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Oxford, October 9 (The Conversation) Hundreds of thousands of bats emerge from their habitats at dusk, a spectacle of nature. Swarms can be so dense they are like smoke rising in the distance. But the aerial antics of the raptors that prey on them are just as surprising.
Studying these behaviors in a remote corner of the Chihuahuan Desert, which stretches from the American Southwest to Mexico, has been a highlight of my nearly 25-year career as a biologist studying animal flight. My team’s research was done in collaboration with bat scientist Laura Kloepper (University of New Hampshire).
Prey often finds safety in numbers, and bats are no exception. Moving within a group dilutes the risk of an individual being attacked. It can also confuse predators, making it harder for them to track down their targets. This is called the confounding effect. Humans can also be disoriented by large groups of objects and animals in this way. Swenson’s hawks tend to attack the Mexican free-tailed bats they feed on when the bats emerge in swarms from the cave. Compared to airspace, bats leave caves at much slower speeds (they can reach speeds of nearly 100 mph if the area is open).
My team found that the Eagles didn’t seem concerned about the chaos effect.
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see it clearly
How do hawks avoid the chaotic effect that confuses us humans when we watch swarms? To answer this question, we captured footage of eagles as they swarm into a stream of bats from a cathedral-like cave. High-definition cameras strategically placed around the entrance to the bat cave allowed my team to reconstruct the 3D trajectories of eagles and the bats they attacked. But recreating a behavior is only the first step in understanding its mechanism. Next, my team analyzed how the Eagles steered their offense.
We used a computer simulation method we first developed in 2017 to study the aggressive behavior of peregrine falcons. This method uses a set of mathematical formulas called differential equations to simulate the behavior of birds.
Our previous work has shown that the Falcon’s attack behavior is similar to a guided missile, using a technique called proportional navigation. To understand how this works, imagine yourself as an aerial predator, looking at its prey while approaching it at high speed. Your prey may try to avoid you. However, if your turn speed is proportional to the speed at which your prey’s compass bearing changes, you will choose a path that is most likely to intercept your prey.
Birds that use this technique to chase lone prey will naturally follow the twists and turns of the target. But the eagles we photographed attacking the swarms didn’t seem to respond to the individual movements of the bats they caught. In fact, the eagle just turns to the swarm with an almost constant radius, flying along an almost circular arc. Instead of picking out a single bat, the birds turned to a fixed point in the colony.
This strategy is hopeless against unstable single targets, but has a good chance of success when dealing with dense prey. It wouldn’t be surprising if bats happened to be hit by arrows shot randomly into the swarm, as did diving raptors. Part of the reason the hawks have successfully avoided the confounding effect may be that they can circumvent the complexities of tracking targets on approach.
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Even so, catching a bat with its claws is not an easy task. It’s a complicated process that often ends in hawkish failure. So how do the Eagles choose which bat to grab when they get close to the swarm?
We use mapping software to track how the line of sight from attacker to target changes during the attack. This analysis revealed a clear answer: Across all attacks, the direction of sight to the target remained virtually unchanged. It changes least in those cases that end with a successful capture.
This geometry applies to any pair of objects during a collision. Sailors are taught to find a constant bearing to avoid collisions with other boats, which drivers can intuitively use to safely merge into busy roads. The swarms of up to 900,000 bats in our study popped up in just a few minutes. For a predator caught in such a dense swarm, with so many targets, at least one will be in the process of a collision. The hard part is identifying and catching it.
A stationary bystander would think the entire swarm was in motion, and the geometry of the collision meant that a moving observer would see that whatever it was hitting was stationary. Herds, flocks and shoals that seem confusing to our own eyes seem more orderly to adventurous mobile predators.
This is true for any attacker, whether it’s a shark ambushing a swarm of tuna, or a drone defending against a swarm of attacking drones. Our findings have implications for understanding how other predators avoid confounding effects and even for designing autonomous air defense systems. Most importantly, our findings are a great example of ingenious ways in which natural adaptation can solve difficult challenges. (conversation)
(This is an unedited and auto-generated story from the Syndicated News feed, the body of the content may not have been modified or edited by LatestLY staff)
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