Guidelines for passive acoustics surveys of bats in woodland
These guidelines bring together research and recommendations from BCT and others regarding passive acoustic surveys of bats in woodland, for site management or monitoring purposes. Guidelines for professional ecological surveys are provided in Bat Surveys for Professional Ecologists: Good Practice Guidelines.
The purpose of the survey
Two common reasons to survey woodlands for bats are to
- Confirm which species are present and their distribution across the site. Survey techniques should seek to maximise detectability and spatial coverage. It is important to detect rarer species. The survey may be a one-off so greater survey effort is often possible.
- Monitor changes to species distribution or activity over time (not considered here). Survey locations should represent an unbiased sample of the site or landscape, but comprehensive site coverage is not required. Survey effort should be consistent and sustainable over time. It is less of a priority to detect rarer species, as the survey effort required to produce robust trends for rare species is often prohibitive.
The guidelines below focus on the first case, confirming the presence and distribution of bat species across a site. We are developing guidelines for monitoring trends in bat distribution and activity in collaboration with Forestry England and Forest Research.
When to survey
Bats activity, and therefore species detectability, is highest and most consistent during July and August, so we recommend survey effort is focused in these months.
How many locations to survey
This will depend in part on the size of the site, and the number of surveyors and detectors available. A simple way to ensure good site coverage is to divide the site into a regular grid, with grid squares of between 200m and 1km in size, and select a monitoring location at the centre of each grid square.
The distance over which UK bat species can be detected in forest understorey ranges from approximately 5m (lesser horseshoe bat and long-eared bats) to 100m (noctule, Barataud et al. 2020). We recommend selecting survey locations no closer than 200m from each other (e.g. using a 200m or larger grid) to avoid unnecessary overlap.
Bats are highly mobile and so it is often not necessary to survey every part of a site to detect all the bats using a site. Foraging activity is concentrated in the area surrounding their roost, known as the core sustenance zone (CSZ). The size of the core sustenance zone varies with species - the smallest CSZ of a UK bat species is 1km, so we would recommend selecting sampling locations no further that 1km apart to ensure all CSZs are sampled (e.g. using a grid no larger than 1km).
Where to site the detectors
To maximise species detection place detectors alongside features mostly likely to be used by bats. These include edge habitats (woodland rides, woodland edges, treelines and hedgerows), close to water and in the woodland interior. To ensure good survey coverage it is important to sample all three feature types.
Additionally survey locations should be:
- Safe to access.
- Secure enough to leave a detector over several nights.
- Not lit be artificial light at night.
- At least 1.5m away from:
- Vegetation or other obstructions that could mask bat calls.
- Hard surfaces such as tree trunks or walls, which can cause echos that distort recordings.
- Water - the reflections of sound off the water's surface can distort recordings.
How high to deploy the detector
To maximise detection, the detector should be elevated above the ground. We recommend deploying the detector at 2m height.
The following options can be used to elevate the detector, secured if necessary using guy ropes or a ground stake and jubilee clip. They can be purchased for a few pounds online or from many high street shops, DIY stores or garden centres:
- Extendable/telescopic laundry/clothes prop (https://images.app.goo.gl/VZWt...)
- Sturdy bamboo cane (at least 2m high, approximately 5cm in diameter at base and no less than 2cm diameter at height).
- Steel garden stake or bean pole
- Ranging pole/rod (https://images.app.goo.gl/2dkm...)
Surveying in the canopy
Research by BCT at Swanton Novern NNR found that detectors placed in the canopy detected a significantly greater level of common pipistrelle and barbastelle activity than detectors placed in the understorey (BCT, unpublished data). However the difference was small and was not significant for any other species. Although it can produce interesting data, deploying and retrieving detectors in the canopy is challenging and for most purposes will not be necessary.
How many nights to survey
If recording continuously (or quasi-continuously, i.e. without triggers) we recommend surveying each location for three nights. This provides a reasonable compromised between detection probability and the volume of data generated. If using Audiomoths, a single set of alkaline or lithium batteries will last three nights, and the recordings will fit onto a single 64GB memory card if using the settings below.
If using recording triggers or thresholds, we recommend surveying each location for at least three nights and up to ten nights, depending on the power and memory capacity of you detector.
BCT research in southwest England suggests that three nights of surveying provides a >90% chance of detecting common pipistrelle, soprano pipistrelle, noctule and barbastelle where they are present (BCT, unpublished data).
Greater horseshoe bat could be detected with 90% certainty where present after five nights of surveys and serotine after six nights, however in our study area serotine was uncommon. It will likely have a higher detection probability in other parts of its range.
We do not yet have detection probabilities for Myotis or Plecotus species in woodland.
There is a trade off between the number of locations surveyed and the number of nights surveyed at each location. We are investigating this interaction across a number of sites in 2021 and will update these guidelines when results are available.
A note on lesser horseshoe bat
Lesser horseshoe bats are challenging to detect in woodland due to their quiet and highly directional call. Our research in southwest England suggested more than 10 nights of surveys are required to provide a >90% chance of detecting this species where present. However, if detected their call is very distinctive. If you suspect lesser horseshoe bats may be present on your site we would recommend targeted surveys to increase the chance of detecting them.
- Lesser horseshoe bats tend to forage within 2km of their roost. If you are aware of a roost location concentrate survey effort within this distance.
- Focus survey effort closer to the roost initially, where activity will be greatest. Detectors can be placed further away as flyways and foraging areas are identified.
- An emergence survey at the roost can be helpful to identify the flyways that bats are using.
- Lesser horseshoe bats do not like to cross open spaces or areas lit by artificial light, so place detectors along linear features (woodland edge, tree lines and tall hedgerows) and away from artificial light.
- Place detectors on the sheltered side of linear features.
- Lesser horseshoe bats forage close to vegetation along woodland edges and in sheltered pastures and wetlands.
Audiomoth recording settings
Make sure you are using the latest Audiomoth firmware
Instructions for configuring Audiomoths are given here
On opening the config app, be sure to select File > Local time, to ensure you are using the correct time zone.
NOTE: These recommendations will be updated from time to time as the Audiomoth firmware is updated. Thank you to Dani Linton for her advice.
Sample rate: 384 kHz
Gain: Medium
Sleep duration: 5 seconds
Record duration: 300 seconds
The following start and end times will capture the main period of bat activity and allow three nights of recordings to fit onto a single 64GB SD card. If you do not have this constraint you may adapt accordingly:
- Start time: 21:00 BST
- End time 4:30 BST
Frequency filtering: Not enabled
Amplitude threshold: Not enabled
We recommend SanDisk Extreme micro SD cards and Energizer Ultimate Lithium AA batteries for use with AudioMoths
How to configure and deploy AudioMoths
Click here for instructions on carrying out an AudioMoth survey.
Notable records
If you are using automated sound classification software to process your recordings then ideally at least one recording of each species on each survey date should be manually verified. This is especially important for notable records, that is records of rare species or species outside their current known range.
For some species, such as Myotis mystacinus, it is not generally possible to conclusively verify an audio recording. If acoustic surveys suggest such species are present the next step is to carry out a trapping survey under licence, to identify the species either in the hand or via DNA analysis of droppings. The location of trapping sites can be guided by the results of the acoustic survey.
References
Barataud, M., Tupinier, Y., 2015. Acoustic Ecology of European Bats: Species Identification, Study of Their Habitats and Foraging Behaviour, 1st ed. BIOTOPE
