The place a bat lives is called its roost. Bats need different roosting conditions at different times of the year and they will often move around to find a roost that meets their needs. Some bats prefer hollow trees, some like caves and some use both at different times. Many bats shelter in buildings, behind hanging tiles and boarding or in roof spaces.
For several weeks in summer, female bats gather in a maternity roost to have their babies. In winter, bats use hibernation roosts. Bats have been discovered roosting in all sorts of places, but there are three broad roost types that are the most common: roosts in trees, roosts in built structures and roosts in underground sites. Bats may also roost in bat boxes.
- UK bats do not construct roosts, but use structures that are already available.
- Bats are not rodents and they don’t nibble on wood, wires and other bits and pieces in buildings.
When the weather gets warmer, usually in early summer, pregnant female bats gather together in warm, safe places to have their babies. These roosts are called maternity roosts. Some groups of bats return to the same site every year.
A bat’s pregnancy lasts between six and nine weeks. The length of the pregnancy depends on the species and can be influenced by weather, climate and availability of food. Bats usually give birth to a single baby (called a pup) each year. They keep their babies close and nurture them carefully. The young bats are suckled by their mothers for four to five weeks until they are old enough to fly. They then begin to venture out from the roost to forage for food.
Bats are very sensitive during the maternity season and may abandon their young if they are disturbed. As warm, dry indoor spaces like lofts are often ideal for maternity colonies, it’s very important to check for bats before carrying out any building or remedial work.
In winter, bats go into hibernation. Hibernation is an extended period of deep sleep (or torpor) that allows animals to survive cold winters with harsh weather. A bat’s body temperature lowers and their metabolic rate slows, meaning they use less energy and can survive on the fat they have stored up instead of trying to forage for food. During hibernation, bats need roosts that are cool and remain at a constant temperature. They often move into underground sites, such as caves.
- Pipistrelles are our most common bats, but we don’t know where they all go in winter! We have not found enough hibernation roosts to account for the numbers we see in the summer months.
- Bats mate during the autumn and sometimes into the winter when they hibernate. The females then store the sperm and do not become pregnant until the spring.
- In North America, white-nose syndrome has been associated with the deaths of 5.5 million bats since 2006. The fungus grows on hibernating bats, irritating and possibly dehydrating them so they wake up. Being aroused from hibernation costs the bats a lot of energy, which makes them lose body fat and can lead to starvation.
Most bats in the UK evolved to roost in trees. Around three quarters of British bat species are known to roost in trees. The remaining species tend to favour human-made structures because of a lack of suitable and available tree habitat.
Trees provide shelter and attract a diverse range of insect species for bats to feed on. Since bats are not able to bore holes or make nests, they use whatever gaps are available – including cavities and crevices made by other animals, the natural decay of the wood or arboricultural methods.
Bats use different parts of the tree for different reasons, depending on the time of year and temperature. For example, in the summer bats might use the higher canopy sites to have their young in warmer temperatures. In winter, they might move deeper and lower into the tree to hibernate. Male bats and non-breeding females tend to prefer cooler conditions throughout the year. Breeding females prefer warmer roosts in spring and summer, when raising their baby (pup) places high demands on their energy levels. Breeding females cluster together to retain body heat, but they also get through this challenging time by gaining 'free' heat from tree roosts in two ways:
- Selecting naturally warm sites, such as sheltered trees receiving some sunshine during the day;
- Selecting highly insulated sites, such as a tree hole with a small space and thick wood.
Trees such as oak, beech and ash are particularly suitable for bats, but any woodland or tree has potential for a bat roost – especially if it has cavities in the trunk or branches, woodpecker holes, loose bark, cracks, splits and thick ivy.
For more information on how bats use trees, advice on what to do if you have concerns about tree work or are planning to fell or prune a tree, download our useful Bats & Trees leaflet.
A collaborative effort between BCT and the Mammal Next Door has taken the Bats & Trees leaflet diagram to produce an electronic poster to help illustrate how important trees are for bat species. This poster can be downloaded here.
Over the centuries, human development has encroached on woodland and other wild spaces, so bats have adapted to roost in buildings. All our British bat species will make use of buildings on occasion, but for some species, buildings are essential as roost sites. Bats can be found in a variety of buildings and structures such as houses, bridges, barns and churches.
Bats often roost in houses, both new and old. You may realise that you have bats roosting in your house during the summer months, when they are more active. If you think you have bats in your house, call the Bat Helpline on 0345 1300 228 and ask for a copy of our Living with Bats booklet, which gives advice and information for roost owners, or the Bats and Buildings leaflet. You can also download the booklet at the bottom of the page.
- Find out more about how and when bats use buildings.
- Learn more about bats and churches.
- Read our tips and advice on how to accommodate bats in buildings.
- Visit the Roost website for case studies on bat roost replacement and enhancement.
Bats hibernate where they are less likely to be disturbed by light, noise and predators. Underground sites like caves, mines, cellars and service tunnels are ideal. Such sites are often referred to as hibernacula and they provide the optimum humidity and stable low temperature that bats require during their winter hibernation. Some bats also use underground roosts during the night in summer for feeding or for mating. Of all UK species, greater and lesser horseshoe bats rely most heavily on caves for roost sites.