Hummingbirds zooming around the garden from flower to flower and sipping nectar probably don’t appear at first glance to be models for instruments of war.
But the tiny thrumming birds are unparalleled aerial acrobats, power in miniature, instantly zipping forward and backward, diving quickly down and soaring back up, pitching, rolling and yawing, and even flying upside down.
Their sophisticated flying abilities have captured the attention of robot designers, especially those studying the use of drones in modern warfare.
“Hummingbirds are the best flyers out there,” said Bret Tobalske, professor of biology and director of the Flight Lab at the University of Montana. “They are extreme in their physiology and flight performance. They are incredibly maneuverable. They are capable of hovering indefinitely” — an adaptation driven by a love of energy-rich nectar.
As vehicles without crews have taken over the skies in conflicts, hummingbirds have become the subject of new research. The Flight Lab, accustomed to looking at the ecology, evolution and biomechanics of bird flight, is part of a broad effort, funded largely by U.S. defense dollars, to build a better robotic hummingbird. Mimicking these highly adapted birds — a phenomenon known as bio-inspiredtechnology — is the holy grail for the makers of flying robots.
“A real hummingbird can fly circles around robotic humming birds,” Dr. Tobalske said. “They can fly circles around real birds too.”
There have been hummingbird robots built — most famously the NanoHummingbird, built by AeroVironment, a private company, with funding from the Defense Advanced Research Projects Agency DARPA — but they have myriad limitations.
“None of these robots can fly fast forward,” Dr. David Letink a professor of Biomimetics at Groningen University in the Netherlands who has studied and built flying robots for decades. “That can only be performed by bigger robots.”
Aircraft design has long been informed by avian flight.
And numerous flight labs around the world study several types of creatures — hawk moths, dragonflies, bats and hummingbirds — to examine the secrets of their flying skills to build better machines. Much of the work is funded by defense agencies, toward the aim of using the knowledge to design better aircraft.
The work at Montana’s Flight Lab is funded by the Office of Naval Research, part of the Defense Department, under a contract totaling a little more than $660,000, part of $2 million over seven years that was allocated to institutions to study the birds. The Defense Department did not respond to requests for comment.
A robot based on a hummingbird might not be likely to carry weapons, but instead could be deployed to scout streets ahead of troops, spy on troop movements or be dispatched to search for wounded soldiers in places where humans can’t go, like inside a collapsed building or a tunnel.
Insects, interestingly, have evolved a very different approach to flying than that of birds and bats, whose wings evolved from arms. They have no muscles or nerves in their wings; instead some have a sophisticated pulley system worked by internal muscles on a hinge that control the wings in a manner similar to strings on a puppet.
“The fly wing hinge is perhaps the most mysterious and underappreciated structure in the history of life,” says Michael Dickinson, professor of bioengineering and aeronautics at the California Institute of Technology who recently published a paper on the topic.
Because pollinators are essential to plant growth, he said, “if insects had not evolved this very improbable joint to flap their wings, the world would be a very different place, absent of flowering plants and familiar creatures like birds, bats — and probably humans.”
Hummingbirds have their own improbable attributes. Though slight — cradling one is akin to holding a tissue — the hummingbird has nimble aerial behaviors that are “enormously rich and complex,” Dr. Tobalske said.
The wings of many hummers beat about 50 times a second, the most of any bird; by contrast a pigeon’s wings beat nine times a second. Hummingbird hearts, the size of a pea, beat 1,200 times a minute when the bird is active. Human hearts, by contrast, beat 60 to 100 times a minute.
The birds are also deceptively strong. “Hummingbirds have more power output gram for gram than any other vertebrate,” Dr. Tobalkse said.
The researchers here are primarily focused on the calliope hummingbird, which weighs less than two paper clips. Yet every fall, the calliope travels an extremely risky migratory path through wind, rain and snow from the Pacific Northwest to southern Mexico.
Much about hummingbird ecology is a mystery, largely because the birds are too small for tracking devices, though it is known that they are very transient, Dr. Tobalske said. “They spend three months traveling south and three months traveling north again.”
Researchers also study rufous and black-chinned hummingbirds.
There are nearly 370 species of hummingbirds in the world, found only in North and South America. When Spanish explorers first came to South America, they were delighted by the brightly colored iridescent, hovering birds and called them joya valadoras (flying jewels).
Despite the time and funding that has gone into trying to fully grasp the complex dynamics of hummer flight, a thorough understanding remains elusive.
A decade ago, researchers took a mobile lab into the Peruvian Amazon, Costa Rica and the Ecuadorean Andes. They studied 200 hummingbirds, and tracked 330,000 maneuvers in their natural habitat.
