The soundscape of bat swarms

Abstract

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.