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. Our results show how predators can simplify the demands on their sensory system by decoupling steering from target acquisition when capturing prey from a dense swarm. We anticipate that the same tactic will be used against flocks and schools across a wide range of taxa, in which case a confusion effect is paradoxically more likely to occur in attacks on sparse groups, for which steering and target acquisition cannot be decoupled.


March 2021
Field-specific reporting Please select the one below that is the best fit for your research. If you are not sure, read the appropriate sections before making your selection. We filmed attacks by wild Swainson's Hawks (Buteo swainsoni) on swarming Mexican Free-tailed Bats (Tadarida brasiliensis), using three pairs of high-definition video cameras fixed in stereo configuration around the cave from which the bats emerged. We tracked the hawks and the bats that they attacked manually in the video data, and used this to reconstruct their three-dimensional flight trajectories using stereo camera reconstruction techniques. We then fitted numerical simulations to the measured flight trajectories under different hypothesised guidance laws commanding the model hawk's steering behaviour.
The research sample was drawn from a population approximately 20 Swainson's Hawks (Buteo swainsoni) hunting Mexican Freetailed Bats (Tadarida brasiliensis) in a maternal colony comprising approximately 700,000 to 900,000 individuals inhabiting the Jornada Caves, Armendaris Ranch, New Mexico, USA.
This was an observational study the sampling strategy of which was to record as many attacks as possible using three fixed pairs of high-definition video cameras mounted on tripods around the caves (called North and South) from which the bats emerged. We set up two camera pairs facing approximately north and south across the South cave for the duration of the study. As the swarm's overall flight direction was variable and influenced by the wind, we positioned the north-and south-facing camera pairs to allow them to be panned from northeast to northwest and from southeast to southwest, respectively. This enabled us to cover most flight directions, except due east (where the bats rarely flew) and due west (which was subject to glare). We set up a third camera pair to view the emergence that occurred from the North cave from the second week onward. When leaving the North cave, the bats usually flew along the lava tube and beneath a rock arch before climbing out of the canyon. We therefore positioned the cameras close to where the swarm began climbing out above the canyon rim, aiming to capture attacks as the hawks swooped low over the canyon.
The bats emerged at a variable time between approximately 18:30 and 20:00 MDT. Each emergence lasted from 10 to 25 minutes, depending on the number of bats present, which increased over the course of the study. As soon as the bats began emerging, the cameras were turned on and left to record, having oriented them in the direction of the swarm's emergence, which varied from day to day. All authors were involved in the collection of the data.
We recorded video of the hawks attacking the bats every evening from 8 to 29 June 2018, except for one evening that had to be missed due to bad weather. The data are collected within the immediate vicinity of the North and South caves from which the bats emerged.
No video data were excluded from the analysis. Attacks were identified by watching the videos after collection, and we aimed to analyse all of the attacks in which the attacking hawk was visible in both cameras.
Qualitatively the same behaviours were observed by the authors in each of the two preceding years, but these were not filmed systematically.
As this was an observational field study, we sought to analyse the totality of all the attacks that we observed. Randomization was therefore relevant only to the construction of the bootstrap confidence intervals presented in relation to the guidance parameters fitted in the trajectory simulations.
Blinding was not possible owing to the need for the video data to be watched and digitized by the authors of the study. However, the basis of this study is to reconstruct the three-dimensional attack trajectories of the hawks, which are then compared to the results of numerical simulations under different candidate guidance laws. It follows that there is little risk of researcher bias, except in the identification of attacks for analysis, as the statistical analysis is far removed from the digitization of the video data.
Data were recorded under dry conditions within the 2h before sunset.
Jornada Caves, Sierra County, New Mexico, USA Access to the Jornada Caves was provided by Armendaris Ranch Inc. No samples were collected in the course of this study.
To begin with, all fieldworkers retreated into make-shift hides as the bats began emerging, but these were gradually phased out for reasons of practicality. The birds quickly became habituated to the fieldworkers' presence, venturing close to the cave even when fieldworkers were present.