Raptors avoid the confusion effect by targeting fixed points in dense aerial prey aggregations.

Collective behaviours are widely assumed to confuse predators, but empirical support for a confusion effect is often lacking, and its importance must depend on the predator's targeting mechanism. Here we show that Swainson's Hawks Buteo swainsoni and other raptors attacking swarming Mexican Free-tailed Bats Tadarida brasiliensis steer by turning towards a fixed point in space within the swarm, rather than by using closed-loop pursuit of any one individual. Any prey with which 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.

Aerial attack strategies of hawks hunting bats, and the adaptive benefits of swarming.

Aggregation can reduce an individual's predation risk, by decreasing predator hunting efficiency or displacing predation onto others. Here, we explore how the behaviors of predator and prey influence catch success and predation risk in Swainson's hawks attacking swarming Brazilian free-tailed bats on emergence. Lone bats including stragglers have a high relative risk of predation, representing ~5% of the catch but ~0.