Abstract1. Most recordings of bats are conducted with fixed equipment, which relies on opportunistic data collection. Unmanned aerial vehicles (UAV; such as drones) are considered inappropriate for recording bats due to ultrasound noise constraints.2. We developed a UAV system that physically isolates UAV noise, so we can record, with 3D manoeuvrability, ultrasonic audio and spatial thermal data of bat flight at altitude.3. We tested the noise of our UAV with various payloads and microphone configurations to characterize the ultrasonic noise of our system, physically isolate drone noise from the microphone, and maximize UAV flight performance.4. Over 84 min of recordings, we captured 3,847 echolocation signals from bats with corresponding thermal data of bat flight. Our system provides a feasible mechanism to capture both acoustic and video data of bats aloft at flexible locations and altitudes.5. We include information on how to extend our method to apply to acoustic recordings in the audible (20 Hz-20 kHz) range for recording sounds of other taxa. K E Y W O R D Sbats, thermal data, UAV, ultrasound This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Aggregation can reduce an individual’s predation risk, by decreasing predator hunting efficiency or displacing predation onto others. 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. Lone bats including stragglers have a high relative risk of predation, representing ~5% of the catch but ~0.2% of the population. Attacks on the column were no less successful than attacks on lone bats, so hunting efficiency is not decreased by group vigilance or confusion. Instead, lone bats were attacked disproportionately often, representing ~10% of all attacks. 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. 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. Most attacks involved one of these two maneuvers, which therefore represent alternative rather than complementary tactics. 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. A hawk’s success instead depends on the flight maneuvers it deploys, including the high-speed stoop that is characteristic of many raptors. 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. 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.

Unmanned aerial vehicles (UAVs) are rising in popularity for wildlife monitoring, but direct recordings of animal vocalizations have not yet been accomplished, likely due to the noise generated by the UAV. Echolocating bats, especially Tadarida brasiliensis, are good candidates for UAV recording due to their high-speed, high-altitude flight. Here, we use a UAV to record the signals of bats during morning roost re-entry. We designed a UAV to block the noise of the propellers from the receiving microphone, and report on the characteristics of bioacoustic recordings from a UAV. We report the first published characteristics of echolocation signals from bats during group flight and cave re-entry. We found changes in inter-individual time-frequency shape, suggesting that bats may use differences in call design when sensing in complex groups. Furthermore, our first documented successful recordings of animals in their natural habitat demonstrate that UAVs can be important tools for bioacoustic monitoring, and we discuss the ethical considerations for such monitoring.

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 Swainson's 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 threequarters 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 (~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.

Brazilian free-tailed bats form large maternal colonies numbering in the millions across the American Southwest. Each night, the bats emerge from their roost to travel to foraging locations. During this emergence, individuals fly together in a dense, linear stream and exhibit collective group behavior. Upon morning return to the roost, the flight behavior of bats change and individuals fly in unpredictable paths, with little to no apparent collective group behavior, creating a swarm. To understand the different sensory challenges each of these flight scenarios pose to bats, we recorded the soundscapes of bats in streams and swarms using four different recording platforms: (1) stationary, ground-based directional and omnidirectional microphones, (2) a zip-line microphone that maneuvered through the bat stream and swarm and was monitored by video, (3) a microphone and thermal camera on a quadcopter that recorded bat flight behavior and signals at high altitudes during swarm re-entry, and (4) a trained hawk that flew through the bat stream while carrying a microphone unit and monitored with unique video. We report on the characteristics and variability of soundscapes for bat swarms, including different noise profiles the bats experience during streaming and swarming and the adaptive time-frequency signatures and flight behavior of individuals during group flight.