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.
Roosts in trees
A Bechsietin's bat roost in a tree (JJ Kaczanow)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.
Roosts in buildings
Brown long-eared bats on timber (JJ Kaczanow)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.
Underground roosts
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.