Something that absolutely fascinates me, is the Nathusius' pipistrelle's (Pipistrellus nathusii) use of unusual social calls outside of the mating season (recordings before the 15th of July), with emphasis on trills and wavy-line calls. These kind of calls are almost non-spoken about in literature. Other unusual social calls to encounter during spring season are part D and part E of the type D social call.
The type D social calls of the Nathusius' pipistrelle can be categorized into different parts, of whom part A, B, C, D and E have been described in literature (Russ and Racey 2007; Hargreaves et al. 2017; Jahelková 2011; Middleton 2022). However, I renamed part E as described in Jahelková 2011 and Hargreaves et al. 2017 to part D0, since the majority of these kind of FM sweeps are in front of a part D motif. Another motif consisting of FM sweeps, but more often used as a stand-alone motif, gained the name part E. This motif has been documented before but wasn't named, since it is shown in figure 8.33.7 in the book Bat Calls of Britain and Europa (Jon Russ, 2021). The Nathusius' pipistrelle can shuffle the different parts to create a syntax with a complex message (Middleton et al., 2020). The sequence "part A, B and C" is the most commonly used. During mating season, part D is also frequently used.
Below are shown snippits of slow trills and calls that show similarity to them. Slow trills can be complemented by (the first half of) a part D1 of the type D social call. I also believe that audio files 913306 and 911808 show 'non-joined components' variations of the slow trill call.
Audio files 911811 and 913306 were recorded at the same location, but with almost a month in between.
Fast trills on the other hand, might possibly be a 'joined components' variant of the start of a part A of the type D social call. This theory first came from the audio file 914575, in whom the fast trills and part A calls emitted by the same individual bat show a lot of similarity. The lower frequencies of a part D and a part E of type D social call are also visible in the first snippit.
Date of recording: 2024-06-19 Recorded by: Levi Laluan Audio file: 914575
However, this theory didn't completely cover the fast trills that had a higher start frequency than end frequency, since the start frequency of a part A of the type D social call is typically the lowest frequency of the call. Altough there are exceptions, as shown in the snippit on the right (also a snippit from the previously mentionded audio file 914575). These extra notes at the start of a part A call is something I will call A0 from this point onward. It is likely that the fast trills are joined part A0's.
Audio files 917429 and 920400 contain very similar looking fast trills where the start frequency is the highest frequency of the call. Besides the fast trill, audio file 920400 also contains another call that is quasi constant. The same call was recorded on audio file 920326. In this audio file it becomes more clear that this call, due to it's strucuture, was most likely a fast trill too. There were 90 meters in distance as the crow flies between the locations of the recordings.
Date of recording: 2024-06-28 Recorded by: Hubert Matuszewski Audio file: 917429
Same audio, but recorded at two different locations:
First snippit: Date of recording: 2024-07-11 Recorded by: Job Hoven Audio file: 920400
Second snippit: Date of recording: 2024-07-11 Recorded by: Sam Harbers Audio file: 920326
Noteworthy might be that the 'fast trills' of the audio files 920400 and 920326 were recorded at the same location as the slow trills of audio files 911811 and 913306, but in July instead of May and June. And at the same location, this wavy-line call was recorded during mating season (August):
Date of recording: 2024-08-17 Recorded by: Gert-Jan Hendriks Audio file: 928168
Wavy-line calls are hard to distinguish. From the structure of the calls it is clear that they are not fast trills. They look similar to slow trills, but have too much of a line-charachter to be such. Most wavy-line calls that I have encountered are just above 10 kHz.
The two wavy-line calls of audio file 911931 were used as a motif in the followig sequences of calls: the first being the sequence "wavy-line + part B" and the second sequence being "wavy-line + cheep + part B". The wavy-line call of audio file 1001463 is used in the sequence "cheep + wavy-line".
Date of recording: 2021-05-10 Recorded by: Sarah Mahie Audio file: 911931
Jahelkova (2011) also described some kind of wavy-line calls, the W-type social call. To cite her description: "In May and June, a continuous social vocalization of quite unusual structure and composition was recorded apart from these standard calls. Most of these calls were designed in a long “wavy line” pattern with the peak frequency of 14–36 kHz, and were accompanied by standard or modified echolocation calls.".
I also had my own encounter with these continuous wavy line calls. They generally appear to be higher in frequency than the stand-alone trills and wavy lines calls that I previously mentioned. Perhaps some of the stand-alone wavy-line calls do belong to the W-type calls as described by Jahelkova, since calls that are higher in frequency might be missed due to the distance of the recording device. On the other side, they also could be a different kind of vocalisation.
One question remains: can documenting these unusual calls give us more information?
This year, 2024, the third official maternity roost of the Nathusius' pipistrelle was found in the Netherlands. The other two maternity roosts were documented in 1994 and 2017. It is assumed that most female Nathusius' pipistrelle leave the Netherlands and that most bats of this species that are witnessed during this season are males. After their first migration, males usually stay in mating roosts along migration routes where they mate throughout the year (Pravettoni 2015; Russ 2021). A female Nathusius' pipistrelle could be able to mate during the spring period, if mating in the autumn was unsuccesfull. It is known that the parturitions are not very synchronous in this species and that both weaned and newborn pups may occur together in one roost (Heise 1984; Schmidt 1985).
