Llanelli Naturalists Newsletter - Winter 1991-1992

Pipistrelle bat Pipistrellus pipistrellus

Pipistrelles are among the smallest British bats with a head and body length of about 40mm (about 1 4") anda wingspan o1 about 200mm (8"). They weigh only about 5g (1/5 oz) - the same as a 5p piece! They are variable in colour, but are usually medium to dark brown on the back and only a little paler underneath.

The pipistrelle can often be seen feeding over water or along hedgerows and edges of woodland. It is also common around gardens and is the most common species in towns and suburbs. It often flies at about or slightly above head height. Its flight appears fast and jerky. as it dodges about in pursuit of small insects, which are caught and eaten in flight. Their main food is midges and caddis flies; they will also take other small flies, such as mosquitoes and gnats, and mayflies, lacewings and occasionally small moths. A single pipistrelie may consume as many as 3000 insects in a single night. Like all our bats, the echolocation sounds used to detect their prey are normally inaudible to humans. but some people can detect parts of these high-pitched squeaks.

Breeding

Mating normally takes place in the autumn at well established 'mating' roosts though it can occur in the spring. The maternity colonies consist almost exclusively of female bats. The maternity roosts are usually occupied between May and August. Bats may remain at a single site for most or all of this time, but more often, particularly in newer housing, they will move irregularly between several sites within a small area. As a consequence numbers at any one site can fluctuate markedly through the summer; they may even change roosts with their babies. The average size of a maternity colony of pipistrelles is about 80, but colonies can number several hundred and have been known to exceed 1000. All females within a roost give birth to their single young (occasionally twins) from early June to mid July, though births as late as August have been recorded. The babies are fed solely on their mothers' milk. Within three weeks the young are able to make their first flights and within six weeks they can forage for themselves. Most colonies disperse as soon as the young are weaned. Females can reach maturity in their first autumn thought most males will not reach sexual maturity until the next summer.

Pipistrelles have been known to live for at least 11 years.

Roosts

Summer

Buildings are the most favoured sites for roosts and more than half of the known roosts are in buildings less than 25 years old. Pipistrelles prefer to roost in very confined spaces around the outside of the building, typical sites being behind hanging tiles or weatherboarding, behind soffit

'and barge or eaves boarding, between roofing felt and roofing tiles, or in cavity walls. Pipistrelles rarely enter roof spaces except in the more stable, well-established, large colonies found particularly in older buildings. There may be a slight smudge around a well-used access hole, otherwise the only evidence of bats being present is droppings and urine marks on windows, window-sills, walls and an accumulation on the ground beneath the favoured entrance/exit hole. The access hole can be a slit as little as 1" wide.

Pipistrelles can be active within the roost during the day especially if young are present and the roost gets very hot. They can also be rather noisy as the time for emergence approaches. They usually start to emerge from the roost about 20 minutes after sunset, though this time can depend on the weather. Bats may be active at their feeding sites for up to four hours, again depending on the weather. When females have young to feed they may make several feeding trips, particularly around dusk and dawn, during which time the young are left behind inside the roost in a group or crèche. Large numbers of pipistrelles circling around the roost entrance at dawn makes a spectacular sight. Pipistrelles are active from March to October or November and they may even be seen feeding during warm spells in the winter months.

Pipistrelles are also found roosting in holes or crevices in trees and in bat boxes.

Winter

Despite being by far the most frequently recorded species in summer, very few pipistrelles are found in winter. Most of the winter roosts are of isolated individuals or small groups in crevices in buildings and trees so that their presence goes undetected. Pipistrelles are only very rarely found roosting in caves or tunnels in winter. They often roost in fairly exposed situations so that they can take advantage of warm periods to feed. Extreme cold weather forces them to change roost and at such times they often appear in houses and individuals in poor condition are often found in unlikely places, such as hanging in the open on a bare exposed wall.

Bats and the Law

Bats and their roosts are protected under the Wildlife and Countryside Act 1981. Should any work be planned that may disturb bats or their roost, the Nature Conservancy Council must be consulted. Local bat groups will also be happy to give help and advice.

British Bats No1