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 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). Brandt's bats produce echolocation calls of frequencies between 35 and 80 kilohertz (5). They emerge during early dusk, and with fast, agile flight they hunt over water or at low levels through woodland (2), feeding on moths, other small insects and spiders (5). Mating tends to occur in autumn (5), but fertilisation is delayed until the following spring (7). Females gather into maternity colonies in summer, typically numbering 20 to 60 females (7). They give birth to a single young in June or July. At three weeks of age the young bat can fly, and it is able to forage independently by about six weeks of age (5). In Britain, only small numbers of Brandt's bats have been found hibernating in caves and tunnels, so it seems likely that the majority of the population spends the winter hibernating in as yet unknown sites (5).