Measurement of bat activity
A Titley Scientific Anabat Swift bat detector (https://www.titley-scientific.com) was deployed to monitor bat calling activity immediately outside the cave. It was set to record full spectrum echo location calls, at a 500 kHz sampling frequency. The detector switched on automatically at 15 minutes before sunset and turned off 15 minutes after sunrise. Monitoring in 2017 was undertaken from 26 July to 29 October, with 14 of the 96 nights within this period being missed due to the equipment failing to activate. Monitoring in 2018 was undertaken from 23 March until 2 November; no nights of data capture were missed. In addition, a Titley Scientific Walkabout hand-held bat detector (https://www.titley-scientific.com) was used occasionally in the nearby woodland, recording at 500 kHz in full spectrum.
Manual identification of echo location calls was undertaken by visual inspection of sonograms using “Anabat Insight” software (https://www.titley-scientific.com) by one observer (SPD), and automated analysis of calls was undertaken using “Bat Classify” software available at https://bitbucket.org/chrisscott/batclassify/downloads. This software was chosen as it reliably identifies the calls of most British woodland bats, including all Myotis species in the study area. This automated software does not attempt to differentiate between calls of whiskered and Brandt’s bats, due to the similarity between the echo location calls of these two species. Only calls that the Bat Classify software identified to species level with 80 % confidence levels or more were used for the purpose of this study.
Individual species within the Myotis genus may have different rates of correct species assignment, as some species may be easier for the software to recognise from their acoustic signature than others, and some species may more frequently give atypical calls at the swarming site as opposed to the calls that they make in their more usual habitat.
Video monitoring of entry and exit of bats at the cave entrance was undertaken on two occasions (28 August and 23 September 2017), using a Canon XA10 video recorder working in infra-red mode with an infra-red light source. Recording were made from 30 minutes before sunset to 3.5 hours after sunset. Numbers of bats entering and leaving the cave were noted.
In parallel with, and before this study, three years of bat echolocation data (2015 -2017) were collected at a nearby cave (“Hobbit Hole” 51.6707, -2.6864) and analysed using a Wildlife Acoustics SM3 Bat detector recording in “zero-crossing” mode (see Davison & Thomas 2017). The resulting sonograms were analysed by visual inspection to identify bat calls to genus level.