Determining recorder distances from multicopters to avoid bat disturbance

Abstract

Introduction: This study examines the impact of two multicopters, differing in size and weight (ConVecDro hexacopter, DJI Mavic 2 Pro quadcopter), on the activity of bats assigned to three distinct echolocation groups. The objective was to define thresholds (in decibels) below which bat echolocation groups show weak or no impact on activity. Using the known noise profile of the multicopter and a model of sound attenuation in air, these thresholds were then used to determine optimal recording device placement distances by estimating frequency-specific sound levels at increasing recording distances. Directional properties of the recording device were also considered. Methods: Bat activity was recorded at distances 10.0 m, 12.2 m, 14.8 m, 17.4 m and 20 m, with the recording device oriented towards and away from the multicopter. We analyzed the effects of noise emissions from the multicopters in relation to the distance between the recording device and the multicopter to identify their influence on the recorded bat activity. Results: The study shows the differences in recorded bat activity between two multicopters with different noise profiles. The larger and heavier ConVecDro produced louder, and potentially more deterrent noise emissions, for Pipistrelloid and particularly for Nyctaloid groups. In contrast, the DJI Mavic 2 Pro had a weaker deterrent effect on bat activity, and only the Nyctaloid groups were affected. Based on our observations of both multicopters, the threshold value below which no echolocation group showed any detectable impact was 4.5 dB at 23 kHz. In our setup, where the recording device was oriented towards the multicopter, it should be placed at 20 m for the DJI Mavic 2 Pro and at 36.0 m for the ConVecDro. Discussion: The findings indicate that multicopter noise exerts a significant influence on bat activity, with the magnitude of the impact varying depending on the echolocation group and the specific characteristics of the multicopter noise profile. The ability to define noise threshold values for the different echolocation groups suggests that beyond a certain point, multicopter noise has no discernible effect on bat activity levels compared to a control group.