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How do hawks target prey inside chaotic swarms?

A new study led by the University of Oxford has investigated how hunting hawks and other raptors solve the problem of intercepting a single bat within a dense swarm. The experts found that, instead of targeting individual bats, the predators would steer towards a fixed point within the swarm, a technique which increases their chances to capture the prey.

Scientists have long argued that being in a large group, such as a swarm of bats, a school of fish, or a flock of birds, offers significant protection from predators, since the presence of so many potential targets may confuse them, making it more difficult to focus on and capture a specific individual. However, empirical evidence for such a “confusion effect” has not been very strong. 

To gain more insight into predators’ hunting strategies when facing large groups of prey, the researchers filmed Swainson’s Hawks (Buteo swainsoni) and other raptors hunting a colony of approximately 700,000 to 900,000 Mexican Free-tailed Bats (Tadarida brasiliensis) as the bats emerged from a cave in the Chihuahuan Desert in New Mexico at dusk. Then, they reconstructed the hawks’ flight paths using 3D modeling and compared the trajectories of the real birds with those modeled by the computer algorithm. 

The analysis revealed that, instead of continuously targeting an individual bat, the raptors would steer towards a fixed point within the swarm. Since any bat on a collision course with the hawk would appear stationary to the predator, the latter may use this to single out a target bat from the swarm. “From the viewpoint of a stationary observer – such as a person stood on the ground – all members of the swarm appear to move erratically. For a mobile observer – like the hunting hawks in flight – any bat that it is on a collision course with it will appear stationary against the background movement of the swarm,” explained study lead author Caroline Brighton, a postdoctoral researcher in Biology at Oxford. 

“Our work shows how the appearance of a swarm depends on the predator’s own motion, so starling murmurations and many other group behaviors that look bewildering to our own eyes may not appear so confusing to a predator taking the plunge,” added study senior author Graham Taylor, a professor of Mathematical Biology at Oxford.

These findings suggest that such a strategy of targeting a fixed point in a group of prey could be a more general mechanism yet to be discovered in other predators. However, such a technique may only be effective when prey aggregations are sufficiently dense.

The study is published in the journal Nature Communications.

By Andrei Ionescu, Staff Writer  

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