The Soprano Pipistrelle Pipistrellus pygmaeus is one of three pipistrelle species found in the North East. It is similar in size to the Common Pipistrelle Pipistrellus pipistrellus and was only separated from it on the basis of their DNA in the mid 1990s (Barrett et al, 1997). Prior to that it had been suspected that there might be two species based on morphological characteristics and John Steele was recording pale and dark morphs at Chatton in 1989. Soprano Pipistrelle typically differs in appearance from the Common Pipistrelle in its colouration, with Soprano Pipistrelle having a pale skin colour on the face and a more ginger colouration, with less contrast between dorsal and ventral fur colour, although many bat workers comment on gradations between the species in this respect. Soprano Pipistrelles also tend to have a strong musky smell though again this is not of itself a definitive characteristic in separating individuals of the two species as this smell is occasionally apparent in Common Pipistrelles as well.
The other main difference between Soprano and Common Pipistrelles is, as their name suggests, in the frequency of their echolocation calls, with Soprano Pipistrelles having a peak frequency around 55 kHz, compared to 45 kHz for Common Pipistrelles. For this reason Soprano Pipistrelle is sometimes known as the “55 Pipistrelle”. However there is some intraspecific variation depending on the environment that bats might be flying in and the pipistrelle that appears to have a peak frequency of around 50 kHz is a frequent source of frustration for bat workers. Consequently it is not always straightforward to separate the two species in the field using the heterodyne recorders that have traditionally been used by bat workers. As a result any records based on a small number of bat passes need to be treated with caution, especially when they contradict other survey results. To take account of this degree of uncertainty, this account has been based on the comments of experienced bat workers from Northumberland, Durham and North Yorkshire Bat Groups rather than taking isolated field records at face value. With regards to their echolocation calls, it is worth noting that both John Drewett and Graeme Smart have recorded pipistrelles with peak frequencies of around 60 kHz in Teesdale and Northumberland respectively.
In Europe the Soprano Pipistrelle is found in more northerly latitudes than the Common Pipistrelle (Dietz et al, 2009). In Britain the relative frequencies of the two species vary between different parts of the country (Swift, 2001) and in parts of central Scotland it can replace the Common Pipistrelle as the commoner pipistrelle. However the suggestion that this is an effect of increasing latitude does not appear to be correct as Scott (2012) points out that it is the Common Pipistrelle that is the commonest in the Highland region. On the face of it, it might appear that a latitudinal cline is apparent in the North East with Soprano Pipistrelles being commonly reported throughout Northumberland, less frequently reported in central Durham and hardly reported in the southeast of our region. However immediately to the south the North Yorkshire Bat Group report Soprano Pipistrelles as being widespread and common in their area (Drewett, pers. comm., 2011), and therefore the effects of latitude do not seem to be a significant factor in distribution across the North East region.
Research points to Soprano Pipistrelles being significantly associated with riparian habitats (Davidson-Watts et al, 2006). This is considered to be a noticeable feature in both North Yorkshire (John Drewett, pers. comm., 2012) and Northumberland (Ruth Hadden, pers. comm., 2012). Of the known roosts in the Durham Bat Group area, only around a quarter are further than one km from a river. Nevertheless there are confirmed roosts where the nearest watercourse is relatively minor, such as the Bedburn Beck through Hamsterley Forest or the Langley Beck at Staindrop; but it may be that in such situations the main factor is the proximity of large areas of mature woodland.
Evidence that the Soprano Pipistrelle is much more restricted in its distribution was provided by the results of a series of surveys carried out by the Durham Bat Group as part of the Bat Conservation Trust’s Bats and Roadside Mammals Survey in 2006. This involved driving transects around four sets of roads in geographically distinct parts of Durham. The surveys recorded a total of 11 Soprano Pipistrelle passes compared to 208 passes of Common Pipistrelle, with the Soprano Pipistrelles only being encountered in two places; Neasham which is adjacent to the River Tees and in Weardale around Hamsterley Forest.
Nevertheless, Soprano Pipistrelle is not an uncommon bat in the region. In particular, it is regularly encountered in Northumberland and the Northumberland Bat Group had 73 known roosts on its database as of 2011. The largest count out of these roosts is one of 660+ bats in Riding Mill though that roost is known to split and move around houses in the surrounding area. Another roost of over 600 is known from Morpeth. Most of the Northumberland records are from the east of the county, though that probably reflects the distribution of bat workers, as a roost of over 100 bats is known from Kielder village in the extreme northwest of the county.
