In order to accurately mitigate for the disturbance or potential loss of lesser horseshoe and greater horseshoe night roosts, day roosts or hibernation sites, it is vitally important to first understand their unique roosting ecology.
Lesser Horseshoe Ecology
Lesser horseshoe bats are the smallest of the two British horseshoe species and one of the smallest UK bat species, being approximately plum sized and having a wingspan of between 200mm – 250mm. They are protected under the EU Habitats Directive (1992). This is due to dramatic population declines in western Europe caused by disturbance of roosts, agricultural intensification, reductions of prey availability as well as losses to foraging and commuting habitats.
Lesser horseshoes are predominantly cave dwellers, showing the largest affinity for roosting and hibernating activity within underground sites. However, in recent years during the summer roosting period the majority of lesser horseshoes have been found in buildings, including larger rural houses, barns and stable blocks. These sites offer large void spaces allowing the bat to access the building uninterrupted where they then hang to roost with their wings folded around their bodies. This is dissimilar to other bats such as Pipistrellus species, that crawl into their roost via crevices.
Maternity colonies in most cases form in buildings, arriving from April/May and producing young by mid June/mid July. The colonies often shift between attics, chimneys and cellars throughout the summer period, depending on the local weather conditions. From September/October to April/May lesser horseshoes go into hibernation and are most frequently recorded in underground sites such as cellars, mines, caves and tunnels. Individuals will often become active within the hibernation site in autumn and spring during warmer weather, when successful feeding may occur. Temperatures inside hibernation roosts are ideally up to 11°C with high levels of humidity.
Greater Horseshoe Ecology
Greater horseshoe bats are one of the largest British bats (approximately the size of a small pear) with a wingspan of between 350mm – 400mm. They are also protected under the EU Habitats Directive (1992), with recent estimates showing declines of around 90% in the last century. Greater horseshoe bat are now confined to areas in south-west England and south Wales. This is a result of a number of factors similarly affecting lesser horseshoes. The roosting ecology between the two species of horseshoe bat in Britain is fairly similar, however, greater horseshoe bats will generally require a larger access point into their roost than a lesser horseshoe.
The key physiological features that helps identify a lesser horseshoe from a greater horseshoe is that the greater horseshoe is larger. They also echolocate at different frequencies.
Night Roosts and Day Roosts
Day roosting in horseshoe bats tends to refer to roosting activity in buildings and in some cases, large numbers of females that form a maternity colony. Within these roosts they congregate to give birth and raise their young. As a result, day roosts are of prime conservation concern as disturbance and destruction has resulted in severe population declines.
Night roosting activity is less understood. Through behavioural research it is known that night roosts are used as ‘pit stops’ between foraging sites and used for resting and digestion between commutes. The loss of a night roost must also be mitigated for.
Factors to Consider While Creating or Retaining a Roost for Horseshoe Bats
While planning successful mitigation strategies for horseshoe roosts or hibernation sites, there are a number of factors that need to be considered:
- Temperature and humidity regimes;
- Aspect and orientation;
- Access points;
- Materials; and
- Vegetation Linkages.
Temperature is deemed one of the most important environmental variables affecting the success of a new roosting facility. Optimum roost temperatures are species-specific.
Aspect and orientation can be manipulated in order to utilise solar energy and maintain optimum temperatures within a roost. In the northern hemisphere, a southerly or westerly facing roost site is best; particularly maternity roosts.
The size of the roost specifically for horseshoe bats should be at least 2.8m in height and 5m in length and width. The void should not be cluttered or obstructed as horseshoes will often fly in spaces between the rafters and ceiling, and ceilings to the floor. A typical truss design should be avoided to successfully mitigate for horseshoes.
Access points are key to the success of horseshoe roost sites as horseshoe bats directly fly into a roost. They need access points of 30cm (width) x 20cm (height) for lesser horseshoes and 40cm (width) x 30cm (height) for greater horseshoes.
As nocturnal mammals, bats are light-sensitive and adapted to low light conditions. Consideration must be made to reduce light spill close to roosting areas. Horseshoe bats typically emerge approximately 30 minutes after sunset and during emergence they often fly in and out of the roost repeatedly before leaving. This behaviour is termed “light sampling” and has an important social function prior to the bats leaving the roost. This behaviour can be seen in the video below. Mitigation schemes can try to create a covered area where bats can undertake this behaviour as shown in the photograph on the right.
The materials used within the roost are critical to its continued success. Non-woven underlay (NWRU) or breathable membranes (BRM) 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. 1F traditional hessian-backed bitumen felt is currently the only roof lining considered to be safe for use with bats. Alternatively, rough timber sarking can be used as shown in the photograph below.
Vegetation Linkages are an important consideration. Roost access points must be linked to vegetation and commuting routes.point/s. Additional hedgerow or tree planting close to the roost access points may be needed to connect to bat commuting or foraging habitat.
Specific Lesser Horseshoe Mitigation
Due to the large access points required by horseshoe bats, ideally either a letter box access or dormer access should be installed for day roosting lesser horseshoe bats. For night roosts, stand alone night roosting facilities are advised.
Letter Box Access – Day Roost Mitigation. These are installed as low as practically possible on a gable wall but at least 0.4m above the level of the deck. This ensures that bats are away from clutter during access and will not be impeded by any potential bird’s nests. Depending on the horseshoe bat that is utilising the site, the size of the entrance way must adhere to their needs (30cm x 20cm for lesser horseshoes and 40cm x 30cm for greater horseshoes). The entrance will be sloped downwards and outwards with waterproofing (e.g. lead lining) below to minimise ingress by rain. An optional canopy above can be used. Additional draught reducing and light deflecting baffles could be used in conjunction with this access feature. Any weather shielding must not restrict roost access. Informal monitoring of mitigation schemes suggests that schemes with access points via letterbox-style entrance slots have a higher rate of success than schemes with dormer access points. The video below shows lesser horseshoe bats emerging from a letterbox-style access point.
Dormer Access – Day Roost Mitigation. Direct access into a dedicated bat loft can also be installed via a custom-built ‘dormer’ opening. The dormer will project out from the roofline and be installed as low as practically possible. Additional draught-reducing and light deflecting baffles could be used in conjunction with this access feature.
Stand Alone Night Roost – Night Roost Mitigation Stand alone night roosts are timber structures with dimensions of at least 1.2m in width, 1.8m in depth and 2.7m in length, constructed from a timber frame and covered with weatherboard, sealed on the inside with traditional bitumen felt which complies with BS747:2007 and BS 5250:2011. They have a steeply pitched roof, covered with tiles or slates with exposed timbers within the roof void. Access points are incorporated as advised above and located away from prevailing wind (south-westerly direction) or otherwise protected via a hedgerow or other scrub planting. Bat access should be at a minimum height of 1.3m above the adjacent ground level. The ideal location of the stand alone night roost is close to existing hedgerow or it should be planted with trees and shrubs linking to a nearby hedgerow to provide a necessary flight line. The structure should be secured to the ground. This could be via either: a post base stand fixed to a concrete pad foundation; a socket which is anchored in a block of concrete; a socket with a base plate bolted to a concrete surface or another suitable method.
Further details of some of the sites that we have worked on involving horseshoe bats can be found here: Forest of Dean Lesser Horeshoe Night-roost Mitigation; St Tewdrics Bat Mitigation
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. For more information call us on 029 2065 0331.
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, Bat Mitigation for Crevice-Dwelling Bats and Bat Mitigation for Hole/roof-void Dwelling Bats.