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Common Pipistrelle

Common Pipistrelle by Terry Coult

Common Pipistrelle is by far the commonest bat species throughout most of northeast England, although literature from before 2000 is confused by Soprano Pipistrelle Pipistrellus pygmaeus. Species separation was first suggested on the basis of their echolocation by Jones and Parijs (1993) and was confirmed by the analysis of mitochondrial DNA by Barratt et al. (1997). However, it took some while for North East bat workers to become confident in the separation of this pair of species.

Common Pipistrelle pelage is grey-brown often, but by no means always, with a darker “bandit mask” through the eyes. This contrasts with the more uniform rufous brown fur of Soprano Pipistrelle. Observations of rescued bats held in captivity show that there are small differences between the summer and winter coats with the summer coats being greyer. This affects young bats as they moult at the end of their first winter and it is harder to separate juveniles of the two pipistrelle species on pelage as a result. Common Pipistrelles have uniformly dark skin whereas Soprano Pipistrelles often show some pink skin on their faces (Schofield, 2002).

The Common Pipistrelle is a small bat with a forearm length of between 28-35 mm (Corbet and Southern, 1977). Weights vary over the course of the year, ranging from over seven grams at the start of hibernation to 3.7 grams for males and 3.9 grams for females at the end of hibernation (Stebbings, 1968). As a general rule, healthy Common Pipistrelles in the North East in summer will have a weight of around six grams and anything less than 5 grams in a rescued bat is regarded as an indicator for special care.

Common Pipistrelles are generalised feeders on small insects which makes them very adaptable (Swift, et al, 1985); where they feed in the same area as Soprano Pipistrelles they resource partition with the Soprano Pipistrelles foraging over the taller trees and the Common Pipistrelles foraging through scrub and underbrush (Vaughan et al, 1997; Swift et al, 2001).

Common Pipistrelle © Ruth Hadden

Common Pipistrelle © Ruth Hadden

Common Pipistrelles require warm locations for their nursery colonies and so are very strongly associated with domestic houses and other heated locations (Swift, undated). It has been observed that Common Pipistrelle colonies are smaller than Soprano Pipistrelle colonies (Barlow and Jones, 1999) but this may be a result of differences in roost type and availability in the areas where these two species are found rather than any specific ecological difference per se. Certainly numbers of individuals present in a roost should not be used as a criterion for species identification.

Common Pipistrelle is widespread across the whole of Britain and has always been considered to be the commonest bat in our region. Gill (in Page, 1905) says “This species is common throughout the region” and George Bolam (1926) suggests that it “ought probably to be regarded as the commonest and most universally distributed bat in our counties”. However, it is clear that both authorities had rather limited amounts of data and their accounts are based more on specimens obtained than observations of nursery roosts and hibernacula. When Durham Bat Group was founded in 1983, they found more about roosts of Whiskered/Brandt’s Bats than pipistrelle and for a short while were convinced that species distribution and abundance was different in the north of the UK from that further south. However, as they started to collect information more systematically, it was clear that Common Pipistrelles were by far the most abundant species in the region.

Common Pipistrelle can occur anywhere that has suitable foraging within flying distance of a suitable roost. Because of the small space requirement for a nursery roost, pipistrelles can exploit roost locations on the exterior of buildings, such as boxed soffits and behind weather-boarding, whereas other species generally require larger spaces in the roof void. When Common Pipistrelles are found in a roof void it is usually juvenile bats exploring from the main nursery roost, which is usually more likely to be located in cavity walls. Both pipistrelle species can exploit post-war buildings which cannot be used by other species of bats, but Common Pipistrelles are the only species which can be found in the urban areas of the region, presumably because they are able to forage effectively in urban habitats, whereas Soprano Pipistrelles have more specific habitat requirements (Vaughn et al, 1997; Swift et al, 2001). Support for this hypothesis is given by the almost total lack of records and absence of any known roosts of other species in the urbanised parts of our region.

If the typical bat habitat across all our region’s bat species is lowland riparian deciduous woodland, Common Pipistrelles thrive in some fairly atypical places. Two nursery colonies spring to mind. The first is Killhope in Weardale, where there is a thriving colony at an altitude of about 500 metres making use of bat boxes in a coniferous plantation. The second is on the now defunct Redesdale Camp at 240 metres on open moorland, but with a coniferous plantation to the south. The camp comprised many prefabricated buildings but with one stone-built boiler house for the showers, which contained a Common Pipistrelle maternity roost. Paul Lupton reports that he persuaded the army from turning off the boiler and believes that their ecologist was trying to fit a heating system in the roost. It is interesting to note that Wardaugh (1992) suggests that there is a strong association with areas below 100 metres in Cleveland and North Yorkshire, which is certainly not the case north of the Tees.

There is evidence to support the suggestion that Common Pipistrelles have a requirement for a high temperature profile in their nursery roosts (Avery, 1991). Wardaugh (1992) noted that the entrance to bat roosts in Cleveland tended not to point north but that the relationship was only just statistically significant. This is likely because the orientation of the entrance is not necessarily an indication of the location or warmth of the roost site. Ruth Hadden (pers. comm., 2011) has stated “I would say from observation that Common Pipistrelles prefer warmer buildings and are one of the bats, in Northumberland, with the closest link to occupied houses. It is rare to find them roosting in cold barns or farm buildings. They are more likely to be in villages wrapped round someone’s Aga flue.” The importance of artificial heat in the cold of the north country is undoubtedly important to Common Pipistrelles. Many roosts use the warmth of modern housing and there are several examples of nursery roosts exploiting direct sources of heat.

