In for the (linear) kill

Sarina Tracy, sarina.tracy@photonics.com

Most people wouldn’t consider birds to be instruments of wartime sabotage, but one species – a top-notch bird of prey – would have to disagree. During World War II, falcons were often released by British forces to kill Nazi carrier pigeons and prevent important messages from being received by the German front.

Falcons have very good eyesight and can spot their next meal up to a mile away. However, until recently, their hunting strategy – a seemingly basic part of the raptors’ behavior – remained a mystery.

“There were computational studies … that simulated this behavior,” said Dr. Suzanne Amador Kane, associate professor of physics at Haverford College in Pennsylvania. But no one had published any behavioral studies. As falcons hurtle through the air and lock sights onto their victims, this aerial attack has obvious lethal strategy. It just wasn’t clear what that was.

Then one day, it hit her: Cameras mounted on birds of prey could give humans a “falcon’s-eye view” of the birds’ flying tactics. It could finally be known how these birds maneuver in the sky to prevail over their victims.

Using personal contacts and social networking, the team connected with falconers around the globe, who attached miniaturized spy cameras to backpacks and tiny helmets worn by their falcons to film encounters during flights. When the footage was completed and sent back to Amador Kane, she and undergraduate student Marjon Zamani located the prey’s position on each frame by hand, then reconstructed each pursuit from the falcon’s perspective.

What they found contradicted preconceived notions of raptor pursuits. The falcons rarely ever flew directly after their prey. Direct flight is inefficient, wasting the predator’s time and energy. In the footage, the prey was never found in the center of the frame, showing that the falcons seldom positioned themselves directly behind their victims.

The team then looked for evidence of falcons viewing their prey at an angle of 40 degrees, a widely accepted strategy proposed by biologist Vance Tucker more than a decade ago. Tucker’s thought was that a falcon would keep its prey in the off-center specialized region of vision and fly in a spiral path toward it. It did so very rarely.

“Falcons have two regions of very acute vision: one directed almost in the forward direction and the other dramatically off to the side – 30 degrees off,” Amador Kane said.

The truth of the matter is that falcons fix prey in their sights and maneuver to keep the image motionless and centered against its surroundings so as to head it off in the least amount of time. This allows the predator to view its prey head-on with its visual field, often flying at speeds in excess of 200 mph during its attack. The prey does not see the predator move until the final instant when the falcon intercepts and strikes. This strategy, although revolutionary to see, is new neither in the skies nor on the ground. Once upon a time, your mom or dad might even have used it on you.

“Think about chasing a toddler around in the playground,” Amador Kane said. “They keep zigging and zagging away from you … you just have to head them off.”

The study appears in The Journal of Experimental Biology.