The South Wiltshire Greater Horseshoe Bat Project

 

The project was launched in autumn 2020 to focus conservation efforts upon the Greater horseshoe bat in south Wiltshire. It is led by Wiltshire Bat Group, and supported by Natural England, Cranborne Chase AONB and horseshoe bat expert Professor Fiona Mathews (University of Sussex, Brighton), as well as Defence Infrastructure Organisation and many landowners across the project area.

Aims of the project The project aims to support greater horseshoe bats by:
1) Improving monitoring and survey of greater horseshoe bats in South Wiltshire and sharing the results, e.g., through roost surveys and acoustic surveys.
2) Enhancing existing roosts, and creating new sites,
3) Providing land management advice to enhance habitats,
4) Assess prey availability, e.g., dung beetles, and provide land management advice to enhance their populations.
5) Promote increased awareness of the incredible bats of south Wiltshire, with particular focus upon the populations of rare bats such as greater & lesser horseshoe bat, Bechstein’s and Barbastelle bat.

Landscape-scale acoustic surveys

Large parts of south Wiltshire have received low levels of bat survey, despite the presence of sites designated Sites of Special Scientific Interest (SSSI) for several species of bat, and their notification as a European Special Area of Conservation, for species such as the Greater horseshoe bat.

In 2021 the project launched landscape-scale acoustic surveys for bats. The project team have been deploying bat detectors to locations across the project area: these detectors record the ultrasonic sounds made by bats, recording them as a sound file, which we can download to a desktop computer, and analyse to identify the bat species present, and their relative activity levels. These surveys are called passive acoustic monitoring, and they generate large volumes of data.

Chilton Foliat Bat Detection

 

How did we do this?
The detectors were deployed at 4 metres above the ground, ensuring that detectors were above the height of grazing animals and out of reach of tampering. These detectors are triggered by ultrasonic sound to automatically record bat echolocation calls. These detectors can be deployed and left to remotely record bat activity for a period of several nights and programmed to begin recording 30 minutes before sunset until 30 minutes after sun rise. The detectors are pre-programmed with the latitude and longitude of the survey location enabling the software to calculate the local sunset and sun rise times automatically (which obviously change incrementally through the year).

All sound data was processed using the British Trust for Ornithology’s Acoustic Pipeline; this recently-launched platform enables efficient processing of large datasets, and identification of sound files to species level, with the confidence levels indicated. It includes classifiers for the identification of bats (and other wildlife such as birds, crickets, small mammals) across Europe from high quality sound files. Development of software that undertakes “automatic identification” of bat sounds is still in its infancy, but the BTO Acoustic Pipeline is one of most advanced at present, and confidence in the platform is high. Nonetheless, we check the identifications of certain rarer species, and a proportion of more widespread species, to ensure that accuracy remains high and that we produce high quality data.

Interpreting the outputs
The output produced for each site is a table detailing the species recorded, and the number of sound files containing each species, on each night. Whilst we cannot estimate the numbers of bats being recorded, the numbers of sound files are a good proxy for the activity levels of each species.

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It is worth noting that each of the bat species may have a different “encounter rate” which reflects their population density, their foraging strategy and how they use the wider landscape, as well as their echolocation characteristics. For example, species such as the horseshoe bats, brown long-eared bat and Barbastelle have a relatively quiet echolocation which reduces the frequency with which they are recorded. Conversely, pipistrelle bats and Noctule bats are common and widespread, but also have very loud echolocation, producing a higher encounter rate. It is therefore necessary being cautious when comparing activity levels of different bat species – pipistrelle bat passes will always dominate the activity recorded (loud and abundant), but even small numbers of passes of horseshoe bats and Barbastelle bats will be very important.

 

Location name: River Kennet, Chilton Foliat, Wiltshire (SU 31673 70216)
Survey dates: 10th-17th September 2025 Deployment: Gareth Harris Analysis: Gareth Harris

Results

 

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Results – bat passes recorded per night:

 

 

 

 

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Comment

A single Wildlife Acoustic Song Meter Mini-Bat ultrasonic detector was placed close to the bridge over the River Kennet at SU 31673 70216. Bat activity over the river was anticipated to be significant, so the detector was not placed immediately over the water’s edge in order to reduce the activity recorded of species such as Soprano pipistrelle bat, and to increase the recording of species foraging in the woodland edge.

The detector was in place for 8 nights.

A total of 38,443 sound files were recorded. From these, the BTO Acoustic Pipeline extracted 118,974 bat passes (because many sound files contained more than one species of foraging bats). After we discarded the lower confidence identifications, and check the remainder for accuracy, we are left with 109,391 bat passes!

I have checked a moderate percentage of the files attributed to Daubenton’s bat, Whiskered/Brandt’s bat, Common pip and Soprano pip. Each of these species is definitely present, but since I have not checked 100% of these files, the actual numbers of passes may vary. (which doesn’t really matter – the numbers relate to bat passes rather than numbers of individual bats). I checked all passes of the other species, so these numbers are accurate.

