Sampling flying bats with thermal and near-infrared imaging and ultrasound recording: hardware and workflow for bat point counts

Abstract

Bat communities can usually only be comprehensively monitored by combining ultrasound recording and trapping techniques. Here, we propose bat point counts, a novel, single method to sample all flying bats. We designed a sampling rig that combines a thermal scope to detect flying bats and their flight patterns, an ultrasound recorder to identify echolocating bat calls, and a near-infrared camera and LED illuminator to photograph bat morphology. We evaluated the usefulness of the flight pattern information, echolocation call recordings, and near-infrared photographs produced by our sampling rig to determine a workflow to process these heterogenous data types. We present a conservative workflow to enable taxonomic discrimination and identification of bat detections. Our sampling rig and workflow allowed us to detect both echolocating and non-echolocating bats and we could assign 84% of the detections to a guild. Subsequent identification can be carried out with established methods such as identification keys and call libraries, based on the visible morphological features and echolocation calls. Currently, a higher near-infrared picture quality is required to resolve more detailed diagnostic morphology, but there is considerable potential to extract more information with higher-intensity illumination. This is the first proof-of-concept for bat point counts, a method that can passively sample all flying bats in their natural environment.

Bat point counts: a novel bat sampling method shines light on flying bat communities

“…We conducted bat point counts for one hour per night in each site. We used a sampling rig with sensors for ultrasound, thermal, and infrared waves; we provide full technical details of the rig and the observation workflow in (K. Darras et al, 2021). Each point count scanned a 120°field of view, directed either towards the road, river, or oil palm.…”

“…Each point count scanned a 120°field of view, directed either towards the road, river, or oil palm. We determined the detection area of the thermal scope and the full-spectrum microphone -fitted with a horn to amplify sounds from the front -with a chirp emitter at 40 kHz (K. Darras et al, 2021). The first three point counts took place in the first hour after the survey started, and the next three point counts happened in the second hour.…”

“…Ultrasound recordings from bat point counts and automated ultrasound recorders were uploaded on the open-source platform BioSounds (K. Darras et al, 2020) to annotate the spectrograms with identitfied bat detections; we included both acoustic as well as thermal-only detections (detections without ultrasound that were vocally mentioned by the observer) (K. Darras et al, 2021). All bat calls were identified using our reference collection of bat calls obtained from captured bats (Chiroptera reference collection in BioSounds (K. Darras et al, 2020)) and literature data (Hughes et al, 2011;Kingston, 2013;Zhu et al, 2012).…”

“…We matched ultrasound recordings and near-infrared photographs from bat point counts to their respective detections using a conservative workflow that discarded uncertain matches (K. Darras et al, 2021). We used data from one additional, incomplete survey night during which our infrared lamp power supply failed to aid with the taxonomic identification.…”

“…It is often advised to combine both methods to reduce the overall sampling bias (Kuenzi & Morrison, 1998), especially where Pteropodidae occur. However, recently, a proofof-concept has been proposed for technologically enhanced point counts to sample flying bats at night (K. Darras et al, 2021). These enhanced bat point counts are an active (i.e., requiring a human operator) sampling method to detect and identify all flying bats within a sampling area at night, combining thermal sensing to detect flying bats, ultrasound sampling to record their echolocation calls, and near-infrared imagery to capture their morphology.…”