Brown Long-Eared Bats (Plecotus auritus)
During building inspection work, ecologists often come across medium-sized, brownish droppings. On closer inspection, the droppings have a golden shine from the moth and butterfly wings they contain. This discovery is usually the first indication that the roost belongs to brown long-eared bats…
These bats often hide inconspicuously in crevices and can be hard to notice, sometimes hiding above the main timber beam along the roof apex, in crevices formed between timber joints, or between roofing felt and roof tiles. In order to confirm exactly which bat species the roost belongs to, the droppings need to be DNA tested. Brown long-eared bats (Plecotus auritus) can be found across most of the United Kingdom, Ireland and Island of Man. However, if we are surveying a site in Sussex, Hampshire, the Isle of Wight, Dorset, Devon or Somerset, we could discover grey long-eared bats (Plecotus austriacus) which are far rarer.
If a building is proposed for development, any bat roosts contained within it must be preserved as best as possible. The preferred option is to leave the roost intact. If this is not possible, the developers, with the assistance of suitably qualified ecologists, must mitigate for its potential loss. At this stage, the ecologists inform the mitigation strategy by relying on their knowledge of the roosting ecology of brown long-eared bats, as well on the information gathered through the preliminary inspection of the building and through emergence or re-entry flight surveys.
Ecology of Brown Long-Eared Bats
Brown long-eared bats, as the name suggests, are named after their long ears which can be almost as long as their bodies. While the bats are resting, they tuck their ears under their wings in order to prevent heat loss. When we observe a resting long-eared bat, their projecting earlobes can be seen sticking out from over their shoulders.
Brown long-eared bats have broad wings, enabling them to fly slowly and with great manoeuvrability. They are even capable of hovering in the air like a kestrel. They often rely on their eyes as well as their ears to locate their prey and do not always use echolocation. Because of this characteristic, it is difficult to detect them with ultrasound detectors while they forage. Consequently, brown long-eared bats are often under-recorded during surveys.
Their preferred foraging habitat is open deciduous woodland or parkland where they mainly feed on moths and butterflies. They often use feeding perches to eat and digest their food. Feeding perches are usually areas with an uneven surface such as rough timber beams, battens or broken roof tiles, enabling them to grip the surface with their claws. The bats can hang freely, using a minimal amount of energy by using a special locking mechanism in the foot. They prefer to roost in stone barns, churches and older buildings. Females are very attached to their roosting sites and will return to them every year. It is crucial to try to preserve the roost as best as possible in its original form, as some of the roosts have been in use for decades.
Brown Long-Eared Bat Mitigation
The first two questions ecologists always ask are: Can modification of the bat roost be avoided? And, can the roosting features be left intact? Often this is not possible due to the nature of the development proposal, therefore the following options need to be considered.
Bat Loft for Brown Long-eared Bat Maternity Colony
Because brown long-eared bats like to roost inside crevices as well as needing a void where they can fly, the best mitigation option when dealing with a brown long-eared bat maternity colony is to recreate the loft void and dedicate part of the new space to them. Ideally, the void height (floor to ridge) should be at least 2m and preferably exceed 2.5m (Razgour et al. 2013), with a length and width of at least 4m (preferably 5m) as detailed in English Nature’s Bat Mitigation Guidelines (Mitchell-Jones 2004).
At approximately 2m intervals along the ridge beam, the bitumen roof felt should have 30 x 100mm slots cut out beside the ridge boards, so that bats can access the underside of the ridge tiles, where they like to roost. A few small torn holes through the felt can be created at several levels from the apex of the roof to halfway down the roof slope. This will allow bats into the space between the tiles and the felt.
In stone buildings, stone crevices are usually recommended for hibernation purposes. Stone crevices can also serve as a summer roosting features for brown long-eared bats inside the lofts. This can enhance a dedicated bat loft with additional roosting places. If a stone wall needs to be re-pointed and the crevices will be lost, some crevices can be left intact so that bats can continue to use them as a roost. Alternatively, any crevices that are lost should be re-created elsewhere, in a suitable location within the building.