There was one particularly adventurous colony in Low Westwood in the Derwent Valley which accessed its cavity wall roost through the concentric air vent and exhaust of a combined boiler. The adults navigated this considerable hazard without trouble but when the juveniles started to fly, several chose the wrong opening and were cremated as a result. Some ingenious work with mesh saved the roost and prevented unwanted cooking smells in the kitchen.

Common Pipistrelles will sometimes travel considerable distances from the entrance hole to the roost site. In one case in Rowlands Gill, the owner of a modern timber-framed bungalow was disturbed by animal noises from behind a radiator. When the roost was investigated the entrance was traced to a point where a few inches of the insect exclusion grill was missing. The bats were travelling some 30 metres around two sides of the building, between the brick skin and plasterboard lining, to get to the hot spot.

One of the largest known bat roosts in the region was a colony of 633 pipistrelles located above the bathroom extension of a terraced house in Gainford. The weight of the droppings made the ceiling collapse whilst the unfortunate owner was in the bath! The roost had long since moved on before the two species had been separated taxonomically. The largest roosts in the region definitely known to be Common Pipistrelle are a roost of 375 at Monk Hesleden in Durham and of 365 at a house in Wooler in Northumberland.

There have been suggestions that there is a difference between the mean sizes of roosts definitely identified as Common Pipistrelle across the region. However, this is not the case. Taking roosts of more than 10 bats, the mean size is 80 (n=74) north of the Tyne and 82.3 (n=66) to the south. It has been noted that relatively large roosts are found on the Magnesian Limestone plateau in County Durham, many in very modest housing in villages such as Wingate. This area is highly cultivated and very well-drained so there is little open water and it is relatively devoid of trees. The available roost sites are thus highly clustered in the pit villages.

MARS Common pipistrelle

Common Pipistrelles are the only species known to have used woodcrete Schwegler bat boxes in the coniferous plantations of Hamsterley Forest whereas seven other species are known to use the wooden boxes. However, nursery use has never been proven. The numbers of bats present has never been higher than 20 and would seem to indicate non-breeding bats and post-breeding dispersal. This further indicates that the bats breed elsewhere and that bat boxes cannot be regarded as mitigation for the loss of a building roost for Common Pipistrelles.

Durham Cathedral has a very important and venerable Common Pipistrelle colony located above the mediaeval timber ceilings of the cloisters, which is only now becoming understood. Bat workers have only had access to the roost after dark since 2010 and it has now become clear that it is a major breeding site which requires daily attendance by bat workers to rescue young bats. Sue Charlton runs the scheme and says that there are a few downed bats in July but there are several casualties a day from the second week in August through into September. The cloisters are also a hibernacula and a post-breeding gathering site for what is thought to be an extended colony of Common Pipistrelles with roosts dispersed over the old buildings of the Durham peninsula.

There are many places where single male Common Pipistrelles have been found in autumn along with a small number of females, but the woodland around the mine workings in Slit Wood, Westgate (Weardale) is one of the few locations where males have been specifically observed singing and holding territory.

Little is known about the hibernacula of pipistrelle species because they do not gather in caves like other bat species. The Common Pipistrelle roost in the drive-through carriage archway at Raby Castle was one of the few pipistrelle hibernacula known prior to 1983 (Robert Stabbings, pers. comm.,1983). Bats are inactive during hibernation, so clues around the roost entrance, such as droppings or bat activity at dusk or dawn are largely absent.

There is currently a healthy debate about how Common Pipistrelles spend the winter. Ian Bond has noticed that the majority of Common Pipistrelles found in hibernation are one or two individuals. John Drewett has observed that Common Pipistrelles will frequently hibernate in relatively exposed locations where the temperature will fluctuate a lot. Veronica Howard has found two Common Pipistrelles hibernating in a hanging basket. Ian Bond has also observed a Common Pipistrelle at the Hart to Haswell railway hanging out of a tree hole on a sunny winter day as if sun bathing, and conjectured that it might be using the sun’s rays to raise its body temperature, thus saving energy.

We do know of a number of cases where a cavity has been opened by accidental damage or to effect building works revealing hibernating bats. When the pipes burst at Prudhoe Castle one Christmas, the plumbers were disconcerted to find a pile of drenched Common Pipistrelles. David Boyson found hibernating Common Pipistrelles whilst repairing a section of dry stone wall at Greenleighton above Fontburn Reservoir in the mid 2000s but cautions that this is the only roost he has ever found in the many miles of wall he has repaired.

Wardaugh (1994) describes two hibernacula. One is “a hibernaculum which contained at least 20 animals (possibly far more) …. in a wall cavity above a timber window frame”. The other is “a large, two-storey building, already known to be used as a nursery roost. Unfortunately the building had to be demolished, this being done in early spring, when it was hoped that no bats would be present. Nevertheless initial dismantling by workers was carried out with care and 11 pipistrelles (nine males and two females) were found behind facia boards.” The winter of 2010/11 was characterized by prolonged heavy snow-fall, and as many houses suffered damage to fascias and weatherboarding the Durham Bat Group dealt with more winter rescues than ever before. These were all Common Pipistrelles and although the rescued animals were individuals, they did include some females.

So whilst the evidence is slim, it does seem likely that some Common Pipistrelles hibernate adjacent to their breeding roosts in secure locations with relatively stable temperatures such as cavity walls, whereas others disperse to roost in small numbers in more exposed locations.

References

Written by Noel Jackson (last updated Nov 12)