Echolocation
Calls are similar in sound to the common and soprano pipistrelle, but the peak frequency is slightly lower at around 40 kHz on average. Pipistrelles with peak frequencies between 36 and 41 kHz can usually be classified as Nathusius’ pipistrelle. The repetition rate is noticeably slower than in common and soprano pipistrelles and more regular; this increases Order: Chiroptera in cluttered environments where it can also tend to sound like dry clicks, more like Myotis calls.
Habits
There is still much to be learnt about Nathusius’ pipistrelle. It was regarded as a vagrant in Great Britain until the mid-1980’s, then upgraded to a winter visiting migrant as it was recorded more frequently. Since the 1990s occasional maternity colonies have been found, in addition to migratory individuals in autumn and winter. The species is now known to be widespread with some resident individuals, and an increasing number of migrating bats in autumn.
It forages along lowland woodland rides and edge meadows, and is most frequently observed over or near water, including lakes, rivers, dykes and marshy areas, feeding mainly on medium-sized flying insects such as aquatic flies, midges and mosquitoes.
Reproduction and life cycle
Summer roosts in continental Europe are almost always in tree holes, but the known British nursery roosts are in buildings. It also roosts in bat boxes. Male bats usually roost alone in summer, and set up roosts close to the maternity roost site during the main mating period from July to early September. Each night they spend long periods singing social calls from their roost to attract females for mating.
Distribution, status and conservation
Nathusius’ pipistrelle is a migratory species and mostly breeds in north east Europe, moving south and west in winter. It may fly long distances, often over 1,000 km from summer roosts to winter hibernacula. With Kent being so close to continental Europe, it is no surprise that this species appears to be found more frequently here than further north.
The first record in Kent was of an exhausted individual on the floor of the customs shed at Dover Docks in 1993, thought to have just flown across the Channel. In January 2002 several individuals were discovered hibernating in a disused building in Chatham, close to the river Medway, and since that time others have been heard increasingly on bat detectors, usually close to water. Records of grounded bats tend to be along the coast in autumn, including one on a curtain in Herne Bay, on the coast and at Sheerness Docks, suggesting they are on migration.
In June 2012 a baby bat was found on a back doorstep under a gable apex near Lydd; as the adults had moved on the pup had to be hand-reared. It was thought to be a common pipistrelle, but when it reached eight grams identification had to be rethought! In August the householder where the pup had been found reported droppings under his neighbour’s gable apex as well as his own bat boxes, and 53 bats were counted out. This was Kent’s first known Nathusius’ maternity roost. A team returned two weeks later and using a time expansion detector recorded social calls from the bat boxes, confirming that at least one box was a mating roost.
More recent surveys show rapidly increasing evidence of resident breeding populations across the UK, not just as migrants. In Kent there have been many records since 2012, including additional maternity colonies and widespread acoustic detections.
