If pursued by a goshawk, make a sharp turn

Humans have been using birds to hunt for thousands of years. And though the practice is known as falconry, falcons aren’t the only birds that have been trained for the hunt. Northern goshawks (Accipiter gentilis) — a species of raptor found across northern North America and Eurasia — have been among the favorites for the sport.

Suzanne Amador Kane and her colleagues at Haverford College in Pennsylvania wanted to know more about how goshawks hunt. Such information is useful for understanding the birds as well as to design things like flying robots. The researchers teamed up with Robert Musters, a master falconer from the Netherlands, and his goshawk, a female named Shinta. Musters made a special fiberglass hood for Shinta to which he attached a tiny camera. Outfitted with the hood and camera, Shinta was set free to fly and hunt as she liked.

The researchers ended up with several hours of video, which included 10 pheasant pursuits, six rabbit pursuits, three pursuits of lures and 10 landings on perches. The pursuits were pretty quick, just a few seconds each, but Kane and her colleagues were able to tease out details of the bird’s motion from the video. Their findings appear January 21 in the Journal of Experimental Biology.

“The initial surprise attack often was followed by an extended pursuit in which the prey fled to avoid capture,” Kane says. “In these scenarios, the goshawk used its exceptional speed and agility in two ways.” When a rabbit or pheasant was stationary or fleeing in a line directly away, the bird flew straight towards its target. “However, when the prey tried to evade capture by fleeing at an angle, the goshawk first flew to intercept” its prey by flying along the most direct path, she says. Only when it got close did the bird swerve to pursue its prey from behind, moving in the same direction as the prey. In both strategies, the goshawk kept its prey fixed in its visual field at a constant angle. From the perspective of its prey, though, the goshawk appears to be stationary, not getting closer — a phenomenon is known as motion camouflage.

When a rabbit or pheasant ran away or swerved, the goshawk was agile enough to keep up — except when the prey turned abruptly sideways. When that happens, the prey “manages to break the predator’s visual fixation,” Kane says. The rabbits and pheasants used these sharp turns in their escape attempts during 8 of the 16 recorded pursuits.