How birds stay in the air

The force a bird exerts to keep itself aloft has always been a bit of a mystery. Engineers are particularly eager for this information because it could inspire designs for futuristic drones. These pilotless flying robots might flap, dart and hover with birdlike grace and agility.

Many researchers have tried to calculate the forces involved in bird flight. Some used lasers, for instance, to measure the flow of air over a bird flying in the lab. Then the experts put those data into a complex mathematical expression — what’s known as an equation — to calculate the wings’ force.

But here’s the problem: When animals — including birds ­­— move, their whole body changes shape. This causes air to flow over it in turbulent patterns. And that makes calculations less reliable. Not only that, lasers can damage a bird’s eyes.

Now, a team of researchers from Stanford University in California has figured out what to do. Leading the effort is David Lentink. This aerospace engineer designed a special box equipped with force sensors.

The box is about the size of a large bird cage. It has two perches and a clear acrylic front. This lets scientists watch as a bird flies from perch to perch.

The box’s floor contains the spring-loaded sensors. The springs work like those in a bathroom scale. The greater the downward force, the more the springs squeeze together. But Lentink’s sensors don’t measure the weight of the bird. Instead, they measure the force generated each time a bird’s flapping wings push on the air inside the box. That air pushes on the box’s floor, transferring the force.

The sensors attached to the box’s floor are super sensitive. They even register air as it circulates inside the lab. So to avoid introducing errors, “We had to turn off the air conditioner during the experiment,” Lentink notes.

The team conducted its tests withtwo Pacific parrotlets: Ray and Gaga. The small white and light-blue parrots were trained to fly from perch to perch. (Each time they did it, the birds were rewarded with a seed.)

On each downstroke, the birds used double the force needed to counteract gravity and lift their weight, the sensors showed. On each upstroke, the bird exerted virtually no force. That means the upstroke wasn’t counteracting gravity at all.

Together, the two wing beats cancelled each other out. This allowed each bird to counteract gravity just enough to keep it aloft.

Lentink is especially pleased to have designed an experiment that does not harm the birds. He now is eager to put his research in motion: “I design flying robots in my lab and I want to improve them,” he says.

Jim Usherwood is an expert in figuring out how animals move on land and in the air. He works at The Royal Veterinary College in London, England. Lentink’s experiment makes good sense, he says. But Usherwood cautions that the new findings won’t apply to all birds.

“Some birds — think of cruising seagulls — act much more like airplanes,” he says. “Their wings are outstretched and support weight even when in upstroke. And some birds produce lift with their wings upside-down during the upstroke when flying really slowly. Hummingbirds do this.”

Lentink agrees that his new findings don’t hold for all birds. In fact, he now wants to measure the force birds use to do tricky maneuvers, such as turning corners or hovering.

Power words

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acrylic  Chemicals and materials based on a compound derived from acrylic acid (which has the chemical formula H₂C:CHCOOH, where H’s are hydrogen atoms, C’s are carbon atoms and O’s are oxygen atoms). Among acrylic materials is a common type of fast-drying paint and a family of clear plastics.

aerospace  A research field devoted to the study of Earth’s atmosphere and the space beyond or to aircraft that travel in the atmosphere and space.

drone   A aircraft or missile that carries no pilot aboard. A drone may fly autonomously, along a preprogrammed path or under the control of a pilot on the ground. A drone also may be called an unmanned aerial vehicle (UAV).

equation  In mathematics, the statement that two quantities are equal. In geometry, equations are often used to determine the shape of a curve or surface.

engineer   A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

force   Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.

gravity   The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

laser   A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.

lift   An upward force on an object. It may occur when an object (such as a balloon) is filled with a gas that weighs less than air; it can also result when a low-pressure area occurs above an object (such as an airplane wing).

parrotlet   Any of several genera of small parrots native to Central and South America. Some species of the birds are kept as pets.

sensor   A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly.

turbulence  The chaotic, swirling flow of air. Airplanes that run into turbulence high above ground can give passengers a bumpy ride.