Bats spend the majority of winter hibernating with a constant body temperature of between 0oC and 5oC. In order to achieve this state of inactivity, the bats must lower their body temperature (from around 40oC to 0oC), slow their breathing rate, and lower their metabolic rate (with their heart beating only 6 times per minute). Typically hibernation roost will form in deep caves where the temperatures are low and fairly constant, however, bats are using increasingly more urban places to roost as we encroach on their natural habitats. As a result of this, any sections of a building that have the required criteria below must be considered to have the potential to support hibernating bats:
- Little or no light disturbance
- Optimum humidity
- Little chance of predator access
Bats are still hibernating but they are reaching the end of their fat reserves. This means that on warmer nights you may see them leaving their roosts in search of food and water but, they will still return to their hibernation roosts.
Bats will start to emerge, though levels of activity will be very low. As the temperatures increase, small numbers will begin to forage. However, if temperatures drop again they may go into a torpid state; a sort of semi-hibernation.
This is the month when all the bats come out of hibernation. Many will have gone almost six months without feeding and so are very hungry; this results in them being very active and out feeding on every suitable night. Despite being desperately hungry bats will not emerge from the roost in high winds or heavy rainfall, on such nights they may become torpid again. They may no longer stick to one roost, simply feeding all night and returning to the nearest suitable place at dawn.
This is the beginning of the bat survey season as recommended by the Bat Conservation Trust. During this month male and female bats separate, the females forming large pre-maternity roosts and the males either roosting alone or in small bachelor roosts. They are fully active and feeding. As soon as the female bats are in back in good condition they fertilize their single egg. The maternity roosts must be warm and dry, the warmth is very important as young bats are not able to thermoregulate. Both male and female roosts are usually high up, along with the ridge beam, under the eaves or squeezed between the tiles and roof lining. Some bats will also roost between stones or bricks where the mortar has warned away. Small bats can enter a roost through gaps as small as 1cm wide.
Female bats usually give birth to a single pup, which they feed on their milk; it is very rare that bats have twins. Young bats are very small (less than an inch) with thin, slightly grey fur, with size comparison it is the equivalent of a human mother giving birth to a 5-year-old. Adult bats will catch thousands of insects each in a night.
Mothers continue to suckle the pups but they quickly become independent. By 6 weeks they will have learnt to fly but after three weeks you may see young bats on the ground as they attempt to fly. As they reach the stage where the mother leaves them the volume of the bat roost increases especially as the mothers leave to feed.
Young bats begin to catch insects for themselves and no longer need their mothers’ milk, as a result, the maternity colonies begin to disperse and bats may move on to mating roosts. From now on the bats, the main aim is to put on enough body fat in order to survive the winter hibernation.
This is when the mating season begins. Males of most species use special calls to attract females, which can include purrs, clicks, and buzzing, they call from inside crevices, tree hollows or under coping stones. Even though the mating occurs now the egg will not be fertilised until May; this is called delayed fertilisation and female bats are one of the very few mammals that are able to store live sperm in their own bodies. A single male may mate with 30 or more females. Female bats give birth at two years old whereas male bats begin mating at one. Bats are also still concentrating on building up fat stores for the coming month.
Mating continues, and building up their fat reserves is becoming crucial in order to survive the winter season ahead. The search for suitable hibernation roosts begins though it is believed that most bats return to the same roost year after year. On colder days bats may start going into periods of torpor in order to conserve the reserves they have collected.
As the temperature drops periods of torpor are lasting longer and longer, some bats will begin hibernation in order to save their energy over the colder months, when insects are harder to find. It’s a hard choice to find the balance between finding more food but using energy to do so or, hibernating and hoping your energy reserves are large enough to last through to the spring.
By now all bats are hibernating. They may roost on their own or in small groups, often in cool, quiet places such as disused buildings, old trees or caves. It is surprising how little we actually know about their hibernation habits. The common pipistrelle is one of the most commonly seen species in the UK, it is recorded in large numbers over the summer but the number of known hibernation roosts does not account for the population. It is important that bats are not disturbed during hibernation; if they wake they will use up valuable energy reserves and then may not survive the winter months; even one person’s radiant body heat can raise the temperature of a roost enough to rouse bats.
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.