Buzzards seem to have developed a successful tactic for picking one out of thousands of potential prey animals in a flock: When hunting in dense flocks of bats, they don't try to pursue a single animal. Instead, they target a fixed point in the swarm and grab the bat, colliding with that spot. In this way, they avoid the so-called confusion effect that occurs when predators have difficulty targeting one given the multitude of possible prey items. Other predators may also use this tactic.
Many animals seek shelter in large groups to protect themselves from predators. One way this protection comes about is through what is known as the confusion effect. Too many potential targets can make it difficult for predators to choose, target, and kill prey. The larger the swarm or herd of prey, the less successful the hunt should be. However, empirical evidence for this effect is inconsistent. In some cases, predators seem to have little trouble picking out their prey despite seemingly chaotic movements within flocks.
Confusion by swarms
But how do the predators manage to avoid the confusion effect? To find out, a team led by Caroline Brighton from the University of Oxford observed common buzzards (Buteo swainsoni) hunting Mexican bulldog bats (Tadarida brasiliensis). In the Chihuahuan Desert in New Mexico, USA, there is a cave system from which huge flocks of 700,000 to 900,000 bats emerge every evening, which are hunted by common buzzards.
"The movements of the individual bats in the swarm are often very erratic and lead to a tangle of interwoven trajectories," the researchers describe. "Such inconsistent behavior makes it particularly difficult to track individual animals, and it can be expected that the confusion effect will be amplified in a dense group." Nonetheless, previous observational studies have shown that common buzzards were no less successful when attacking bats , flying within the column rather than choosing individuals flying outside the flock.
Buzzards on a collision course
So what tricks do buzzards use to successfully capture prey, even in large swarms? Brighton and her colleagues observed this using a series of cameras that recorded the flight patterns of the bats and buzzards. The researchers reconstructed the respective trajectories on the computer. In addition, they used an algorithm to calculate how the trajectories would have to be if the bird of prey sighted the bat it finally caught right from the start. This calculated trajectory differed significantly from the one actually observed. On the other hand, if the researchers modeled the trajectory under the premise that the buzzard aims at a constant point in the swarm, this coincided with the real trajectory of the birds of prey.
Brighton and her team conclude that the buzzards don't even try to focus on an individual animal. Instead, they thrust into the swarm and catch whatever they can get their hands on. "Any prey with which the predator is on a collision course appears to remain in a constant position, so that the choice of target follows naturally from the geometry of a collision," the researchers explain.
"Our results show how predators can simplify the demands on their sensory system by decoupling control from target acquisition when capturing prey from a dense swarm," the authors say. "We suspect many other predators are using the same tactics. In this case, paradoxically, a disorientation effect is more likely in attacks on sparsely populated groups where control and targeting cannot be decoupled.”
Source: Caroline Brighton (University of Oxford, UK) et al., Nature Communications, doi: 10.1038/s41467-022-32354-5