Animal behaviour: Hunting hawks target the swarm

Hawks that hunt swarming bats steer towards a fixed point in the swarm rather than targeting individual bats, reports a study published in Nature Communications this week. The findings aid our understanding of how predators select and track a target among thousands of potential prey.

It is generally thought that being in a large group — such as a swarm of bats, flock of birds, or school of fish — provides protection from predators. One type of protection is the ‘confusion effect’: that large numbers of potential targets will confuse predators, making it more difficult for them to focus on and capture a specific individual. If predators get confused, then their success rate in capturing prey should decrease as the number of available prey increases. However, empirical evidence for a confusion effect has been mixed.

Caroline Brighton and colleagues observed 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 evenings. By using an array of cameras, they were able to reconstruct the 3D flight trajectories of the raptors and bats. Their analyses revealed a solution to the confusion effect; instead of targeting an individual bat, raptors would target a fixed point within the swarm. A bat on a collision course would appear to the raptor to be on a constant bearing, singling it out of the swarm they suggest.

The authors suggest that this strategy of targeting a fixed point in a group of prey may be a more general mechanism yet to be discovered in other predators. However, they indicate it may only be effective when prey aggregations are sufficiently dense.