Bats are the only true flying mammals. In Britain they are insectivorous (eat insects), and contrary to popular misconception they are not blind; many can actually see very well (6). All British bats use echolocation to orient themselves at night; they emit bursts of sound that are of such high frequencies they are beyond the human range of hearing and are therefore called 'ultrasound' (7). They then listen to and interpret the echoes bounced back from objects, including prey, around them, allowing them to build up a 'sound-picture' of their surroundings (7). Leisler's bats produce echolocation calls of frequencies between 18-45 kHz (5). They emerge just after sunset, and with fast, high flight they hunt for moths, beetles and a range of flying insects, making frequent dives (2). Mating tends to occur towards the end of August and early September (2), but fertilisation is delayed until the following spring (6). During the mating season, a male holds a 'harem' of up to 9 breeding females in a mating roost (2). Hibernation occurs between the end of September and early April (2). The following summer, maternity roosts of 20 to 50 females form, with births occurring after mid-June (2). Leisler's bats are known to live to a maximum of 9 years (2).