Birds versus bats: attack strategies of bat-hunting hawks, and the dilution effect of swarming

Aggregation is often thought to reduce predation risk, whether through dilution, confusion, or vigilance effects. Such effects are challenging to measure under natural conditions, involving strong interactions between the behaviours of predators and prey. Here we study aerial predation on massive swarms of Brazilian free-tailed bats Tadarida brasiliensis by diurnal raptors, to test how the behavioural strategies of predators and prey influence catch success and predation risk. The Swainson9s hawks Buteo swainsoni that we observed achieved high (31%) catch success without any morphological specializations for bat-hunting, but showed clear evidence of adaptive behaviour: the odds of catching a bat were increased threefold when executing a stoop or rolling grab manoeuvre, one or both of which were observed in three-quarters of all attacks. Catch success was several times higher against the column than against lone bats, so we found no evidence for any vigilance or confusion effect. Attacks on lone bats were infrequent (c. 10%), but were >50 times more common than the lone bats themselves. Because of their preferential targeting, the overall risk of predation was >20 times higher for lone bats. Dilution is therefore both necessary and sufficient to explain the higher survival rates of bats flying in column formation.