Panel Style Roost
Another type of mitigation is a panel-style roost. This can be installed into the loft space when dealing with a non-maternity colony of brown long-eared bats. A space with an approximate surface area of 1 square metre can be created between the rafters and separated from the rest of the loft space with plywood or hardwood boards. It is not possible for the bats to access the roof space from inside the panel-roost. Appropriately designed bat access points lead from the panel roost to the outside of the building. This can be a single raised tile, a gap under the ridge tile or access underneath a raised bargeboard. Details of different access points are presented in the article about crevice dwelling bats here. It is crucial that the roof lining used is made out of bitumen and not the breathable membrane.
Eaves Bat Roost
If a dedicated bat loft is not possible and we are dealing with a non-maternity brown long-eared roost, the third option is to install a roost into the eaves. This option is often used when we are converting a loft into living space. In this scenario, the majority of the existing loft space will be lost. A chamber or compartment is built at the eaves, extending back under the roof and beyond the cavity wall. Bats gain access to the roost via gaps beneath fascias or soffits at the eaves. A hatch can be installed in the internal wall of the roost to facilitate monitoring and carry out maintenance.
A similar option to creating an eaves bat roost is to create a roost along the internal gable end of the building. Dependant on the size of the loft, the compartment can have different dimensions (for example, 1m to 3m in length) and extend from the eaves up to the ridge board. Access to the roost can be provided via gaps beneath the adjoining fascias or bargeboards.
Increasing the Number of the Internal Roosting Opportunities
Any exposed woodwork in a roof void or artificial roost should be left with a rough surface, allowing bats to cling onto it. Roughened timber cross battens with tiles or plywood squares secured to the battens can also be attached to the rafters and walls to provide extra roosting opportunities and increase the range of crevices and niches available.
As long-eared bats roost in crevices, tending to roost inside the ridge tiles or against the ridge board (Razgour et al 2013), consideration can be given to installing double or triple ridge boards to increase the availability of roosting sites. Multiple ridge boards can run parallel with each other, maintaining a 20–30mm gap between each board. This creates a roosting crevice running along the length of the ridge inside the loft.
Bitumen Vs. Breathable Membrane
Internal roof spaces must be insulated with bitumen lining rather than breathable membranes which must not be used!
As the name suggests, breathable membranes (commonly abbreviated to BRM) is highly breathable! BRM’s allow moisture to escape the roof which helps regulate the humidity and temperature of a building. BRM’s are fast replacing the more traditional bitumen roof linings which have been commonplace for several decades.
Breathable membranes are made from tiny filaments which bats can hang onto. However, over time, the long fibres start to unwind which can entangle and trap bats. There is mounting evidence highlighting this issue. For example, Stacy Warring reports on her web page of dead bats being found caught in breathable membrane from where they were not able to escape.
Consequently, it is essential to use bitumen inside bat lofts rather than BRM’s. Using BRM’s in a bat loft is akin to creating a bat trap. Using a combination of breathable membrane and bitumen felt in different areas of a loft is also not advisable, as it is hard to completely isolate the areas with breathable membrane from the parts with bitumen felt. In the long term, roof tiles will become raised or fall out and bat accesses points will be created giving access to the areas constructed with a breathable membrane.
In summary, the roof of a bat loft or any roof spaces available to bats must be lined internally with 1F traditional hessian-backed bitumen felt which complies with BS8747:2007 and BS 5250:2011.
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.
Further details of some of the sites that we have worked on involving brown long-eared bats can be found here: St Tewdrics Bat Mitigation
Further information on mitigation for long-eared bats can be found in the publication Conserving Grey Long-Eared Bats in our Landscape or English Nature’s Bat Mitigation Guidelines (Mitchell-Jones 2004).