Sound and Vision – A New View of the Behaviour of Woodland Bats

The colony of rare barbastelle bats in the woods of the Bovey Valley has been the subject of active research over recent years. This work has used radio tracking technology to monitor the movements of the bats between roost sites in the oak woods and their foraging grounds along hedges, woodland edges and rivers. Revealing discoveries of the species’ roosts have shown which areas of ancient woodland, and which cracks and crevasses, are preferred by the barbastelle, giving the nature reserve managers more information on which woodland features to conserve. But, in addition to all this valuable information, another hi-tech scientific study has emerged from the woods. Local volunteer, Susan Young, used her skills in CCTV filming to record some behaviour and calls around the roost trees that opened up a whole new area of research.

Four barbastelles emerge from the roost

Working in a team with scientists from the University of Bristol, Susan’s research paper explains that, “We monitored a barbastelle (Barbastella barbastellus) maternity roost for four months using a portable CCTV system, time synchronised with ultrasound recorders, and we discovered three patterns of vocal activity not previously described.”

2016 setup
Setup showing the roost tree and recording equipment

Bats are well known for using high frequency echolocation calls to orientate themselves in their surroundings. Finding their way through a tangle of ancient woodland branches to locate their insect prey relies on great skill, speed and accuracy and we have known for a long time that their calls change as they fly. On approaching their prey, the calls become more rapid to pinpoint an individual insect; often a moth, in the case of a barbastelle.

Other ‘vocalisations’ are used as ‘social calls’ which tell other bats in the area about their intentions and this is where the crucial combination of synchronised video and audio recordings made a breakthrough. Susan had initially set up the CCTV cameras to identify which of the bats on the videos were actually barbastelles, but explained, “I was studying the barbastelle recordings in detail to measure how long the calls lasted, how they were structured and what their frequency was, but I kept seeing strange groups of calls that did not look like anything I had seen before. Initially I discounted them, but on further examination, noted that there were three distinct groups of the strange calls. In each case they seemed to be related to the bat behaviour seen at the same time on video. For example, when a bat approached the roost tree it emitted a very quiet string of pulses of sound. When groups of bats were swarming they made two types of strange calls, one of which was extremely loud.”

dawn swarming barbastelles

The research paper describes these calls as, “three vocalisations not previously reported for barbastelles. These are: approach echolocation calls, which took place when a bat approached the roost tree; swarming echolocation calls, produced when several bats flew in an energetic manner near the roost tree; swarming ‘honking’ calls, produced when several bats were flying in a swarming manner, which we hypothesise to be a warning to other bats of an impending collision.”

Investigating barbastelle
Swarming bats at speed appear blurred
A. Standard barbastelle echolocation B. Barbastelle approach echolocation
Audio trace A: standard barbastelle echolocation B: barbastelle approach echolocation

Only the combination of CCTV video and synchronised audio recording could have led to this discovery and a lot of hard work and inquisitive minds combined to produce such clear results.

After a few years of field work and writing up her research paper, Susan reflected on the project saying, “There were technical challenges as I had to design the setup, without there being any prior work of this type. Physically, it was a challenge, carrying a heavy battery up and down a steep hill from the road to the woodland site,” and the report concluded, “We believe that the ability to relate the vocal behaviour to direct observations shows that CCTV is a valuable addition to roost monitoring.”

Andy Carr from the School of Biological Sciences at the University of Bristol added that, “The ability to observe the behaviour of bats while monitoring their echolocation calls is important as bat researchers mainly infer behaviour depending on the characteristics of sound emitted. Susan’s work helps to confirm which echolocation types are connected to the flight behaviour of bats while at roosting sites.”

by Matt Parkins
I
mages: Susan Young, Andy Carr and Paul Moody

The research was funded by the Heritage Lottery Fund through the Moor than meets the eye Landscape Partnership incorporating Natural England, the Woodland Trust and Dartmoor National Park Authority

The full research paper, “CCTV enables the discovery of new barbastelle (Barbastella barbastellus) vocalisations and activity patterns near a roost” will be published by Acta Chiropterologica later in 2018

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