Hurtling through the air, a falcon locks its sights onto a victim as they engage in mortal combat. Intrigued by how flocks of birds respond to aerial attack, Suzanne Amador Kane from Haverford College, USA, had realised that she couldn't interpret how flocks react to raptors until she understood the predator's hunting strategy. But when she investigated the literature, it was clear that little was known about how falcons pursue their prey. 'There were computational studies [...] that simulated this behaviour', recalls Amador Kane, but no one had published any behavioural studies. Amador Kane was stumped until she and her team saw a BBC documentary and realised that she could mount minute cameras on birds of prey to get a falcon's eye view to understand their lethal strategy. They publish their discovery that falcons head off their prey by flying so that the target appears stationary in the falcon's visual field in The Journal of Experimental Biology at http://jeb.
Resorting to personal contacts and social networking, Amador Kane linked up with falconers around the globe who were happy to attach miniaturised spy cameras to backpacks and tiny helmets worn by their falcons to film encounters during flights. Then, when the movies rolled in, Amador Kane and her undergraduate student Marjon Zamani painstakingly located the prey's position on each frame by hand before reconstructing each pursuit from the falcon's perspective. Eventually, the duo simulated three possible strategies that the falcon could use to find out which agreed best with their observations.
In the first strategy, the falcon would simply fly directly after the prey, but this is usually inefficient, wasting the predator's time and valuable energy when the victim takes evasive action. Calculating that the prey would always be found at the centre of each frame in the movie if the birds used this approach, it was clear that the falcons rarely followed the victim's path, ruling out the strategy.
Amador Kane and Zamani then tested the second strategy, which had been proposed by Vance Tucker over a decade earlier. 'Falcons have two regions of very acute vision: one directed almost in the forward direction and the other dramatically off to the side, 30deg off', explains Amador Kane. Tucker had suggested that raptors would fly so that their prey was always at an angle of 40deg to them, allowing the predator to keep the victim in the off-centre specialised visual region. However, if that were so, the falcon would fly in a spiral path towards the prey. The duo looked for evidence that the falcon viewed the prey at angles greater than 30deg, but found that the birds did so only very rarely.
So the scientists tested the final strategy, where the falcon fixed the prey in its sights and manoeuvred to keep the prey's image motionless against the background in order to head it off in the least amount of time. Amador Kane explains that there are two advantages to this strategy: first, the predators can view the prey head on with the central visual field, rather than off to one side; and second, the victim does not see the predator move until the final instant when the predator strikes. Analysing the video footage and simulations, Amador Kane and Zamani realised that this is exactly what falcons do.
So, falcons aim to head off their prey during pursuit, and it turns out that bats and even humans do this too. 'Think about chasing a toddler around in the playground: they keep zigging and zagging away from you... so you just have to head them off', says Amador Kane, laughing.
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REFERENCE: Amador Kane, S. and Zamani, M. (2014). Falcons pursue prey using visual motion cues: new perspectives from animal-borne cameras. J. Exp Biol. 217, 225-234.
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