Bat Surveys
Legislation for Bats
All bat species are fully protected by the Habitats Regulations 2010 (as amended) as they are listed on Schedule 2. Regulation 41 prohibits:
- Deliberate killing, injuring or capturing of Schedule 2 species (e.g. All bats).
- Deliberate disturbance of bat species in such a way as:
a) To impair their ability to survive, breed, or reproduce, or to rear or nurture young;
b) To impair their ability to hibernate or migrate;
c) To affect significantly the local distribution or abundance of the species.
- Damage or destruction of a breeding site or resting place.
Bats are afforded the following additional protection through the Wildlife and Countryside Act 1981 as they are included on Schedule 5:
- Intentional or reckless disturbance (at any level).
- Intentional or reckless obstruction of access to any place of shelter or protection.
Development activities such as building renovations or demolition and tree pruning or felling could result in the damage, modification or destruction of bat roosts or disturbance to bats.
Survey Methodology
The methodology for bat surveys will depend upon the survey objectives. Some of the most common bat survey methodologies are described below.
Preliminary Roost Assessment
A Preliminary Roost Assessment (PRA) is the first stage in the bat field survey process. It will comprise a daytime bat walkover survey of the site to identify potential bat roosting, foraging and commuting opportunities within the site boundary and surrounding habitat.
Built Structures
Built structures will be subject to an external inspection for the presence of Potential Roost Features (PRFs) including (but not limited to) lifted roof tiles, hanging tiles, gaps in brickwork, fascia boards and timber cladding. An internal inspection will also be completed, including any roof voids or cellars.
Trees
Trees will be subject to a Ground Level Tree Assessment (GLTA) for the presence of PRFs including (but not limited to) cavities, raised bark and ivy cover.
Foraging and Commuting Habitat
The value of the site for foraging and commuting bats will be assessed based on the habitats present on the site and in the surrounding landscape and taking into account other factors such as light spill and habitat connectivity.
Survey Aids
Ladders, torches and endoscopes will be used to undertake a close inspection of PRFs, where accessible. Evidence of bats including live or dead individuals, droppings, urine staining, scratch marks and feeding remains will be recorded, where present.
Droppings may be collected and sent to a laboratory for DNA analysis to determine species.
Assessment of Suitability
Built structures will be assigned a value based on their suitability for roosting bats, from negligible to high.
Trees will be classified according to whether PRFs were identified (and whether the PRFs could support individual or multiple bats if this can be determined from ground level) or if they could be present but not identifiable from ground level.
If evidence of roosting activity is identified, the built structure or tree will also be classified as a confirmed roost.
Habitats will be assigned a value based on their suitability for foraging and commuting bats from negligible to high.
Bat Emergence and Re-entry Surveys (Built Structures)
Where built structures are assessed to have low to high roost value, they may require a minimum of one to three bat emergence and/or re-entry surveys respectively to determine the presence or absence of roosting bats.
Surveys of each built structure will be spaced at least three weeks apart. Emergence surveys commence 15 minutes before sunset and continue for at least 90 minutes after sunset. Re-entry surveys commence at least 90 minutes before sunrise and continue for 15 minutes after sunrise.
Full coverage of the potential roost features on each built structure will be required via appropriate placement of surveyors, accompanied by bat detectors with recording capability, and night vision aids such as infra-red or thermal cameras.
Where roosting activity is identified, the time, location, number and species of bat and roosting behaviour will be recorded on survey forms and site plans.
Footage from the night vision aids will be reviewed as soon as possible following each survey, using video playback software to identify roosting activity.
Acoustic analysis of the bat calls recorded by the bat detectors will be completed using acoustic analysis software to identify bat species.
Hibernation Surveys (Built Structures)
Where built structures are assessed to have suitability for hibernation roosts, targeted hibernation surveys may be necessary.
Hibernation surveys will comprise a minimum of two visits, spread at least four weeks apart.
An inspection of the built structures and any potential roost features will be undertaken for hibernating bats, which may include the use of ladders, an endoscope and torch as survey aids.
During each survey automated bat detectors will be deployed for a period of at least two weeks to record bat activity.
Acoustic analysis of the bat calls recorded by the bat detectors will be completed using acoustic analysis software to identify bat species.
Bat Activity Surveys
Where a site is assessed to have low to high foraging and commuting value, they may require a minimum of three bat activity surveys to determine the level of bat activity at the site.
Bat activity surveys comprise night time bat walkovers and the deployment of automated detectors.
The night time bat walkovers will commence at sunset and continue for at least two hours after sunset. For the first 30 minutes of the survey, surveyors will be positioned at vantage points. After the first 30 minutes, a pre-defined transect route will be walked. Surveyors will use bat detectors with recording capability.
Bat activity including behaviour, direction, bat species and number of bats will be recorded on survey forms and site plans.
Alongside the night time bat walkover, automated bat detectors will be deployed for a minimum of five consecutive nights per survey in suitable bat foraging and commuting habitat within the site boundary.
Acoustic analysis of the bat calls recorded by the bat detectors will be completed using acoustic analysis software to identify bat species.
Statistical analysis will be completed to determine the level of bat activity at the site.
Key Survey Considerations
PRAs can be completed all year round, however, other types of bat surveys may be restricted to a specific seasonal window. Bat emergence and re-entry surveys must be completed between May and September, though the optimal period is May to August, whilst bat activity surveys must be undertaken between April and October. Hibernation surveys must be completed between November and March but the optimal period is December to February.
Bat emergence and re-entry surveys and bat activity surveys must also be undertaken in weather conditions which are conducive to bat activity and must avoid periods of strong wind, heavy rain and cold overnight temperatures.
Additional Requirements
Subject to survey findings, additional roost surveys may be necessary to enable the characterisation of a roost to a particular type (this may include maternity, hibernation, satellite, feeding, day, night and transitional roosts ).
A greater level of bat activity survey effort may also be required for sites with moderate to high value for foraging and commuting bats or where the assemblage of bats is of particular interest.
The approach to identifying bat roosts in trees, following a PRA, differs to that of built structures. Trees that have been assessed to have suitability only for individual bats, following a PRA, may not require further surveys. However, where trees contain PRFs that could support multiple bats or whether the presence and/or extent of PRFs cannot be determined from ground level, further surveys may be necessary. This will typically comprise one or more aerial climbing surveys to enable a close up inspection of PRFs, however bat emergence and/or re-entry surveys may be acceptable if aerial climbing surveys are not possible (e.g. for health and safety), as long as night vision aids are used.
Where bat roosts will be impacted, a bat mitigation licence may be required from the relevant statutory authority. There are three different approaches to bat mitigation licensing including the traditional European Protected Species Licence (EPSL), a Bat Mitigation Class Licence (BMCL) and Earned Recognition (ER).
The most appropriate licence approach for a site will depend on the type and number of roosts and the bat species present. The level of mitigation that will be required is dependent upon the type of licence that is obtained.
Impacts to maternity roosts are not permitted under any licence during the breeding period May to September. Likewise impacts to hibernation roosts are not permitted during the hibernation period November to March (there may be some variation subject to climatic conditions).
Further Survey Information
The survey will be undertaken in accordance with the following publication:
- Collins, J. (2023). Bat Surveys for Professional Ecologists —Good Practice Guidelines, 4th edition, Bat Conservation Trust, London.