These Hawks Have Figured Out How to See the Bat in the Swarm

People of the same kind stick together. And so do bats and fish and all the little creatures hoping that getting lost in the crowd will keep them off the dinner menu. But predators need to eat. So they in turn found clever ways to break through the visual clutter of a churning mass of panicked prey to sharpen their aim.

Apparently, for hawks searching for food in huge swarms of bats, the trick is to simply dive in and snag the hapless flier that happens to be sharing your trajectory. This is the result of a study in the journal Nature Communications.

Graham Taylor: Intercepting even a single maneuvering target is a challenging task in itself.

Hopkins: Graham Taylor is Professor of Mathematical Biology at the University of Oxford. He wondered how predators like hawks and hawks manage to isolate and intercept a moving target when it’s part of a flock or flock… a tight assemblage that looks like it’s offering optimal protection to the prey.

Taylor: For one, it can reduce your individual risk of being attacked in the first place. And it might also make it harder for a predator to catch you, if it launches an attack at all.

Hopkins: This is thanks to the so-called “confusion effect”.

Taylor: This is the idea that having multiple similar targets makes it harder for a predator to single out and pursue a single individual.

Hopkins: But are flocks as confusing to a hawk as they may seem to you or me? To find out, Taylor teamed up with Laura Kloepper, a bat biologist at Saint Mary’s College in Indiana, and headed to New Mexico.

Taylor: It’s a very remote spot in the Chihuahuan desert where there’s a bat cave from which almost a million Mexican free-tailed bats emerge every night. And when the bats emerge in that huge stream pouring out of the cave, there are some Swainson’s hawks waiting, diving into the flock and trying to snatch bats out of it.

Hopkins: So Taylor and his team strategically positioned video cameras around the cave. And that revealed that… from the bat’s point of view… swarming actually reduced a given person’s risk of being caught.

Taylor: However, we also found that the hawks were no less successful at catching flocks of bats than they were at catching solitary ones. What that means is that the hawks don’t suffer from any kind of confusion effect.

Hopkins: But how do the hawks avoid becoming visually disoriented when attempting to snatch a lone bat from a sea of ​​erratically beating wings? To find out, the researchers compared the trajectories of predator and prey. And they saw that attacking a swarm required a different strategy than pursuing an isolated individual.

Taylor: If you analyze the way a hawk chases a single target, it really closely follows the twists and turns that the target makes as it tries to avoid the hawk.

Hopkins: But not like that when you go to the crush.

Taylor: Instead, the best model from the data was one that said the hawks simply turned into the flock at a constant radius, or perhaps more generally aimed at a fixed point in the flock.

Hopkins: As it steers toward that fixed point, the hawk can then reach out with its claws and pick up any bat flying along the same path. And because they share the same trajectory, to the hawk the bat looks like it’s not moving at all.

Taylor: The portion of the flock that the hawk is heading towards will appear virtually stationary compared to the movement of the rest of the flock. So this provides a mechanism by which the bird diving into the flock is able to see which bit it’s going to hit, but then also pick a target within it without suffering the confusion effect.

Hopkins: Taylor says the same should apply to other predators that hunt prey in large herds or swarms or murmurations. But for now, it’s safe to say that the mayhem of the Flock of Swainson’s Falcons doesn’t seem to turn heads…although it’s totally insane.

For Scientific American’s 60 Second Science, I’m Karen Hopkin.

[The above text is a transcript of this podcast.]