Why Do I Need Mitigation?
If the bat surveys undertaken for your development reveal that you have bats roosting or hibernating in your building, the next step is to do an impact assessment and set out a mitigation plan. The aim is to ensure that the development can go ahead with no detrimental effect to the conservation status of bats by maintaining and preferably enhancing the affected bat populations.
The level of required mitigation will depend on the size and nature of the impact the development will have on the resident bats, as well as the importance of the roost in terms of its nature conservation value. This is very site specific and depends on the species recorded and number of bats present. An impact assessment undertaken by a suitably qualified ecologist will lead to one of the following three outcomes which will go on to inform the mitigation strategy:
- Avoidance – If appropriate avoidance measures are taken, then negative impacts on the roost as a result of the development can be avoided. Avoidance measures can include re-designing elements of the development that may impact bats or carrying out works at certain times of the year to avoid disturbance;
- On-site mitigation – To maintain the roost on-site, mitigation is required. This will involve either improvement of the existing roosts or the provision of new roosting opportunities within the site or building. This can include modifying parts of the building design such as the soffits or eaves in order to accommodate bats;
- Off-site compensation – When on-site mitigation is not deemed to be possible and the existing roosts will be permanently lost as a result of the development, off-site compensation is required. This will involve the creation of new roosts nearby in an appropriate location.
Once a mitigation plan has been completed, it will consist of the following four elements which are discussed in detail below:
- Avoidance of deliberate killing, injury or disturbance of bats. This requires the developer and those working on site to take reasonable steps to ensure works do not harm individual bats by altering working methods or timing of works.
- Roost restoration, creation or enhancement (to provide appropriate replacements for roosts being lost or damaged).
- Long-term habitat management or maintenance to maintain foraging and commuting opportunities. Bats follow well used linear features (such as hedgerows) to navigate around the landscape. It is therefore important that these features are maintained or enhanced.
- Post development population monitoring (to assess if the scheme has been successful at maintaining the conservation status of bats).
Avoidance of Deliberate Killing, Injury or Disturbance
The precise details of this part of the mitigation plan are very much site and roost dependent. Generally, suitable roost provision (usually in the form of a bat box) for any bats displaced during the works must be put in place before any works can commence. However, this approach is not suitable for horseshoe bats which do not use bat boxes.
The timing of the works is also an important consideration. The seasonal nature of bat roosts provides a window in the year (usually over winter) when bats are unlikely to be present in a range of building types.
If bats are found to be roosting within a building, it is standard practice for a suitably qualified ecologist to brief site workers (usually termed a ‘toolbox talk’). This briefing will inform site workers of the construction practices which could affect bats, explain the legal status of bats, highlight the likely places to find bats and outline the working practices required to minimise or avoid harming or disturbing bats. Site workers are also informed that if bats are found during works, then works need to cease until the bat can be safely removed by an ecologist. It may also be necessary for the ecologist to supervise a ‘soft strip’ of the parts of the building that provide opportunities for bats to roost. For example, the roof, including the access and roosting points identified during the surveys would be removed using hand tools, under direct supervision of the ecologist.
The precise details of this part of the mitigation plan are very species specific as different bat species have a range of roosting requirements. For any roost restoration, creation or enhancement to be successful, the design and installation of the roosts must be specific to those species found on site.
Our bat species can be divided into three different categories with regard to their roosting requirements:
- Crevice-dwellers (pipistrelle species, whiskered bats, Brandt’s bats, barbastelle and serotine);
- Hole/roof-void dwellers (long-eared, Natterer’s, Daubenton’s, Bechstein’s, Leisler’s and noctule);
- Bats that need internal flight space and flying access (greater horseshoe and lesser horseshoe).
Of the crevice-dwelling bats, you are most likely to encounter common pipistrelle, soprano pipistrelle, whiskered bat and Brandt’s bat roosting in buildings; all can be found in a wide range of building types and are often found in houses. A soprano pipistrelle only needs a crevice the width of your finger nail to roost in. Crevice-dwelling bats are often found in stone crevices and under lifted bargeboards and roof tiles and so replacement roosts usually take a similar form.
