This helmeted hawk may look ready for battle, but she's actually about to do some serious science. Researchers who strapped a camera to a goshawk's head have discovered that these birds of prey use a clever strategy to try to snag their fleet-footed meals.
The findings, described in the Journal of Experimental Biology, could also shed light on why certain evasive tactics work better than others, and could help engineers looking to design future flying robots.
Scientists have long studied how certain small animals maneuver to avoid becoming a hungry raptor's next meal, but relatively little research had been done on how these raptors managed to hunt their prey down, said lead author Suzanne Amador Kane, a physicist at Haverford College in Haverford, Pa.
"No one had really studied experimentally how they used visual guidance to find their prey, and what kind of trajectories they used to fly after them," Kane said.
There were a few ideas as to how goshawks would move to capture their meals, Kane said. An earlier theory posited that because the birds' sharpest vision is off to the side rather than straight ahead, the birds would track prey in a curving flight path. Another option was that the birds used classical pursuit – that is, they hunted prey by keeping the target directly in the center of their visual field.
To get a bird's-eye view of what was really happening, Kane and her colleagues outfitted a goshawk with a helmet-mounted camera (a helmet built by falconer and engineer Robert Musters in the Netherlands, and worn by his goshawk, Shinta). Separately, British-based falconer David Burn filmed goshawk flight paths, to give some ground-based context for their hunting tactics.
The researchers quickly ruled out the side-eye theory. When a goshawk was going after a target (say, a rabbit or a pheasant) that was standing still or running directly in front of them, they used classical pursuit: The bird looked straight ahead, keeping its prey in the center of its field of view.
From the bird's perspective, if the prey is in the center of its field of view, it doesn't move; the animal just looms larger and larger until it nearly fills the goshawk's field of view. That's when the raptor is close enough to go in for the kill.
But life doesn't always give you straight lines. What about when the birds of prey had to chase an animal that was running at an angle? In these cases, the goshawks would keep their target (say, a rabbit) at a specific point in their field of view. This point wasn't in the center, but at an angle that allowed the hawks to take the fastest path toward the rabbit, while still keeping the rabbit in the same unmoving position in their field of view. (Where does that point lie in their field of view? It depends on the particular angle at which the animal is running away.)
Staying on this special point, at this special angle, has other potential advantages. Since the rabbit isn't moving in the goshawk's field of view (only appearing larger as it gets closer), it isn't moving from the rabbit's perspective, either. This phenomenon is called motion camouflage.
If the rabbit doesn't sense any movement in the distance, that might make it easier for the goshawk to sneak up on the rabbit – and there are studies to support this idea. But there are plenty of papers that say the opposite – that prey animals specifically react to predators coming in at this fixed, "motionless" angle, because they know the predator's coming in for a direct hit.
"I've been trying to think of experiments that might resolve how to separate this issue from the looming cues, which complicate such studies," Kane said. "So, no clear answer on this one."
The scientists called this efficient strategy "constant absolute target direction." Though it sounds strange, it's actually very intuitive, Kane said – humans do it too. She recalled kayaking on Lake Natoma in northern California with her kids 2 1/2 years ago.
"I told my kids, 'Don't go far away.' And of course what do they do? They just take off in their kayaks. And so I realized, as I was chasing them, that I was doing this -- without even thinking about it," Kane said of her pursuit strategy. "I was just paddling for all my life was worth, and I have my [teenage] children going off in the distance, and I'm paddling to keep them at this constant angle as I'm chasing after them. So it's a very innate reaction."
For the goshawks, there are pros and cons to both strategies. Classical pursuit, for one thing, works best when a target is sitting still, or is traveling in a straight line directly in front of the goshawk. Using classical pursuit on a moving target, however, means constantly readjusting as the animal moves at a slant, forcing the hawk to move in a more curving, less speedy path.
"If you have the prey animal moving in such a way that it's moving at an angle, or if it's constantly maneuvering, then this classical pursuit method is going to lead to a curving, very, very non-optimal trajectory," Kane said. "So you won't be able to get there in time."
Constant absolute target direction, on the other hand, gets these high-speed aerial hunters to the rabbit much more quickly than classical pursuit does. The problem is, the method is so fast that the raptor has a very short window to nab its prey. Any mistakes, and it misses its meal.
As it turns out, the goshawk doesn't just use one tactic or the other when it's tracking moving rabbits or pheasants. It uses a hybrid strategy, in order to harness the advantages of both.
When the goshawk first spies a tasty-looking animal, it swoops toward its prey using constant absolute target direction, guaranteeing that it will get to the animal's position fast. But when the hawk gets close enough, it switches to classical pursuit. That way, it can keep the animal in the center of its sight line, so that it can more slowly (and carefully) home in on its prey.
(You can see it in the video above, where the goshawk first leaps off his handler's arm and directly plunges at the fleeing rabbit, quickly closing most of the distance – but then appears to spend several yards gradually moving in toward the hapless animal.)
In a small minority of the cases, the ground video showed that goshawks used neither classical pursuit nor constant absolute target direction. They're not sure exactly what's happening in those cases, Kane said, but it's an area ripe for study, perhaps by outfitting birds with GPS sensors or using 3D video to study the trajectories in detail.
"My group is starting to work on the second approach," Kane said.
Still, given that the goshawks often use classical pursuit at the end of the hunt, this could explain why some fleeing animals seem to dart suddenly sideways, or fly upward at a steep angle, to avoid capture.
After all, if the goshawk's vulnerability was physical (say, that it's large and can't quickly change direction), then the best escape tactic would be to turn and run in the opposite direction -- toward the goshawk, as it were. But the animals tended to do quick, sudden sideways jumps instead. So the researchers think that the fleeing animals are taking advantage of a visual perception trick: Darting to the right or left (at a 90-degree angle) is the most effective way to break the goshawk's target-centering system.
The findings may also shed light on the best ways to design the hardware and software for flying robots, Kane said.
People looking to build animal-inspired robots want to understand a number of things, Kane said: how to use the robot's sensors to interact with its environment, avoid obstacles, interact with its robot peers, even how to land.
"Having insights into how actual animals perform these tasks is extremely useful when you're trying to design a robotic solution to these tasks," Kane said.
Andrew Biewener, a comparative biomechanist at Harvard University who was not involved with the paper but who serves as an editor on the journal, agreed that roboticists could draw upon common navigation principles from across the animal kingdom.
"Natural selection has evolved similar strategies across very different kinds of animals," Biewener said. "The eyes operate differently in an insect than they do in a bird, and they have different brain structures and ways in which they process the information, and yet natural selection has shaped those sensory and information processing systems to utilize similar principles of guidance and navigation."