on nectar. Now, through herculean efforts, researchers are one step closer to figuring out what makes the animals so nimble. The new work not only helps explain their complex choreography, but it may also lead to more maneuverable robots and drones.
Biologists have clocked how fast hummingbirds can fly and how long they can hover, but maneuverability—all that zipping back and forth—is “notoriously difficult to study,” says Peter Wainwright, an evolutionary biologist at the University of California (UC), Davis, who was not part of the new work. That’s because “it involves a complicated set of possible movements, and it’s very spontaneous.”
That didn’t stop Paolo Segre, then a graduate student at the University of British Columbia in Vancouver, Canada. He decided to try by filming hummingbirds in the wild, which are less inhibited about flying than their captive counterparts. To prepare, he spent the better part of a year perfecting and miniaturizing a four-camera, computer-coordinated system for high-speed filming.
Two months later, Segre was in Peru. He and his team hiked up mountains and crashed through jungles to find the perfect site. Once they set up camp, they constructed a large cage outfitted with the solar-powered camera system and started testing their hummingbirds, one by one. The researchers filmed each bird for about 30 minutes as it flitted between perches and visited a nectar-feeding station inside. Then they let the bird go and repeated the process. Segre and his team set up stations in three other locations: the Ecuadorian Andes, and high- and low-elevation camps in Costa Rica.
Getting the data wasn’t easy. In Peru, the team’s testing site was swarmed with army ants for 2 days straight. In Costa Rica, Segre and his colleagues had to wade across crocodile-infested waters—at night—in the middle of a lightning storm. “We were mostly terrified by the lightning,” recalls Segre, now an ecophysiologist at Stanford University in Palo Alto, California. The scientists eventually made videos of 207 birds belonging to 25 species.
Once they had the data, Segre's labmate, postdoc Roslyn Dakin, now at the Smithsonian Migratory Bird Center in Washington, D.C., developed sophisticated software with her colleagues to analyze them. Because there were four cameras, the researchers could reconstruct the flight pattern of each bird in three dimensions, measuring the number of times it accelerated, decelerated, turned, rolled, soared, or dove, among other maneuvers. Each of those simple moves repeated and combined into predictable patterns. “More complex maneuvers were made up of sequences of simpler maneuvers,” Segre explains.
When the researchers compared flight patterns among species, they found that each one tended to stick to the maneuvers it was best at (something especially true of turns). But they were surprised to find that heavier hummingbird species were generally better at accelerating and making tight turns. Based on studies in birds and bats, the team had expected the exact opposite. “But larger hummingbird species were actually more maneuverable,” Dakin says. The reason: Those heftier hummers had relatively bigger muscles and wings than smaller species, she and her colleagues report today in Science.
Several other trends emerged. Maneuvering behaviors that differed from species to species generally came down to structural and physiological traits such as wing size, wing surface area, weight, and muscle mass. Finally, when the team grouped the birds based on their flight patterns, they found the clusters reflected the hummingbird family tree: More closely related species had similar flight patterns.
Dakin says this new maneuverability “framework” could help roboticists understand how to tweak their flyers to be less clumsy and fragile. Particularly useful is hummingbirds’ ability to generate rapid wing movements, which helps with agility, says Andrew Biewener, a biomechanist at Harvard University. As a result, adds Robert Dudley, an organismal biologist at UC Berkeley, even more engineers are now studying animal flight than biologists.