The term “crevice-dweller” is used to describe bat species that often utilise small crevices for roosting. Crevices can include natural features such as stone fissures, holes in trees and under loose bark. When found in buildings, crevices can be found underneath raised tiles, within cavity walls, between a roof lining and tiles and under fascia boards. The small size of crevice dwelling bats means that the crevices they can utilise for roosting can be as small as approximately 10mm in size.
Species that are termed “crevice-dwellers” include common pipistrelle (Pipistrellus pipistrellus), soprano pipistrelle (Pipistrellus pygmaeus), Nathusius’ pipistrelle (Pipistrellus nathusii), Brandt’s bat (Myotis brandtii) and whiskered bat (Myotis mystacinus).
Common Pipistrelle and Soprano Pipistrelle
Common pipistrelle and soprano pipistrelle bats are the two most abundant UK bat species and both have a wide distribution across the UK. Consequently, they are the two bat species which most often require mitigation implementation during a project or a development.
Habitat requirements of these two pipistrelle species differ slightly. Due to their feeding habits, the soprano pipistrelle tends to be more associated with foraging close to water bodies whilst the common pipistrelle is thought to prefer urbanised areas. Both species are known to forage in natural habitats such as woodland edges, hedgerows and parks.
Whiskered and Brandt’s Bats
Whiskered and Brandt’s bats are another two very similar species that can be hard to tell apart from one another based on their morphological features. Both of these species are commonly found in buildings and generally prefer older buildings. They can also be found roosting within trees. Whiskered and Brandt’s bats are not able to tolerate disturbance as much as the pipistrelle species and for this reason, are less likely to be found in urban areas. Common habitats that these species utilise are countryside, grasslands and forest areas.
As with any species, the best form of mitigation is avoidance, i.e. the development proposals should be designed in such a way that any negative impacts on wildlife are avoided at the outset. However, this is not always possible. For example, small scale developments may not always have the option or ability to change the development proposals due to land ownership or cost issues. If it is not possible to avoid negative impacts to bats, then these impacts (e.g. loss of roosts or access to roosts) will need to be compensated for.
Crevice dwelling bat species are often the easiest species to compensate for within a development. Below is a brief outline of the five most common ways in which the negative impacts on crevice dwellers can be mitigated:
Fascia, Soffit and Bargeboard Roosts
Crevice dwelling bat species often exploit gaps in or under fascias, soffits and bargeboards, to gain entry into buildings. If these features are being removed, replaced or made inaccessible to bats during the development work and bats are known to be using these features, then compensation within the new fascia, soffit or bargeboard should be implemented. This can be as simple as cutting holes in the new soffit box or bargeboard. The holes must be located next to the wall of the building as bats often require a surface (often a wall) to land on before crawling through the access. Examples of this are shown below:
Alternatively, gaps can be left between the wall and the fascia, soffit or bargeboard, by raising parts of the feature off the wall by approximately 20mm.
Raised Roofing Tiles
Another area within a building where crevice-dwelling bat species are often found is between the roof tiles and the roof lining. It is very easy to allow bats to continue using this space after the development by installing bat access within the roofing tiles. The number of raised roof tiles required depends on factors such as the size of the building, the size of the roost and how many tiles had gaps under them before the development. Installing an appropriate number of raised roof tiles is essential. Installing too many can alter the environmental conditions within the roost and make it unfavourable for bats. Putting in too few access holes can also be problematic within larger roosts. One simple method for raising a tile is to place a small amount of cement or mortar under the bottom of the tile, this slightly lifts the tile from the one below.
Alternatively, spacing two adjacent slates so they sit slightly apart from one another is another simple way of providing bat access into the roof of a building.
Certain companies supply specially designed ‘bat access tiles’ which come in a variety of different designs. These bat access tiles aim to be as aesthetically pleasing as possible whilst providing suitable access points for bats. The positions of the new tiles should be as close as possible to the original tile the bats were using to access the building.
Ridge Tile Access
The number of ridge tile access points created for bats will depend on a number of factors, such as the size of the building and the roost within it, and how many ridge tiles had gaps under them before the development. Selecting the right number of access points is critical. Having too many openings can alter the temperature or stability of a roost making it unfavourable for bats. Having too few access points could prevent bats from being able to leave larger roosts, trapping them inside. There are several different ways that bat access under ridge tiles can be created. The first is by leaving gaps in the mortar under the ridge tiles. These gaps should be 20mm x 100mm. A second approach is to modify standard ridge tiles by cutting a small section out from one edge.
Alternatively, another option would be to place a ridge tile so that it sits on top of its two neighbouring tiles. However, because this approach makes the access point most noticeable, it is often undesirable to developers.
Schwegler 1FR Bat Tubes and Surface Mounted Bat Boxes
If compensation for the loss of the bat roost needs to be located within the wall of new development (perhaps to mimic the loss of a crevice in the stonework), then a bat tube can be installed into the fabric of the building.
Bat tubes are often desirable for developers as they sit flush with or beneath the rendered surface of the wall so only the entrance hole is visible. Another advantage is that they can be painted with air permeable paint to match the colour of the building. Bat tubes also provide good roosting conditions for bats as they hold a high thermal mass due to being located within the wall of the building. Alternatively, surface mounted bat boxes can be installed.
Bat boxes can be useful in certain situations to provide artificial roosting features for bats. They are often used to house bats that are discovered during development work. They can be used when compensation needs to be installed on trees or when compensation for lost tree crevices needs to be made. Below is just one example of the many bat boxes that are available on the market.
Further details of some of the sites that we have worked on involving crevice-dwelling bats can be found here: St Tewdrics Bat Mitigation
More information about bats and survey techniques can be found in our articles – Bat Droppings, the Bat-Year, Architectural Terms for Bat Surveyors, a Guide to Bat Mitigation, Horseshoe Bat Mitigation and Bat Mitigation for Hole/roof-void Dwelling Bats.