Natterer's bats are found throughout most of the British Isles. Recent records have extended its range in Scotland, north to the Great Glen fault. Generally it is a scarce and poorly known species. It is widespread in Europe, north to southern Scandinavia.

The UK population of Natterer's bats is probably of international importance. For the conservation of summer roosts the conversion of barns and the maintenance and remedial treatment of other older buildings needs to be carefully monitored. With a significant reliance on underground sites for hibernation, important wintering sites should be protected. Its feeding habitat requirements are poorly known.

Flight & ultrasound

Natterer's bats have a slow to medium flight speed, sometimes over water, but more often amongst trees where their broad wings and tail membrane give them great manoeuvrability at slow speed. They normally fly at heights of less than 5 m but occasionally my reach 15 m among the tree canopy.

Much of the prey is taken from foliage and includes many flightless or day-flying insects. Sometimes larger prey is taken to a feeding perch.

The echolocation calls of the Natterer's bat are very quiet. Their frequency range is 35 - 80 kHz with a peak about 50 kHz. On a heterodyne bat detector the calls are heard as irregular rapid clicks, with a sound similar to that of stubble burning.

Breeding

Mating occurs in the autumn but has been observed in all winter months. Maternity colonies of adult females are formed from May to June through to July and sometimes until September - October. They may changed roost site frequently. The female gives birth to its single young at the end of June or early July. For the first 3 weeks the young bat feeds only on its mother's milk and is left in a crèche inside the roost when its mother goes out at night to feed. During this time the young may make its first flight inside the roost and within 6 weeks is fully weaned and able to forage for itself.

Summer roosts

Relatively few summer roost sites are known. However, most known summer colonies are in sold stone buildings with large wooden beams such as castles, manor houses and churches, or older large-timbered barns. Crevices in beams or gaps in beam joints are common roost sites. One colon y is known to roost in a stone garden wall and two in the entrances to mines. They also roost under bridges. Although colonies are rare in houses, they occur occasionally in the roof space or directly under ridge tiles, where they are often hidden amongst timber or tiles.

Access to roost sites is often by direct uninterrupted flight through a permanently open aperture or at the eaves; but is sometimes via tortuous routes through hollow walls or behind tiles. There are usually a few droppings below the access point. The emergence of Natterer's bats from their roost reaches a peak about one hour after sunset. There is then a lull in activity before the bats begin to return. They have been observed returning an hour or two before sunrise but when young are present they may do so soon after emergence.

Natterer's bats are traditionally tree-roosting bats and it is likely that many still roost in trees. They will occasionally use bat boxes.

Winter roosts

Natterer's bats start to arrive at their hibernation sites in December with peak numbers in January or early February. Most leave by early March. They show a preference for the cool entrance areas of caves and mines, but will hibernate in any underground shelter.

Natterer's bats are one of the species most frequently found in any small cave-like site or even exposed rock crevices. They are usually solitary but small groups are not uncommon and may include other species. An exceptional cluster of about 150 bats is regularly found at one site. In their efforts to lodge in small crevices they can be found in almost any position, including lying on their back or sides, or even resting on their heads. Individual Natterer's bats are occasionally found hibernating in churches, in crevices between beams.

In Great Britain, all bats are fully protected under Schedule 5 of the Wildlife and Countryside Act (1981) as amended, and by the Conservation (Natural Habitats etc) Regulations (1994). An agreement on the Conservation of Bats in Europe (EUROBATS) under the auspices of the Bonn Convention, also known as the Convention on Migratory Species (CMS) is in force, and all European bats are listed under Appendix II of the CMS .