The Northumberland Nathusius’ Project conducted surveys for Nathusius’ Pipistrelle Pipistrellus nathusii using Anabat detectors across more than 20 sites in Northumberland in 2011. Both Soprano and Common Pipistrelles were found at all of the sites surveyed. Tony Martin has collected very large levels of Anabat monitoring data over a couple of years from a site in north Northumberland (east of Cheviot and in the upland fringe) and found that the numbers of Common and Soprano Pipistrelle passes there were about equal. He estimates that Soprano Pipistrelles would account for around 20% of bat passes in the wider rural Northumberland area. This is in line with an estimate by Ruth Hadden, the Northumberland Bat Group recorder, who considers that she encounters foraging Soprano Pipistrelles on around 25% of the field surveys that she undertakes in the county. Sam Talbot has calculated that around one third of the bat roosts that she has encountered as a Natural England bat warden in Northumberland have been of Soprano Pipistrelle. However, Soprano Pipistrelle roosts might give rise to more frequent requests for bat warden visits than other bat species due to their strong smell and tendency to form large roosts.
While the Soprano Pipistrelle features regularly on bat warden visits in Northumberland, this is not the case in Durham and Cleveland, particularly when it comes to rescuing individual bats that can then be identified in the hand. Noel Jackson, who has 30 years experience of bat work across the whole of County Durham, has only rescued two Soprano Pipistrelles in that time. Similarly in 15 years of bat work in south Durham and Cleveland the author can only recall rescuing Soprano Pipistrelles on two occasions and then from the same area of Darlington. Taking into account that we will have rescued bats numbering well into three figures between us in that time, the ratio of Soprano to Common Pipistrelles would be lower even than the results of the Bats and Roadside Mammals Survey mentioned above.
Although apparently not as widespread as it is in Northumberland, the Soprano Pipistrelle can be encountered quite regularly in Durham though its distribution seems to be localised. The first record of Soprano Pipistrelle in Durham was by Noel Jackson at Barnard Castle in April 1997. It is now known to be present throughout the middle stretches of the River Tees with several roosts between Middleton St George in the east and Middleton-in-Teesdale in the west, with the latter being the most westerly record for the species in the county. Prior to the separation of the two species several large roosts, numbering several hundred bats, were recorded along the Tees near Darlington and it is thought that these are likely to have been Soprano Pipistrelles. To date the largest known roost in the Durham area is the one at Staindrop mentioned above, from which 600 bats were counted in 2011.
North of the Tees it occurs about four km upstream on the River Skerne at South Park in Darlington but then there are no confirmed records until the River Wear. It is regularly encountered in the central Wear Valley and also the Derwent Valley with several roosts known in both areas, including one of nearly 300 bats in the flat roof of a building in Blackhall Mill (Fran Mudd, pers. comm., 2012). On the Wear it is found as far east as the Wildfowl and Wetlands Trust, east of Washington. So far it has not been found as far west on the Wear catchment as it has on the Tees, with the most westerly roost on the Wear being at Witton-le-Wear with some field records a little further west on the River Gaunless. Soprano Pipistrelles do not appear to be present in upper Weardale or upper Teesdale though survey effort in those areas has been limited. On the Magnesian Limestone Plateau Natural Character Area, which is largely east of the A1(M), there are no known roosts and no confirmed field records. This is perhaps not surprising as this area is characterised by very low woodland cover and no sizeable watercourses other than the inter-tidal section of the Wear.
In the area of the former county of Cleveland there have been a few field records claimed though most of these are of one or two brief encounters in areas that have otherwise only turned up Common Pipistrelles, so they should perhaps be viewed with caution. The species is present in south Cleveland with field records around Nunthorpe and field and roost records at Kirkleatham. It may be more widespread but to put this into context, the author has carried out numerous bat surveys in Cleveland over more than a decade and has never encountered the species there. The paucity of Cleveland records may be due to avoidance of the large urban conurbation that forms the heart of the former county. It is worth noting that there are no records for the large urban conurbations of Sunderland and Tyneside; likewise Northumberland Bat Group has no records for central Newcastle.
On the other hand the species’ affinity with water is perhaps underlined by what is surely the most unusual North East bat record, which was of foraging Soprano Pipistrelles (as well as an unidentified Myotis species) recorded over Coquet Island by Claire Snowball in 2011. Coquet Island is a mere 400 metres x 200 metres in area. It has no fresh water but it is surrounded by the North Sea, which separates it from the coast of Northumberland about one km away.
Written by Ian Bond (last updated Nov 12)