Bats in buildings
With the clearing of woodland, the numbers of most bat species have declined. In response to this they have adapted to living in buildings and now rely so heavily on these for roosting that their conservation depends largely on our tolerance and good-will.
Like house martins and swallows, bats are usually seasonal visitors to houses and are typically present for only four to five months of the year. They tend to form maternity colonies during May and June and then leave during August and early September once the young bats are independent. The colonies are often most obvious during July, when the young are starting to fly; soon after this the adults will start to leave, followed by the young. Although this seasonal pattern usually applies, different species have slightly different habits, notably brown long-eared bats, which can sometimes occupy the same building throughout the year.
The typical size of a maternity roost, occupied by either of the two widespread pipistrelle species in Scotland, is between 100 and 200 bats. Exceptionally, colonies may contain over 1000 bats. In general, colonies at the smaller end of this range mostly comprise common pipistrelles, while the very large ones are formed by the closely-related soprano pipistrelle.
To maximise warmth, maternity roosts are often located on the south and west of houses or close to sources of heat such as chimneys and boilers. Most species prefer to roost in quite small spaces and are not usually found in open draughty areas like barns. The common and soprano pipistrelles are our smallest and most common bat species. They are generally found in the inaccessible parts of the roof structure and around its edges and rarely enter the loft space. Both species colonise new buildings quite readily and frequently roost in houses built since the 1960s. By contrast, the brown long-eared bat, our third most common species, prefers older building with large roof spaces and is often seen in clusters at the top of the roof ridge inside the loft.
Where bats are seen in buildings during the winter, they tend to be alone or in small scattered groups, hidden in crevices or under slates and away from sources of heat.
House Roost Sites
Bats and people
Bat colonies usually live happily with their human landlords, but occasionally problems or concerns arise. However, most bats are seasonal visitors and will leave of their own accord at the end of the breeding season. Scottish Natural Heritage is happy to provide advice on how to deal with any problems, the most common of which are outlined below. Scottish Natural Heritage should, by law, be consulted before you take any action that may affect bats or their roosts. Details of the how the law protects bats can be found on page 18.
Bats in rooms
Bats generally come and go from their roosts without any trouble, but occasionally they appear inside the house. This seems to be most common when baby bats, which are tiny and often grey, crawl out of their roost and find a way into the rooms. There are also cases of young, inexperienced, bats flying in through windows by mistake. If a bat is flying inside a room, open doors or windows to allow it to escape. If it has settled, and remembering you should not touch bats, phone your nearest Scottish Natural Heritage office or the SNH bat helpline on 07774 161219. If you need to remove a bat from a room or anywhere else, place a small box or other container over it and slide a piece of cardboard underneath to trap it. Alternatively, cover the bat with a soft cloth or towel and gather it up carefully. In both cases you should wear protective gloves. Carry the box or cloth outside and release the bat away from houses, cats and people, preferably at dusk.
If more than one bat appears inside, it is most likely that they are crawling through from a roost, so block up any holes around pipes or gaps in ceilings or floors from inside the house. If you are not sure what to do, phone Scottish Natural Heritage or your local Bat Group for advice.
Noise
Problems of noise can occur occasionally; if they do, then specialist advice may be needed. Large colonies of pipistrelles can be noisy during the summer, especially if they are within the wall of a bedroom. While this may last for only a short time it can be disturbing - Scottish Natural Heritage can advise on possible solutions that will help both you and the bats.
Droppings and smell
Bat droppings can be unsightly, though in Britain there is no known health risk associated with them. In the loft, the simplest and most cost-effective measure is to cover stored goods with dust-sheets, which can be brushed off every now and then. Outside, droppings stuck to the wall and windows are usually only a seasonal problem, but if a remedy is required, deflector boards fitted above windows and doors can provide a solution. Bat droppings do not normally smell strongly, but may do so if they get wet, either because they are in a very confined space with little ventilation or because rain water is entering the roost and the building.
Damage
It is very rare for bats to cause any damage to houses. Unlike birds, they do not bring in material to build nests and, unlike mice, they don't gnaw wood, electric cables, or entrance holes; they merely take advantage of existing gaps or holes. Once inside the roost they cling onto the timbers or squeeze themselves into cracks and crevices. Most bat colonies are small, with a colony of 50 bats weighing in at well under 500 grams.
Disease
British bats have few diseases that could affect humans and problems are very rare indeed. In recent years a tiny number of bats have been detected with a rabies-like virus called European bat lyssavirus, which can infect humans. For this reason, you should not touch bats or have contact with bat saliva. Bats rarely come into contact with people, but if you are bitten or scratched by a bat, you should wash the wound thoroughly with soap and running water for at least 5 minutes and seek immediate medical advice. An effective post exposure rabies treatment is available. Even if you have bats in your roof, there is minimal risk to you, provided you do not handle them.
Bats and household maintenance
Because they depend so much on buildings, bats need our tolerance and help to survive. This is mainly a matter of leaving them undisturbed. However, repairs, maintenance and alterations to buildings can adversely affect bats and their roosts. In these cases, a little forethought can minimise the problems. Remember the legal position is that advice must be sought from Scottish Natural Heritage before anything is done that could affect bats or their roosts.
Repairs, re-roofing and alterations
If you think you have a bat roost and want to carry out work such as re-roofing, roof repairs, alterations in the loft, wall repointing, or repairs to eaves or cladding that might affect the bats, please contact Scottish Natural Heritage for advice before starting the work. In most cases, it is just a matter of organising the timing of the work to avoid the bats' breeding season. We may also ask you to leave access points so the bats can return in the following years.
Remedial timber treatment
Treatment of woodworm or rot can be a major hazard to bats if carried out with the wrong sort of chemicals or at the wrong time of the year. Older-type insecticides such as lindane leave poisonous deposits on the treated surfaces which can kill bats even if they're not present at the time of treatment. Fortunately, this situation has now been improved by the introduction of more modern insecticides like permethrin or cypermethrin, which are much less poisonous to mammals. However, timber treatment should not be carried out when bats are present. If you think bats use the building, tell your timber treatment company and request advice from Scottish Natural Heritage on the type of chemicals to use and when to apply them. Likewise, there is a range of fungicides for treating wet and dry rot which are approved for use in bat roosts.
Burglar alarms
Sometimes bats flying in and around buildings can set off burglar alarms, including those that use light beams, microwave or ultrasonic detectors, or passive infra-red sensors. Persistent problems can usually be solved by moving the detectors, installing additional sensors or changing the system, for example to modern pulse-counter detectors, which are resistant to false alarms.