Echolocation
Echolocation calls range from 15 to 45 kHz, peaking at 25 kHz. Two very loud calls are produced in a bubbly ‘chip-chop’ sound when the bat detector is tuned to about 25 kHz. The repetition rate of calls is much slower than smaller species.
Noctules’ calls are similar, but the lower part of their call usually has a peak frequency of 21 kHz or less. Leisler’s bat can be confused with serotines when flying in cluttered habitats with the peak frequency increasing up to about 30 kHz.
Habits
Leisler’s bats forage over parkland, cattle pasture, woodland and meadow and also forage around white street lights. Above water bodies they may dive quite close to the water’s surface, and sometimes forage close to the ground along lanes and well lit roads, feeding on flies, moths, caddis flies and beetles.
Radio-tracked bats in Kent foraged preferentially along woodland margins, even when these were along major roads.
Although naturally a forest species, these bats are much less dependent on trees for roosting than noctules, using a wide range of buildings, both old and new.
Leisler’s bats hibernate in tree holes and cavities in buildings.
Reproduction and life cycle
In summer, maternity colonies of females, usually around 20 to 50 bats, gather in tree holes and buildings. In houses they have been found in a variety of situations, including around gable ends in lofts, between tiles and underfelt and behind hanging tiles, often moving roosts every few days.
Mating occurs from late summer to mid-autumn, when males emerge from tree holes at dusk and fly around slowly calling loudly for females. Unlike their echolocation, these calls are audible to people.
Distribution, status and conservation
Although they are found throughout the British Isles, Leisler’s bats are rare in most areas, but recent BCT and NBMP data indicates small but slowly increasing populations in parts of southern England. Only in Ireland is this species common, where the population is of international importance, holding the bulk of NW European populations.
In London and parts of the southeast of England it has been recorded more often in the last few years than previously, as noctules have declined. Noctules are very dependent on trees for roosting, and as more and more trees are felled, it is suggested Leisler’s bats may now be at an advantage as they also roost in buildings.
