So good at hunting they’re named after their prize, birds of prey have a whole lot to teach us about flight and airborne pursuit. But for whatever reason - perhaps because they’re so quick - very little research has been done to figure out their hunting strategies. Exactly how do they catch their equally agile prey in mid-air?
Also known as raptors, the birds of prey group includes hawks, eagles, vultures, osprey, secretary birds and falcons. As the smartest of the bunch, falcons are the perfect place to start deciphering raptor pursuit tactics, so Suzanne Amador Kane, Associate Professor of Physics and Astronomy at Haverford University in Pennsylvania and undergraduate student, Marjon Zamani, found four willing falconers to mount tiny video cameras on a handful of these baby-faced killers and press play.
They collected video footage between 2009 and 2013 of six peregrine falcons (Falco peregrinus) and two hybrid falcons (gyrfalcons crossed with Saker falcons) hunting wild crows in parts of the US, the UK, the Netherlands and Belgium. The video cameras were mounted to the falcons’ backs and a few to their little hoods to provide a real bird’s eye view, and as you can see below, it’s virtually impossible to figure out most of what’s going on because these birds can hit speeds of up to 322 km per hour:
“Raptors have incredibly fast visual processing abilities, measured at about 90 to 100 times per second - several times that of humans. Events that seem a meaningless blur to us are easy for them to analyse and respond to,” says Amador Kane.
So they slowed down the footage, which was shot at 30 frames per second, and analysed the images frame by frame to track the position of the prey. From this they could figure out how the position of the prey related to the falcon’s field of vision, and then reconstruct each pursuit from the falcon's perspective. Reporting their findings in today’s edition of the Journal of Experimental Biology, Amador Kane and Zamani proposed three strategies that the falcons were likely use in their pursuits and used computer models compiled from the video footage to determine which one was the best fit.
The first prediction was based on the simplest form of pursuit - flying directly towards their prey, which would keep it in the centre of the falcon’s field of view. Bees, flies and tiger beetles prefer this tactic, but Amador Kane argued that it would waste the falcons’ time and energy, and sure enough, the falcons in the video footage rarely followed their victim's path.
So then it was on to strategy number two - falcons adjust their movements to best suit their large, forward-facing eyes. Proposed in 2000 by Victor Tucker from Duke University, this strategy is based on the raptor eye having two foveae, which are tiny depressions in the retina that allow for acute vision. We have one in each eye and they help us with activities such reading and driving. Raptors have one deep fovea and one shallow fovea in each eye, the former pointing forwards and at approximately 45 degrees to the right or left of the head axis, while the latter also points forwards but at approximately 15 degrees to the right or left of the head axis.
Raptors don’t really rotate their eyes in their sockets, possibly because this would mess with the proper functioning of the foveae, so instead they sometimes physically move their heads to the side to get the best view using their deep foveae. Observing falcons, hawks and eagles, Tucker found that when looking at objects that were closer than 8 metres away, these birds would look at them front-on, but as the distance increased to 21 metres, they started to turn their heads to the side. At distances of more than 40 metres, Tucker reported that they looked at the objects side-on 80% of the time.
So based on this, he suggested that to get the best view of their prey from a distance, raptors could dive in a logarithmic spiral formation (like a snail shell) with their heads straight and one of their eyes always looking sideways to keep their prey at the optimal 45 degree angle for the deep foveae. This would make more sense than flying straight towards their prey with their heads on the side, which would create unnecessary drag. "Although the spiral path is longer than the straight path, a mathematical model for an 'ideal falcon' shows that the falcon could reach the prey more quickly along the spiral path because the speed advantage of a straight head more than compensates for the longer path,” Tucker reported in the Journal of Experimental Biology.
But when Amador Kane and Zamani compared Tucker’s strategy to their third proposal - interception - it lost out as a slower and less effective method. The interception strategy predicted that the falcons would fix their prey in their sights and then manoeuvre themselves in a way that makes the prey appear motionless against the background so they could quickly fly towards the likely interception point.
"Researchers studying bats and dragonflies have found these animals use an interception strategy called 'motion camouflage', in which the predator keeps its prey at a fixed visual angle chosen to match the prey’s velocity to achieve the shortest interception time. That’s what we observed in our studies,” says Amador Kane.
This strategy also works best, she says, because motion camouflage helps conceal the predator’s motion on the prey’s visual field. “In addition, the range of angles we measured corresponds with the falcon’s other preferred angle for vision. Tucker had a good point: the falcons like to sneak quick peeks at their prey with the sideways vision during chases!”
Not that crows or similar prey animals are forever doomed to death by interception. Amador Kane said that in their footage, the prey would often double back and fly almost towards the falcons, or fly sideways towards the falcon rather than fleeing away from it. "To escape, prey need to out-manoeuvre the predator by flying (preferably unpredictably!) so it can’t change course and follow them,” she said. “However, our work suggests that another goal may be to prevent the predator from achieving a visual fix. Indeed, we see that the prey can achieve this by flying cross-field and toward the predator."
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