Paolo Segre, an assistant professor at the University of Wisconsin, Green Bay and Roslyn Dakin, associate professor at Carleton University, divided the maneuvers into three categories — pitching up and down; accelerating and braking; and complex, sharp turns to veer off in a different direction.
One of their discoveries, counterintuitively, was that larger hummingbirds can maneuver more easily than smaller birds because they have more muscle mass and a larger wing size in proportion to their bodies.
One aspect of the bird’s flight dexterity is the way it harvests the inertia from wing thrusts to power its extraordinarily agile maneuvers.
Dr. Tobalske and a doctoral student, Rosalee Elting, have spent years studying the hummingbird’s escape maneuvers — how it rapidly flees from predators.
(Although he also displayed a photo of a hummer whose escape efforts failed — a praying mantis waiting at a feeder captured and devoured the bird.)
To study escape behaviors, Dr. Tobalske and Ms. Elting moved toward a bird enclosed in clear plexiglass to frighten it. As it fled, they took extremely high speed video with cameras developed by the Pentagon to film and study ordinance blasts. That allows researchers to slow the videos down to parse the smallest components of flight in high resolution.
The birds, as they escape, pitch up and then roll and face away from the feeder and then pitch down and fly away. “It happens in 120 milliseconds, so very short,” Ms. Elting said.
She recently embarked on a separate project to unpack maneuvers that occurred during battles between male hummingbirds as they fended off invaders approaching flowers and feeders in their territory.
The birds are uncannily aggressive. “They’ll chase someone off five times for every one time they use the feeder,” she said.
Four distinct battle maneuvers have been identified as the birds hover: chasing and signaling (like charging at an invader); a face off; spiral flight; and jousting.
One aspect of the study of how hummingbirds fly involves measuring the air velocity, pressure and viscosity that are flowing around beating hummingbird wings and support flight. Equations that explain this phenomenon are so complex that it takes a supercomputer weeks to devise a model of the aerodynamic forces.
The birds’ bodies, especially the wings and tails, morph repeatedly and undergo numerous changes as they fly.
That complexity is difficult to replicate. For robot designers, “the biggest challenge is to change the shape of the wing and the tail like a bird does,” Dr. Letink said.
“This has not really been cracked by engineers,” he said. “Making the wing shape change like you see in a bird, not only sweeping motion, but the twisting motion and the folding, to do all of that is super hard for an engineer.”
Another difficulty is recreating filoplumes that provide sensory input — the tiny, palm-tree-shaped feathers that monitor wind speed, temperature and the condition of the feathers and then send that information to the bird’s brain.
While many studies examined wing motions, the tiny brain that drives these flying abilities largely remains a black box. One recent study found that hummingbird neurons are more sensitive to stimuli than those of other species, which helps them process objects faster and enables their nimble flying.
There are other major technical hurdles to overcome for robot makers — short battery life and the sound a robot makes, which would be a dead giveaway that a hummingbird was a robot. Response to sensory information is hard to build into a robot — how a hummingbird reacts to the scent of a flower or the sight of a praying mantis.
Another major obstacle to replicating a hummingbird’s flight lies in the bird’s singular adaptation to wind, or gust tolerance.
“We like to fly our drones on calm days because they don’t like wind,” said Moble Benedict, who builds robots at Texas A&M University that are based on the kind of research the Tobalske lab does. “It’s the exact opposite with birds. You see more in the sky on windy days. They are not trying to fight winds, they are flying with the wind.”
It’s especially important because as the climate warms turbulence is increasing.
In one study of perturbations at the Flight Lab, Remy Delplanche, a doctoral student, placed a Eurasian-collared dove with a tiny, rare earth magnet on its back in a wind tunnel between two other magnets. As the bird flew and cameras recorded, the magnets were activated, which pulled the bird off course.
Researchers filmed the experiment to dissect how quickly and reflexively the bird recovered — in split seconds. At some point, it may be possible to design a robot that can continue flying in the wind, but not yet.
Insects also provide a model for robots, especially the hawk moth, bumble bee and dragonfly. Small robots would most likely be used as spies — a hawk moth could land on flowers at a restaurant patio and keep tabs on enemies eating dinner, for example.
“Minority Report,” the 2002 dystopian film based on a short story by Philip K. Dick, featured a swarm of heat-seeking robotic spiders that sought to find the character played by Tom Cruise, who hid in a bathtub of ice water.
Developing a hummer robot that could fly like the real thing is possible but still years away, experts say.
“I think, ultimately yes,” Dr. Tobalske said. Accomplishing that feat would demand a considerable commitment of time and substantial funding, he estimated, and even then could take a decade or two.
It might be far easier, he joked, “to think about raising a hummingbird and training him with a little backpack.”
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