Photo Copyright John Kaczanow

The Pipistrelle has probably declined as a result of modern agricultural practices. Its reliance on buildings makes it vulnerable to renovation work, exclusion and toxic remedial timber treatment chemicals.

Only recently have scientists recognised that two separate species have been confused under this one name. The two species were originally identified by differences in their echolocation calls and later confirmed by DNA studies. Researchers are now busy working to identify any differences in the ecology and behaviour of the two species. 

The names given to the two different species are Common Pipistrelle (Pipistrellus pipistrellus) and Soprano Pipistrelle (Pipistrellus pygmaeus).

In a 2001 study of the European distribution of the two types it was found that the ranges of both species largely overlap (with the exception of Sweden, where only Pipistrellus pygmaeus was found). It is thought that Pipistrellus pygmaeus may be a specialist around riparian (waterside) habitats with Pipistrellus pipistrellus being a generalist feeder. Although both species occur together there is thought to be no hybridisation (cross-breeding).

The Soprano Pipistrelle (shown in the top of the picture) has a non-contrasting brown face that merges more into the fur colour. It has an 'open' expression due to the more exposed appearance of the eyes and the fur is far more of one colour. It also, allegedly, has a distinctive 'perfume'.

The Common Pipistrelle (shown at the bottom of the picture) usually has a jet black face mask, black ears and black wing membrane. Its fur is at least two-tone (black rooted with brown outer which shows up best when the fur is parted).

Flight & ultrasound

Their flight appears fast and jerky as they dodge about pursuing small insects, which are caught and eaten in flight. A single Pipistrelle may consume up to 3,000 insects in a night. Like other small species of bat they tend to follow linear features of the landscape on their commute from their roost to feeding areas, but they do periodically pause along the way to feed. Typically, they fly at about head height (2m) but they can be found feeding higher in the tree canopy earlier in the evening and come down lower as the temperature drops. If they need to cross open spaces they will often fly faster and increase their call rate.

The ultrasound calls range from 40 to 60kHz. The peak of the Common Pipistrelle's call is about 45kHz and that of the Soprano about 55kHz. On a heterodyne bat detector a series of clicks turns into 'wetter' slaps towards the bottom of the frequency range. To identify the species of Pipistrelle use headphones and, without looking at the dial, quickly rotate the frequency dial of the bat detector up and down between 37 and 60 kHz until the pitch of the 'wet slap' is at its lowest. (You are looking for the lowest pitch of the sound not its loudness). If the lowest point is below 48 kHz it is most likely to be a "45" or Common Pipistrelle. If it is above 52 kHz then the bat is probably a "55" or Soprano Pipistrelle. If the lowest point appears to be between 49 and 51 kHz then it will not be possible to assign the bat to either species. If you find the deepest note is heard below 40 kHz then it may be a Nathusius' Pipistrelle. Pipistrelles make their most distinctive sounds when flying in open spaces, so try and stand in the most open place where bats are flying.

Their 'social' calls are emitted between 20 to 30 kHz  and are heard as 'chonks'; these calls can be heard by some adults and children.

The intensity of their call is staggering. Just 10cm in front of them levels can reach 120 decibels which is the equivalent of holding a smoke alarm to your ear!

Breeding

Mating occurs during autumn at well established mating roosts and occasionally in spring. Maternity colonies consist almost exclusively of female bats and are occupied between May and August but sometimes into September. Females give birth to their single young which weigh in at about 1 gramme (occasionally twins - especially in Scotland) from early June to mid-July, though births as late as August have been recorded.

The young are fed solely on their mother's milk and females with young to suckle may make several feeding trips during the night, leaving the young inside the roost in a group or crèche. Within three weeks the young make their first flights having more than tripled their birth weight to around 3.5 grammmes, and by six weeks they can forage for themselves. They reach their adult weight of about 4 grammes in 50-60 days. Most colonies start to disperse soon after the young are weaned.

Pipistrelles may remain at a single site during this time, but more often, particularly in newer housing, they more irregularly between several sites within a small area. As a consequence numbers at any one site can fluctuate markedly throughout the summer.

Females can reach maturity by their first autumn but most males will not reach sexual maturity until the following summer.

The soprano Pipistrelle (55kHz) tends to form larger roosts.

Summer roosts

Buildings are the most favoured roost sites and more than half of known roosts are in buildings less than 30 years old. Pipistrelles prefer to roost in very confined spaces around the outside of the building; typical sites being behind hanging tiles, weather boarding, soffit and barge or eaves boarding, between roofing felt and roof tiles or in cavity walls. They 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 is the presence of droppings beneath the favoured entrance, on windows, windowsills and walls. They are also found roosting in tree holes and crevices, behind ivy and in bat boxes. The access hole can be a slit as little as half an inch wide.

Pipistrelles can be active within the roost during the day, especially if young are present and the roost gets very hot. They may also be noisy as the time for emergence approaches. They usually start to emerge from the roost about 20 minutes after sunset. Large numbers circling around the roost entrance at dawn (known as 'swarming behaviour') can be a spectacular sight.

Winter roosts

Despite being by far the most frequently recorded species in summer, few Pipistrelles are found in winter. Most winter records are of isolated individuals or small groups in crevices in buildings and trees so their presence often goes undetected. They often roost in fairly exposed situations to take advantage of warmer weather to feed. They are only very rarely found roosting in caves or tunnels.

Extreme cold weather forces them to change roost and at such times they often appear in houses. Individuals in poor condition are found in unlikely places such as hanging in the open on a bare exposed wall.

Information kindly provided by The Bat Conservation Trust

General characteristics