Nathusius’ Pipistrelle migrates across Europe from east to west in autumn and back again in spring. Records of animals from gas and oil platforms in the North Sea suggest it occurs in the UK as a migrant and this was confirmed when an animal tagged in south-west England was recovered in Holland. In Northern Ireland the species occurs in large breeding colonies and there is evidence it breeds in England but the overwhelming majority of animals caught in a recent trapping based survey here have been males. It may be that males arrive earlier than females to establish ‘singing posts’ from which they emit social calls to advertise for passing females. The social calls given by males from roosts are longer and more complex than those given in flight (when there is not time to emit a long call between wing beats and it is particularly energy expensive) but it is not known how the call types function. The roosts of calling males can be found in close proximity to each other and these aggregations are sometimes referred to as leks but the full details of reproductive behaviour in the species in the UK are still unclear. Research in continental Europe has shown that two or three strategies can be employed by males searching for females.
The distribution of Nathusius’ Pipistrelle in the UK is frequently tied to very large waterbodies and the species has been recorded at Draycote Water in Warwickshire (the largest water body in the county) on several occasions. In 2013 it was recorded on the north shore and bat boxes were erected in the hope Nathusius’ Pipistrelle may occupy them. In 2014 the box checks failed to find any as did searches along the north shore and in Thurlaston village. The species was recorded in the south-west corner of the reservoir though and Dr. Jon Russ (who discovered Nathusius’ Pipistrelle in Northern Ireland and undertook post-doctoral research on it there) heard social calls.
Jon determined to organise a trapping effort for 2015 and field work began on 12/09 after the first scheduled trapping night was cancelled due to poor weather. Four mist nets and a Harp Trap with an acoustic lure were set up between Windsurfers and the end of Hensborough Bank (the locations chosen being areas where Nathusius’ was recorded in 2014).
Harp Trap with acoustic lure
The disappointment of the early setback was more than made up for with two male Nathusius’ Pipistrelle caught along with Common and Soprano Pipistrelle, two Daubenton’s and a Natterer’s Bat.
I happened to assist with the removal of what turned out to be one of the Nathusius’ from the net, its large size raising some early suspicion/hopes that we may have caught our target species. Identification of Nathusius’ Pipistrelle is extremely difficult. The species is large and heavy for a pipistrelle and has some different wing measurements to Common and Soprano Pipistrelles, but use of these in identification is neither diagnostic nor straightforward and is based on the calculation of a ratio. Close inspection of the teeth is also required and perhaps the only feature which is comparatively easy to asses is only of use for males, which have extensive white hairs around the penis. It is hoped that research work will slowly start to answer some of the questions about Nathusius’ Pipistrelle in England and to this end one of our males was christened Adam, fitted with a radio transmitter and ringed.
Nathusius’ Pipistrelle Draycote Water
Nathusius’ Pipistrelle Draycote Water
Later that night Jon picked up the signal from the tagged bat on the north shore some 2km from the capture site but initial searches for the daytime roost failed to locate it. On the evening of the next netting session on 15/09 I drove round to the north shore once the nets were up and found the bat pretty much where Jon had last heard it. I could see Adam foraging over the grass at the edge of Biggin Bay and some two hours later his signal was very strong a little to the east, exactly where Louise Sherwell and I recorded the species in 2013. This area lies to the south of Thurlaston village and Tricia Scott picked up a signal from there the following morning, narrowing the search for the roost locality down to a very small area. After some hiccups the roost was eventually located in the corner of a house roof in the village by Katrena Stanhope. On the evening of 22/09 Jon and I confirmed that at least two (and possibly as many as four) Nathusisus’ Pipistrelles had emerged (including Adam) and social calls were heard which Jon recorded. Unfortunately the house owner was away so further investigation was not possible. At the beginning of October I left the UK for a month and so missed the last of the trapping nights. I recently called Jon for an update but the only really significant development was that he had discovered another male social calling from a roost in the rangers offices at Draycote Water after I left. Further fieldwork is planned for 2016 including efforts to locate roosts in surrounding villages and surveys throughout the summer to try and establish the pattern of seasonal use of the site.