Why airborne hunters don't dive straight in
THE mystery of why birds of prey spiral in towards their victims may have been solved. They do it to make the most of their pin-sharp sideways vision, according to an American biologist.
Vance Tucker of Duke University in North Carolina, has long been puzzled by the way birds of prey approach their dinner. In studies of peregrine falcons in Colorado, he and his colleagues found that the birds almost always follow a curved path once they get within 1.5 kilometres of their prey. "We had observed these curved paths for years, and I was racking my brains to explain that," says Tucker.
He suspected the explanation lay in the birds' vision. Tests have shown they see objects in front of them most clearly when they turn their heads about 40 degrees to one side. But turning their heads in mid-flight might increase the birds' aerodynamic drag, slowing them down.
To test this idea, Tucker placed life-sized models of peregrine falcons and red-tailed hawks in a wind tunnel. Sure enough, force sensors showed that, at a wind speed of 42 kilometres per hour, the drag on birds whose heads were turned 40 degrees was more than 50 per cent greater than on those looking straight ahead. To avoid this, Tucker concludes, the birds keep their heads straight and follow a path called a logarithmic spiral. That way, they can keep one eye fixed on their prey. And while a spiral path is longer, Tucker calculates that the speed advantage more than compensates.