These are obviously astonishing numbers. Firstly, they reflect the incredible activity of foraging bats at the survey site. Remember that these “bat passes” reflect each time a foraging bat triggers the bat detector when it flies past – the numbers we generate are not numbers of individual bats. The problem arises when large numbers of foraging bats forage continuously in proximity to the detector – i.e., it is constantly triggered!

These surveys recorded many of the common & widespread bat species in the county, with a total of ten species recorded. This is not considered a final species list for the site – further surveys are likely to add additional species. This is however a great outcome for early Autumn.

In the following table, I have indicated the percentage of total bat passes recorded, for each species. This is just to give you an idea of the relative activity levels of each species. But please note the paragraph above discussing encounter rates for different bat species. Since different bat species have different encounter rates, it’s not “fair” to compare them like for like. Daubenton’s bat, Soprano pip, and Common pip echolocate loudly, and are very abundant at this location, so they will always account for the majority of activity. Surveys of this type, will however, under-estimate the quieter species.

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Some conclusions:
40% of activity related to Soprano pip, 30% to Common pip, and 27% to Daubenton’s bat. These are abundant species, and species frequently found foraging in and around wetlands. Such a high activity demonstrates an abundance of insects and suggests the habitat health is good. (I’m sure more detailed riverfly monitoring and aquatic invertebrate survey will give a more detailed comment on river health than this!).

Other common and widespread bat species recorded were Whiskered/Brandt’s bat, Natterer’s bat, Noctule, Serotine, and Brown long-eared bat.

I have asterisked the Whiskered/Brandt’s bat records – there is some difficulty in accurately and confidently differentiating the closely related Myotis species, particularly Whiskered and Brandt’s bats. Usually, we will aggregate acoustic records of Whiskered and Brandt’s bats.

Species of greater interest include Barbastelle bat and Nathusius’ pipistrelle bat. These species are listed on the UK conservation red list as Vulnerable and Near Threatened, respectively. Serotine bat is also listed as Vulnerable.

Barbastelle bat - Barbastelle bat is considered rare, and infrequently encountered by most observers, but our surveys across Wiltshire highlight that this species is remarkably widespread in the county. This species has a low detectability in these types of survey, so the relatively low number of passes recorded does not mean this is insignificant. It is also worth stating that Barbastelle bats fly very fast and therefore cover ground quickly. The roost site may not be that close to the survey site but is clearly within a kilometre or so. Barbastelle bats forage over a very wide area, within individuals foraging over 20km from their roosting sites on favoured feeding patches. They are truly a landscape-scale bat species, using a variety of habitats each night.

We have studied the Barbastelle bats of the Savernake Forest and wider area for many years – the bat group initiated these studies in the late 1990’s, and we continue them today. This included some ground-breaking radiotracking studies in 2024.

The Savernake and wider area support several breeding groups of Barbastelle, and radiotracking has shown that these bats often forage in the Kennet valley too.

Nathusius’ pipistrelle bat –
Nationally rare, and rare in Wiltshire but quite well-known in the north of the county, throughout the Cotswold Water Park. This is a species favouring lakes and wetlands, often in proximity to woodlands but it’s also a migratory bat, so we think that some migrate through the UK (and Wiltshire) in spring (heading north) and autumn (heading south). Large numbers of this species breed in Scandinavia, but winter in southern Europe. Occasional records of this species can turn up during spring and late summer/autumn and presumably relate to migrating individuals. Concentrations of records and activity would deserve further exploration!

During these surveys at Chilton Foliat, we recorded 6 confirmed passes of this species. More are of course possible, buried in the deluge of similar Common pipistrelle bats! This likely relates to migratory bats.

Species assemblage -
Species such as Barbastelle, Natterer’s bat, Whiskered/Brandt’s bat and Noctule bat indicate the wooded nature of the landscape here, with small woodlands and hedgerows that these species will utilise for commuting across the landscape and then foraging. Noctule and Serotine bat are larger bat species, which characteristically often hawk high over pastures and meadows for aerial insects such as small flies and dung beetles, as well as woodland edge, parkland and mature woodland.

In this way you can perhaps imagine different species foraging at different heights, with species such as Daubenton’s bat and Pipistrelle bats foraging low over water and grassland, Natterer’s and Brown long-eared bats feeding around hedgerows and tree canopy, whilst high in the sky are Noctules and Serotine bat “dogfighting” like spitfires for insects.

The high species diversity reflects the diversity of habitats within the site and nearby, ranging from wetland habitats such as river corridor & wet woodland, pastures, the hedgerow network, broad-leaved woodland and farmland habitats. This wide range of habitat offers diverse foraging grounds throughout the year, as well as a well-connected, “permeable” landscape for foraging and seasonal movements to hibernation sites and mating sites.

Thanks again for supporting our work by hosting our visits and surveys. I attach additional information for background too. There is a huge amount of information here, so please let me know if you have further questions.

13th October 2025
Gareth Harris

Further information: Gareth Harris southwiltsghbproject@gmail.com / wiltsbatrecords@gmail.com