Falcons use guile to track down prey

Computer modelling of pursuit strategies reveals birds use motion camouflage to deceive their targets
Peregrine falcon
Fighting ace … the peregrine falcon. Photograph: Peter Barritt/Corbis

Falcons are well known for the speed at which they dive (up to 322 km/h), but until now no empirical work had been done on how they track their prey, because it was technically too difficult to record their trajectory in three dimensions. While modelling raptor pursuit strategies, Suzanne Amador Kane, associate professor of physics and astronomy at Haverford College, Pennsylvania, watched wildlife documentaries and became convinced that fitting the birds with miniature video cameras could overcome this difficulty. The Journal of Experimental Biology published the conclusions of her research last month.

In partnership with professional falconers, Kane fitted gyrfalcons (Falco rusticolus), peregrine falcons (Falco peregrinus – pictured) and hybrids with miniature videocameras – on their head or back, depending on size. She was thus able to film 48 pursuits targeting carrion crows, bustards and other species in various parts of rural Belgium, Great Britain and the US, and the desert in Dubai. "We analysed the video images, one at a time using software, in order to determine the prey's apparent position in the bird's visual field, as well as other factors such as their relative speed and the distance between them," Kane says. The data was then interpreted using computer simulations based on predator-prey pursuits involving insects and mammals. The results differed from what was expected.

"The simplest way to pursue a prey is to fly straight at it. So you expect to find the prey in the middle of the video image," Kane explains. "Another idea, proposed by the biologist Vance Tucker in 2000, was that falcons fly along a spiral trajectory so as to keep their prey at a roughly 45-degree angle in their visual field, the angle at which they have the highest visual acuity."

Both these assumptions proved incorrect and the researchers turned to bats and dragonflies to gain an understanding of how falcons operate. "Previous work shows that these animals use a pursuit strategy known as motion camouflage," Kane adds. "The predator anticipates the point at which it will intercept the prey based on its speed and sets its course for that spot, keeping the prey at a constant angle in its field of vision. Which is exactly what we observed with the falcons."

This is a highly effective strategy because it fools the prey. The predator's apparent position in its target's visual field does not change, giving the latter the impression that the threat is still at a distance, until it is too late. Sailors are familiar with this manoeuvre, steering a course at a constant angle to the vessel they aim to board. The collision course falcons deliberately adopt is exactly the same.

However, despite its apparent sophistication this pursuit strategy fails three-quarters of the time. If, for instance, the prey turns and flies towards the predator, it upsets the pursuit. Kane now plans to study other raptors – goshawks and sparrowhawks – that have developed their own pursuit strategies.
