A week before the planned Bat Walk, organised by the Chilton Foliat Wildlife Group, a Wildlife Acoustics Song Meter Mini-Bat ultrasonic detector was placed close to the footbridge over the River Kennet by Gareth Harris of the Wiltshire Bat Group.
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 deployed at 4 metres above the ground, ensuring that it was 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. They can be 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 detector was in place for 8 nights.
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.
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.
A total of 38,443 sound files were recorded from which the software extracted 109,391 bat passes!
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 Wiltshire, 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 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.
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.
Other common and widespread bat species recorded were Whiskered/Brandt’s bat, Natterer’s bat, Noctule, Serotine, and Brown long-eared bat.
Rare 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 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, with 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 1990s, 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.
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.
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 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
