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.
Highlights
Bats are nocturnal, flying mammals found in all continents except at the poles, and they perform a wide range of ecosystem functions (Kunz et al, 2011)
We conducted three 10-minute bat point counts in the first hour after nautical twilight, and three 10-minute bat point counts in the second hour
For the first time, we show that it is possible to combine thermal, ultrasonic, and infrared sensors to comprehensively sample both echolocating and non-echolocating bats flying at night with bat point counts, and we propose a conservative workflow for taxonomically discriminating or identifying most bat detections
Summary
Bats are nocturnal, flying mammals found in all continents except at the poles, and they perform a wide range of ecosystem functions (Kunz et al, 2011). Detecting, counting, and identifying bats in their natural environment is challenging, because many bat species are not distinguishable morphologically (Mayer & von Helversen, 2001) or acoustically (Russo et al, 2017) Their ultrasonic calls - which are identification features - are mostly outside the human hearing range. To sample active bat communities effectively, it is currently advised to combine capture methods with ultrasound recording (Flaquer et al, 2007; O’Farrell & Gannon, 1999). Both methods have limiting drawbacks but are required to identify species that have similar calls, those that have similar morphology, and especially those tropical bat species that do not emit echolocation calls at all (e.g., the vast majority of Pteropodidae). A comprehensive sampling of flying bats requires considerable resources
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