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.