![]() ![]() ![]() It is an open question whether and how the confusion effect may operate in other ecological contexts. 2020), and may also be true of raptorial predators that have more opportunities to capture individual prey items when striking at a dense shoal ( Nottestad and Axelsen 1999). This is true, for example, of large pelagic predators such as baleen whales that exploit the shoaling of their prey during engulfment ( Cade et al. In such cases, hunting may actually be more efficient against a denser group of prey. However, whereas confusion may well impact the outcome of a directed chase, a predator lunging or plunging into a dense prey aggregation need not be targeting any one individual. Within this category fall, the shared benefits of group vigilance ( Lima 1995) and the confusion effect occurring when the presence of multiple prey make it harder for a predator to target any one individual ( Landeau and Terborgh 1986 Quinn and Cresswell 2006 Duffield and Ioannou 2017). In contrast, the second class of mechanisms decreasing individual predation risk does so by reducing predator hunting efficiency. 2017 Rayor and Uetz 1990) but see ( Parrish 1989). These closely related phenomena are known as selfish herding and marginal predation, respectively, and are widely observed across taxa (Duffield and Ioannou 2017 Ioannou et al. Likewise, individuals at the periphery have a domain of danger extending outward from the group, so are expected to suffer higher attack rates than those in the centre ( Hamilton 1971). Analogous effects apply within a group, where widely spaced individuals have a larger domain of danger than tightly spaced individuals, so are expected to be attacked more often if predators attack whichever prey is closest ( Hamilton 1971). The net effect is then to displace the burden of predation onto lone individuals or smaller groups, in a phenomenon known as attack abatement. The former is often attributed to the simple numerical dilution of per-attack risk among the individuals within a group ( Foster and Treherne 1981 Morgan and Godin 1985), but group members will only enjoy a reduction in predation risk if the predator’s attack rate increases less than proportionally with group size ( Turner and Pitcher 1986 Wrona and Dixon 1991). The first category of mechanisms encompasses the distinct phenomena of dilution, selfish herding, and marginal predation. Whereas a bat’s predation risk depends on maintaining its position within the column, the catch success of a hawk depends on how it maneuvers itself to attack, and is maximized by executing a high-speed dive or rolling grab maneuver.įlocking, shoaling, and swarming behaviors can all serve to reduce an individual’s predation risk, by either 1) displacing the burden of predation onto others within or outside the group or 2) decreasing predator hunting efficiency ( Krause 1994 Rieucau et al. Swarming bats emerging from a massive desert roost reduce their predation risk by maintaining tight column formation, because the hawks that predate them attack peripheral stragglers and isolated wanderers disproportionately. A hawk’s success instead depends on the flight maneuvers it deploys, including the high-speed stoop that is characteristic of many raptors. ![]() Hence, whereas a bat’s survival depends on maintaining column formation, a hawk’s success does not depend on attacking lone bats-even though their tendency to do so is sufficient to explain the adaptive benefits of their prey’s aggregation behavior. Most attacks involved one of these two maneuvers, which therefore represent alternative rather than complementary tactics. In contrast, the hawks’ catch success depended only on the attack maneuvers that they employed, with the odds of success being more than trebled in attacks involving a high-speed stoop or rolling grab. Swarming therefore displaces the burden of predation onto bats outside the column-whether as isolated wanderers not benefitting from dilution through attack abatement, or as peripheral stragglers suffering marginal predation and possible selfish herd effects. Instead, lone bats were attacked disproportionately often, representing ~10% of all attacks. Attacks on the column were no less successful than attacks on lone bats, so hunting efficiency is not decreased by group vigilance or confusion. Lone bats including stragglers have a high relative risk of predation, representing ~5% of the catch but ~0.2% of the population. Here, we explore how the behaviors of predator and prey influence catch success and predation risk in Swainson’s hawks Buteo swainsoni attacking swarming Brazilian free-tailed bats Tadarida brasiliensis on emergence. Aggregation can reduce an individual’s predation risk, by decreasing predator hunting efficiency or displacing predation onto others. ![]()
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