Bats are the only true flying mammals. 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' (5). 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 (5). Brown long-eared bats produce echolocation calls at frequencies between 25 and 50 kHz, which are very quiet, and have earned the species the alternative name of 'the whispering bat' (4). They emerge only after nightfall (2), and their broad wings allow them to fly slowly, but with high manoeuvrability (4). They hunt for flying insects such as moths, beetles and flies whilst on the wing, but also take spiders, earwigs and other invertebrates from leaves or tree bark (known as 'gleaning'), and may even land on the ground to deal with awkward prey (4). Their sense of hearing is so acute that they can home in on prey by listening for the sounds made by the insect as it moves around; they can also hunt by sight (4). Large prey items may be taken to a perch, which can be identified by the heap of insect remains on the floor below (4). Mating takes place in the autumn (2), but fertilisation is delayed until the following spring (6), as females store sperm inside their uterus (womb) during the winter hibernation (6). In April and May maternity roosts typically of 10 to 50 females form, and unlike many species of bats, males also occur in these roosts (2). A single young (rarely two) is produced around the middle of June (2). When the mother goes out to hunt, the young are left in a 'crèche'; they can fly after three weeks, and are independent at six weeks of age (4). The brown long-eared bat can live to a maximum of 22 years, but the more likely average life span is 4.5 years (2).