Why Don't Birds Crash? It's All in Their Posture

Birds crash into things sometimes, whether it's windows, cars, plans, or even unfortunately the occasional wind turbine. But for the most part, it's smooth sailing for these expert fliers. And given that navigating a cluttered environment at high speed one of the greatest challenges in biology, researchers wanted to learn how birds have managed this feat. It turns out it's all in their posture.

Specifically, there are two highly stereotyped postures that birds have mastered to avoid obstacles in flight, a new study describes in the journal Proceedings of the National Academy of Sciences.

"This was somewhat surprising to us," lead author David Williams said in a statement. "In lower-order animals like insects, we think of these very stereotyped motor programs where you stimulate your muscle, and the passive dynamics of your exoskeleton or the tendons attached to that muscle control most of the motion."

"But when you look at higher-order animals," he added, "it's common to expect that those motor programs are going to be more complex, and there's going to be more subtle gradations in those programs. So it was surprising to see a very high-order animal like a bird using very simple motor programs. Biology is optimized to be just good enough to work, so what that indicates is those are very effective motor programs."

Unlike past studies, this one focuses on how birds adjust their in-flight posture not when it's clear skies, but when they encounter gaps between obstacles, big or small. To better understand the birds' in-flight postures, Williams and colleagues built a 20-meter-long flight corridor - essentially an obstacle course consisting of a number of vertical poles - for pigeons to fly through.

While the research team expected that birds would adopt a myriad of postures to fit in between various objects, they were surprised by the reality.

"What we actually found was there are two very distinct, stereotyped postures that are adopted," he said. "We thought there would be body rotation. We thought there might be intermediate stages where they would pull their wings in a little bit. We thought there would be stages where they might have one wing up and one down. We thought it would be catch-as-catch-can, and it's not." (Scroll to read on...)

One of these postures is referred to as "wings paused." This is when the birds hold their wings wide out, at the top of the upswing of their flapping. In the second posture, the birds tuck their wings back against their bodies, almost as if they were perched on a branch.

"The paused posture ... interrupts their wing beat cycle for shorter periods of time, so they tend to lose less height, and their wings are ready to hit the air running, so to speak," Williams said. "We thought maybe they were using the ballistic posture in cases where they want to get smaller, but it doesn't actually make them much smaller from a forward angle. What it does do is reduce the amount they're going to rotate or be disrupted if they do collide with anything."

What's more, previous research has revealed that pigeons in particular are actually actively making the decision of which of their two postures they want to use to best avoid an object mid-flight.

It was thought that perhaps which way the birds chose was simply based on the timing of their flaps, but that does not seem to be the case.

Birds are pretty impressive fliers - even better than anything humanity has ever built. And now this new study has shed more light on the method behind their madness.

In addition, the researchers hope their findings could open the door to new ways to program drones and other unmanned aerial vehicles to avoid similar obstacles in the future.

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