Swirly Bird Gets the Prey

The quickest route to dinner is not a straight line--at least for some predators

• Mark Cheater
• May 01, 2001

The quickest route to dinner is not a straight line--at least for some predators. Peregrine falcons and some other raptors diving on distant prey follow a curved path known as a logarithmic spiral.

"That bothered us," says Duke University biologist Vance Tucker, who has studied peregrine flight patterns with colleagues in Colorado. "We thought, 'Why in the world would that happen?'"

Tucker says it may be due to the raptors' peculiar visual system. The birds have their sharpest distance vision at 40 degrees off-center. Since the birds' eyeballs can barely rotate in their sockets, they either need to turn their heads or fly at an angle to see clearly distant objects ahead of them.

Tucker tested raptor models in a wind tunnel and found that head-turning is similar to air-braking in airplanes--it increases drag. He says it is actually faster for the birds to dive along a curved path with their heads straight and their prey at 40 degrees until close enough to see their prey clearly straight ahead. Then the victim may get a fleeting and final chance to look its enemy straight in the eye.