Warwickshire Bat Group

Natterer's

Natterer's bat (Myotis nattereri) is a medium-sized species. The ears are narrow, fairly long and slightly curved backwards at the tip; pink at the base, a little darker at the tip. The inner lobe of the ear (the tragus) is long, narrow and sharply pointed. A characteristic feature of this species is a fringe of very stiff bristles along the trailing edge of its broad tail membrane. Its rather pinkish limbs give rise to its old name of 'red-armed' bat.

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.

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. It prefers foraging in semi-natural broad-leaved woodland and tree-lined river corridors.

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.

  Natterer's bat call on a Time Expansion bat detector.

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.

Description
Head and Body Length	40 - 50 mm
Forearm Length	36 - 43 mm
Wingspan	245 - 300 mm
Weight	7 - 12 g
Colour	Fur light buff brown on back, white underneath. Bare pink face.
Life Cycle
Mating Period	Autumn and winter.
Maternity Colonies	Late spring. Young: 1 (occasionally twins) born end of June to early July, weaned at 6 weeks.
Colony Size	20 - 1,000 plus
Longevity	Up to 20 years
UK Status	Vulnerable.
Habitat and Food
Summer Roosts	Old stone buildings and large-timbered barns, tree holes.
Winter Roosts	Caves, mines, most underground sites.
Feeding Habitat	Open woodland, parkland, hedgerows, along waterside vegetation.
Food	Flies, moths, spiders, other small insects.

further reading

"Natterer's bats prefer foraging in broad-leaved woodlands and river corridors" Journal of Zoology Volume 275 Issue 3, Pages 314 - 322 (May 2008).