Camera traps with white flash are a minimally invasive method for long‐term bat monitoring

Abstract

Camera traps are an increasingly popular survey tool for ecological research and biodiversity conservation, but studies investigating their impact on focal individuals have been limited to only a few mammal species. In this context, echolocating bats are particularly interesting as they rely less on vision for navigation, yet show a strong negative reaction to constant illumination. At hibernacula, camera traps with white flash could offer an efficient alternative method for monitoring threatened bat species, but the potential negative impact of white flash on bat behavior is unknown. Here, we investigate the effect of camera traps emitting white flash at four hibernation sites fitted with infrared light barriers, infrared video cameras, and acoustic recorders over 16 weeks. At each site, the flash was turned off every second week. We quantified whether flash affected (1) nightly bat passes using generalized linear mixed models, (2) flight direction of entering bats using permutational multivariate analyses, and (3) latency of the first echolocation call after the camera trap trigger using randomization tests. Additionally, we quantified and corrected for the potential impact of confounding factors, such as weather and social interactions. Overall, white flash did not influence short‐ or long‐term bat activity, flight direction or echolocation behavior. A decrease in nightly bat activity was observed with an increasing proportion of hours with rain. Moreover, flight direction was affected by the presence of other bats, likely due to chasing and avoidance behavior. Our findings highlight the potential of camera traps with white flash triggered by infrared light barriers as a minimally invasive method for long‐term bat population monitoring and observation of species‐specific phenology. Such automated monitoring technologies can improve our understanding of long‐term population dynamics across a wide range of spatial‐temporal scales and taxa and consequently, contribute to data‐driven wildlife conservation and management.

Counting in the dark: estimating population size and trends of bat assemblages at hibernacula using infrared light barriers

“…At each hibernaculum, we collected 15 weeks of video data from August 1 to 4, December 2020 (here referred to as ‘autumn’), and another 15 weeks from 20 February to 4 June 2021 (here referred to as ‘spring’) using self‐built infrared video cameras (as described in Krivek et al, 2022; see https://gabik-bat.github.io/FlederCam/ for construction details). From the resulting 11 868 hours of raw infrared video recording, 6‐second‐long video clips were isolated for each event registered by the light barrier.…”

“…At each hibernaculum, we collected 15 weeks of video data from August 1 to 4, December 2020 (here referred to as 'autumn'), and another 15 weeks from 20 February to 4 June 2021 (here referred to as 'spring') using self-built infrared video cameras (as described in Krivek et al, 2022; see For each site, we indicate the type of underground structure and if it was accessible or not for visual surveys (roost type), the light barrier model installed at each entrance at the site (Liba-4/16/16k), and the positioning of the light barriers relative to the entrance (integrated within the opening, or mounted on the wall at the back of the opening). Species composition refers to the bat species recorded during the last winter count (NA: sites are inaccessible); bold species represent 25% of the visually counted population.…”

“…At sites with up to several thousand bats, a camera trap triggered by a light barrier can be used to obtain relative species abundances. Along these lines, we have previously shown that such custom-built camera traps are minimally invasive and do not noticeably affect bat behavior (Krivek et al, 2022). At large and remote sites, camera trap monitoring may not always be feasible due to power limitations and data volume.…”

BatNet: a deep learning‐based tool for automated bat species identification from camera trap images

“…1B–D). Moreover, bats do not change their behavior in response to the fast, white flash of such camera traps (1/5500 s, 1/16 power), making these photo‐monitoring systems suitable as a minimally invasive method for bat monitoring (Krivek et al., 2022). These camera traps can either be installed on the inner side of the light barrier and be triggered by each bat entering the hibernaculum (i.e., ‘entry’ camera), and/or on the outer side and be triggered by each bat leaving the hibernaculum (i.e., ‘exit’ camera).…”

Sensor Fusion of 3D Time-of-Flight and Thermal Infrared Camera for Presence Detection of Living Beings

“…The use of radar sensors in wildlife monitoring typically results in a lot of FP when there is wind or rain [9]. By using infrared light barriers to detect living beings, a detection occurs when the light barrier is crossed, limiting the camera's triggering to a very limited space [10].…”