Whiskered/Brandt’s bats are also crevice dwellers but are usually associated with the roof voids of buildings, as they are known to move around in the roost before emerging. A roof void with enhancements for crevice dwelling species or an eaves roost are usually good options for these species.
Of the hole-dwelling bats, you are most likely to encounter long-eared bats and Natterer’s bat roosting in buildings; both are often found in traditional stone barns. Long-eared bats are often found roosting in the hole between the wooden ridge beam and the ridge tiles in buildings with an uncluttered ‘A frame’ type roof construction. Long-eared bats are light sensitive (i.e. they tend to avoid areas with high levels of artificial illumination such as street lighting) and will need a bat loft with ridge beam roosting opportunities and a suitable access point that is not illuminated. Natterer’s bats are also a light sensitive species and require an uncluttered roof void. They are often found roosting in holes in the mortise joints of old barns which are thought to mimic tree holes, they are also often found in churches. For roost creation, mortise gaps can be created to mimic those found in old barns. Alternatively, a specialist type of bat box can be used. However, mitigation schemes for Natterer’s bats have often been unsuccessful. It is therefore important that the replacement roost matches the original roost as closely as possible.
Our two species of horseshoe bat have fairly similar roosting requirements but are quite different from our other species’. They are the only bats in the UK which hang freely to roost rather than crawling into crevices and holes. It is for this reason that they usually require a roof space and an open access point like a letterbox access square or a small dormer. They are often found in cellars and corrugated metal outhouses with timber and stone surfaces which they prefer for perching. They are usually found in dark locations close to good foraging habitat. Horseshoe bats are considered to be our most light-sensitive species. They will not use bat boxes as they are unsuitable for their requirements. There are a range of mitigation options available with dedicated loft voids and purpose-built stand-alone timber roosts being the most popular choices.
Long-term Habitat Management or Maintenance
Bat roosting features should be retained and maintained in perpetuity to allow continued use.
Landscaping and planting are important considerations when planning the long-term maintenance of bat foraging habitat. Landscaping and planting can enhance the habitat by encouraging a greater number of night flying invertebrates. A variety of plant species should be planted as this will bring a wider range of invertebrates for bats to consume. Plants which provide a rich source of nectar are considered to be the most suitable, for example lavender and honeysuckle.
Another long-term consideration if the development requires a new roof or repairs to the existing roof is the roof lining. Non-woven underlay (NWRU) or breathable membranes (BRMs) must be avoided in bat roosts as they tend to rip and can give rise to dislodged fibres in which bats can become tangled, trapped and eventually die. 1F traditional hessian-backed bitumen felt which complies with BS8747:2000 and BS 550:2011 is currently the only roof lining considered to be safe for use with bats. Read our article on BRMs here.
Post-development Population Monitoring
The period of time during which post-development monitoring will be necessary will depends entirely on the specific development, the type and number of roosts and the species present. Monitoring is required to determine if the mitigation is successful or not so that it can be improved if necessary.
Monitoring can comprise anything from a single site visit to check the recommended mitigation features are in place, to dusk emergence or dawn re-entry flight surveys and remote monitoring. This will help to assess the ongoing use of replacement roosts in future years. As maternity roosts are considered to be the most important type of roost in terms of conservation value, schemes involving maternity roosts may require further monitoring and modification of the mitigation strategy if they are not deemed to be successful.
Bat mitigation need not be the inconvenience that it is often made out to be.
Our licensed bat survey specialists are able to undertake all aspects of bat surveys including initial bat scoping surveys, dusk emergence and dawn re-entry flight activity surveys and remote monitoring. We have several years’ experience in the design and implementation of mitigation strategies for bats including the construction of bespoke bat roosts, bat exclusions and bat roost enhancement, as well as applying for European Protected Species licences for bats.
Various publications have been produced which provide a more in-depth look at bat mitigation. Links to some of these are given below.