High detectability with low impact: Optimizing large PIT tracking systems for cave‐dwelling bats

Abstract

Passive integrated transponder (PIT) tag technology permits the “resighting” of animals tagged for ecological research without the need for physical re‐trapping. Whilst this is effective if animals pass within centimeters of tag readers, short‐distance detection capabilities have prevented the use of this technology with many species. To address this problem, we optimized a large (15 m long) flexible antenna system to provide a c. 8 m2 vertical detection plane for detecting animals in flight. We installed antennas at two roosting caves, including the primary maternity cave, of the critically endangered southern bent‐winged bat (Miniopterus orianae bassanii) in south‐eastern Australia. Testing of these systems indicated PIT‐tags could be detected up to 105 cm either side of the antenna plane. Over the course of a three‐year study, we subcutaneously PIT‐tagged 2,966 bats and logged over 1.4 million unique detections, with 97% of tagged bats detected at least once. The probability of encountering a tagged bat decreased with increasing environmental “noise” (unwanted signal) perceived by the system. During the study, we mitigated initial high noise levels by earthing both systems, which contributed to an increase in daily detection probability (based on the proportion of individuals known to be alive that were detected each day) from <0.2 (noise level ≥30%) to 0.7–0.8 (noise level 5%–15%). Conditional on a low (5%) noise level, model‐based estimates of daily encounter probability were highest (>0.8) during peak breeding season when both female and male southern bent‐winged bats congregate at the maternity cave. In this paper, we detail the methods employed and make methodological recommendations for future wildlife research using large antennas, including earthing systems as standard protocol and quantifying noise metrics as a covariate influencing the probability of detection in subsequent analyses. Our results demonstrate that large PIT antennas can be used successfully to detect small volant species, extending the scope of PIT technology and enabling a much broader range of wildlife species to be studied using this approach.

A PIT‐tag–based method for measuring individual bait uptake in small mammals

“…It is unknown whether the remaining four individuals had lost their tags, left the study area before the trial or whether they were present but would not enter the hoppers. Low rates of tag loss and mortality have been found when PIT‐tagging bats (van Harten et al., 2019) and ground squirrels (Schooley et al., 1993), and this study provides further evidence that PIT‐tagging is a reliable method for gathering behavioural data on small mammals in their natural environment.…”

The Use of Intrinsic Markers for Studying the Migratory Movements of Bats

“…Despite the promise of this combination, the previously documented low recapture rates for marked bats and decreased survivability associated with banding of certain bat species may hinder the widespread feasibility of these coupled techniques for bat biologists [ 9 , 24 , 179 ]. Recently, however, researchers have developed passive detection mechanisms for PIT tags that result in increased recapture rates [ 180 ] without implicating flight behavior of otherwise affected species [ 181 ]. Another important and newly developed mechanism for inferring migratory direction is the circular release box for bats (CRBox), which allows inferences to be made about orientation behavior and flight direction [ 182 , 183 ].…”

Social networks based on frequency of roost cohabitation do not reflect association rates of Myotis lucifugus within their roosts

“…PIT tag readers autonomously and continuously record the unique identity of tags when they come within range of antennas, making them seem ideal for passive studies of bats at known roosts (van Harten et al, 2019). However, PIT tags used in wildlife research employ low-frequency (125-135 kHz) technology, which is hampered by a radio-frequency identification (RFID) limitation known as tag collision.…”

“…Tag collision occurs when >1 tag is present at an antenna and results in readers failing to detect tags due to a blockage of transmissions (Klair et al, 2010;Shih et al, 2006). The effects of tag collision are well recognized in the fields of RFID (Klair et al, 2010;Shih et al, 2006) and animal agriculture (Adrion et al, 2018;Reiners et al, 2009;Thurner et al, 2009), but are rarely discussed in wildlife studies despite the limitations they create (Smyth & Nebel, 2013).…”