Attack frequency, attack success and choice of prey group size for two predators with contrasting hunting strategies

The selective benefits of living in groups, for example predator confusion and collective detection, may increase with group size but this will depend on predator species or hunting mode. We tested whether the strength of selection for flock size in redshanks depended on hunting mode used by two contrasting predators – sparrowhawks and peregrine falcons. Both predators benefit from the element of surprise, but sparrowhawks prefer to stalk and ambush prey from cover and almost always catch their prey on the ground, or shortly after escape during short chases. In contrast peregrine falcons normally attack from the open and use aerial pursuit, often resulting in long chases before catching prey. When hunting by surprise, attack success rate should always decline with group size because of vigilance effects, and so should be similar for both predators. When not hunting by surprise, attack success should decline with group size for peregrines because confusion effects apply, but not necessarily for sparrowhawks who give up their chase before confusion is likely to apply. Consequently peregrines should be more likely to attack smaller flocks than sparrowhawks to minimize confusion. Observations of 883 attacks, including 117 captures, confirmed these predictions. The decline in success rate with group size was the same for both predators for surprise attacks but only peregrines showed a decline with group size for non-surprise attacks. Sparrowhawks attacked larger flocks relative to peregrines, although both species attacked small flocks with similar frequency. Our results suggest that the selective benefit of group size is dependent on predator type and hunting mode, we assume because of variation in the importance of vigilance and confusion, and is also dependent on relative predator abundance. Behavioural responses by prey involving group size change may then change the strength of selection on vigilance and confusion effects by altering the relative attack frequency by different predators.