These are the biggest bats in our area, weighing 15-49g and with a 330-450mm wingspan. They are sleek animals with dark skin, gingery fur and long narrow wings built for speed. They fly high over trees and rivers well before dusk and can often be mistaken for swifts, although their low slow calls of ‘chip-chop, chip-chip-chop’ at around 25kHZ on a heterodyne detector will quickly distinguish them from birds! Some people can hear the lower harmonics of their calls without detectors.
They eat beetles, moths and a range of other insects, stooping mid-flight to catch them. They roost mainly in hollow trees and nursery roosts are usually around 20 individuals. They also hibernate in trees, but man-made structures are occasionally used for summer or winter roost sites. They can migrate short distances and occasionally use bat boxes.
Leisler’s bat (Nyctalus leisleri)
This bat is a very rare bat in Britain, but some recordings of ‘noctule’ bats in Northumberland have subsequently transpired to be Leisler’s bats! They do have very similar calls, the difference being apparent only on computer analysis. The bat is very similar in many ways to the Noctule, but is slightly smaller, 10-20g, wingspan 260-340mm. It uses buildings more frequently and with less of a dependence on tree holes.
They are fast fliers and hunt along woodland margins, also using rivers and pastures to gather flies. Rather than swarming at cave entrances to meet mates as the Myotis bats do, Leisler’s males sing to their females from tree top perches in late summer and autumn.