The soundscape of dense bat swarms: Differences based on location within the group

Abstract

The laboratory work of Jim Simmons and colleagues has provided key information on how bats use harmonic structure to identify targets from background clutter. Although advantageous for detecting targets straight ahead, the question remains as to how bats modify echolocation, if at all, when identification of objects in the periphery may be needed, such as when flying in a dense group. Based on the work of Dr. Simmons and colleagues, we predicted that free-tailed bats in central positions of swarms may use different echolocation signals than bats on the periphery. We investigated this by quantifying the soundscape based on location within the group. To overcome the challenge of recording inside a dense swarm, we developed a microphone/camera unit carried by a trained Harris hawk that flew through the bat swarm. Frequency spectra were extracted and analyzed based on the frequency-dependent amplitude compared to position in the swarm. We found a significant difference in the soundscape between the ‘Edge’ and ‘Middle’ of the swarm for frequencies above 40 kHz, suggesting bats in central positions of the swarm produce echolocation signals with more energy in higher frequencies, which may aid in distinguishing their calls from conspecifics.