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Like a ‘missile’ at a swarm — Hawks target fixed points to catch prey from flock, study finds

It is widely believed predators are confused by large groups like swarms or flocks. But a new study shows hawks use navigational geometry to overcome this effect.

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Bengaluru: Safety in numbers might not be all that it’s cracked up to be, at least when it comes to a swarm of bats being targeted by a determined bird of prey.

In a study published this week in the journal Nature Communications, researchers from the University of Oxford and the University of New Hampshire showed how hawks home in on a single bat within a massive swarm.

The study challenges the prevalence of the “confusion effect”, which is the notion that being part of a large group, like a flock of birds, school of fish, or swarm of bats, offers increased protection since predators find it hard to focus on a single individual within a dynamic, moving mass.

For their research, the team went to the Jornada Caves in New Mexico, where there is a colony of up to 900,000 free-tailed bats, as well as a population of Swainson’s hawks, medium-sized raptors, living nearby. The researchers used several video cameras to observe the flight trajectories of bats and the hawks hunting them to ascertain whether the ‘confusion effect’ works and what strategy birds actually employ to successfully capture an individual from a swarm.

Their analyses showed that instead of targeting an individual bat, raptors target a fixed point in space, within the structure of the moving swarm, and fly towards it. They display no evidence of confusion or of targeting any individual bat.

Instead, as a hawk approaches the swarm, flying near-parallel to it, any bats in the spot they are targeting appear to be at constant bearing or in a collision course. The hawk eventually flies alongside and catches whichever bat is in that position at the time.


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Camera captures & 3D reconstructions

The researchers observed and recorded hawks attacking bats for 21 days, using six high-definition video cameras. Fixed around the cave where these bats lived, these cameras captured them emerging in the hundreds of thousands every evening, when hawks would hunt them.

The researchers analysed the recordings, tracking hawks and the specific individual bats they caught or tried to catch and recreated the trajectories of the hawk and the bat in 3D.

In total, they reconstructed 62 attacks where both animals were confirmed, and 26 attacks where the bat could not be identified. They even did reconstructions for two peregrine falcons that attacked the bats.

 

Video sequence of a Swainson’s hawk attacking swarming Mexican free-tailed bats at a bat cave in New Mexico, USA | Caroline Brighton (Oxford Flight Group)

The analysis showed that there was no closed-loop pursuit or targeted attack on the single individual bat who was ultimately captured by the hawk. The hawk’s path seems to be solely based on where the target would be at the final position, rather than instantaneous positions during the flight.

The researchers confirmed this by examining data on other birds of prey that attacked lone targets. These birds tracked their prey by modifying their trajectories according to the prey’s instantaneous location, rather than the final position.

“We find no evidence that hawks attacking swarming bats use closed-loop pursuit of an individual bat — in contrast to raptors attacking singleton targets. Instead, the hawks appear first to turn into the swarm, and then to extend their legs in a grab manoeuvre directed at whichever bat they find themselves on a collision course with as they close range,” the study said.

Geometry of the hunt

Hawks are able to hone in on their prey using navigational geometry that is similar to what is used during missile-guidance or ship navigation.

As the hawk moves, focusing on a point within the structure of the swarm, it flies near-parallel to its prey, maintaining a line of sight.

“Any prey with whom the predator is on a collision course will appear to remain on a constant bearing, so target selection emerges naturally from the geometry of a collision,” explain the authors.

‘Constant bearing’ is a navigational term that means another ship is in the same relative position to a ship even as it moves; if the two ships are not moving in parallel, they are on a collision course.

Constant Bearing, Decreasing Range (CBDR) is visualised like two arms of a triangle that represent the path taken by the two ships, until they collide. The line of sight between the objects remains constant, indicating they are on constant bearing; but their distance or range decreases heading into a collision.

While sailors hope to avoid CBDR collisions, torpedoes attacking underwater utilise it for the kill, just like the hawks do with bats.

Where does this leave the confusion effect?

Dense aggregations of individuals — flocks, schools, swarms — are widely thought to confuse predators.

It is believed that when any predator targets a single individual, they would experience a sensory overload due to the overwhelming number of individuals in the group and their movement dynamics. This is commonly called the confusion effect.

However, the evidence for confusion effect has not been established and it has been unclear which predators experience it, especially when they are visual hunters like hawks and other birds of prey.

Even the nature of protection provided by groups to individuals, often called the ‘selfish herd theory’ (where individuals seek to be in the middle of a group) is unclear in situations where predators can attack in three dimensions, from above and below.

The confusion effect is generally thought to work when a predator encounters a smaller group where it may seem feasible to launch a targeted ‘closed-loop’ attack. From older studies, it is thought that beyond seven members in a group of prey, confusion effect might not apply at all to a predator.

(Edited by Asavari Singh)


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