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

The selective benefits of living in groups 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, Tringa totanus, depended on hunting mode used by sparrowhawks, Accipiter nisus, and peregrine falcons, Falco peregrinus. Sparrowhawks prefer to ambush prey from cover, whereas peregrines normally attack from the open and use aerial pursuit. The success rate of surprise attacks should always decline with group size because of vigilance effects, and so should be similar for both predators. The success rate of nonsurprise attacks should decline with group size for peregrines because confusion effects apply, but not necessarily for sparrowhawks which quickly give up their chase. Consequently, peregrines should be more likely to attack smaller flocks than sparrowhawks to minimize confusion. Observations of 895 attacks, including 120 captures, confirmed these predictions. The decline in success rate with group size was the same for both predators for surprise attacks but was seen only for peregrines for nonsurprise attacks. Sparrowhawks attacked larger flocks than peregrines, although both species attacked small flocks with similar frequency. Our results suggest that the selective benefits of group size depend both on predator type and hunting mode, we assume because of variation in the importance of vigilance and confusion, and on relative predator abundance. Changes in prey group size may then change the strength of selection on vigilance and confusion effects by altering the relative attack frequency by different predators.