For a male to be able to mate during spring period, a female must be available in the area to participate. So, one would expect a male Nathusius' pipistrelle to choose it's location with care. This sprouted a question in me: would we be able to locate possible maternity colonies of the Natusius' pipistrelle based on the locations where unusual social calls are emitted?
Since I suspect that the documention of the specific locations of where unusual social calls of the Nathusius' pipistrelle were recorded outside of mating season, might give more insight in potential mating behaviour during spring and/or migration routes, I started to visualize the encounters of 2024 (and my only recording before that year, one of 2021) on a map. "Unusual social calls" are part D and part E of the type D social call, slow trills, fast trills and wavy-line calls.
Source image: Russ, J. (2022). Nathusius’s Pipistrelle Pipistrellus nathusii (Keyserling and Blasius, 1839). In: Hackländer, K., Zachos, F.E. (eds) Handbook of the Mammals of Europe. Handbook of the Mammals of Europe. Springer, Cham. https://doi.org/10.1007/978-3-319-65038-8_68-1
Orange dots = Locations where bats were recorded during the spring of 2024.
Blue dots = Locations where unusual social calls were recorded during spring.
Red diamond = Officially documented maternity roost and corresponding year.
Since the majority of Nathusius' pipistrelle roosts are located close to large freshwater lakes, I would not be surprised if the fourth maternity roost of Nathusius' pipistrelle in the Netherlands would be located in or near De Biesbosch National Park. The national park harbors a large network of rivers and smaller and larger creeks with islands. When taking the locations of the unusual social calls in spring into account, perhaps the south of the national park might be a promising place to look.
Another guess for a maternity roost location is De Alde Feanen National Park. A Natura 2000 area that contains morasses, lakes, forests, peat and meadows. Even though there is only one location in the area where unusual social calls were recorded (Grou), the unusual social calls were recorded during all three visits in spring.
List of locations of the 2024 map:
Giessenburg (2021) - wavy-line
Alphen aan den Rijn (2024) - part E (2 occ.) + wavy-line
Fijnaart (2024) - part D
Gouda (2024) - part D + slow trill
Grou (2024) - part E + slow trill + fast trill
Hoofddorp (2024) - part E
Klundert (2024) - part D + part E + slow trill + fast trill
Nieuwegein (2024) - fast trill
Roelofarendsveen (2024) - part E
Stampersgat (2024) - part E
Vianen (2024) - part E
Zwijndrecht (2024) - part D
Hardenberg (2024) - Wavy-line
Additional locations of the 2025 map:
Den Helder (2024) - wavy-line call
's-Graveland (2025) - wavy-line + part D + part E + fast trill
Zwarte Waal (2025) - part E (+ possible part D)
Vlaardingen (2025) - W-type call + part D + part E + fast trill
Zuidoostbeemster (2025) - W-type call + part E
Noordbeemster (2025) - wavy-line call + part D + fast trill
Oudeschild (2025) - W-type call
Vollenhove (2025) - part D
Giethoorn (2025) - part D
Sint Jansklooster (2025) - part D + part E
Updated map (latest update 2025):
All recordings are licensed under the following Creative Commons Attribution-NonCommercial-NoDerivs 4.0 license and in courtesy of Sarah Mahie.
All sonograms are screenshots of the recordings imported in the ultrasound analysis software BatExplorer 2.2 (Elekon, Switzerland).
Bilbiography:
- A. Schmidt (1985). Zum Jugendetwicklung und phänologischem Verhalten der Rauhhautfledermaus, Pipistrellus nathusii (Keyserling u. Blasius, 1839) im Süden des Bezirkes Franfurt/O. Nyctalus 2: 101–118.
- Bat Conservation Trust (2025). Nathusius' pipistrelle. https://www.bats.org.uk/about-bats/what-are-bats/uk-bats/nathusius-pipistrelle
- Daniel Hargreaves, Helena Jahelkova, Oliver Lindecke and Guido Reiter (2017). Bat Species of the Year 2015: Nathusius’ pipistrelle (Pipistrellus nathusii). Facts compiled for BatLife Europe.
- Erik Broer (2024). Zeldzaam: derde kraamkolonie ruige dwergvleermuis ooit in Nederland gevonden. Natuurmonumenten; Bureau Viridis; Zoogdiervereniging. Retrieved from: https://www.zoogdiervereniging.nl/actueel/nieuws/zeldzaam-derde-kraamkolonie-ruige-dwergvleermuis-ooit-nederland-gevonden
- G. Heise (1984). Zur Fortpflanzungensbiologie der Rauhhautfledermaus (Pipistrellus nathusii). Nyctcdus 2: 258–260.
- Helena Jahelková (2011). Unusual social calls of Nathusius' pipistrelle (Vespertilionidae, Chiroptera) recorded outside the mating season. Institute of Vertebrate Biology, Czech Academy of Sciences. Folia Zoologica, 60(1): 25-30. https://doi.org/10.25225/fozo.v60.i1.a4.2011
- Jon Russ and Paul Racey (2007). Species-specificity and individual variation in the song of male Nathusius’ pipistrelles (Pipistrellus nathusii). Behavioral Ecology and Sociobiology, 61(5): 669-677. DOI:10.1007/s00265-006-0295-9
- Jon Russ (2021). Bat Calls of Britain and Europe: a Guide to Species Identification. Pelagic Publishing.
- Neil Middleton, Andrew Froud and Keith French (2022). Social Calls of the Bats of Britain and Ireland (second edition). Pelagic Publishing.
- Riccardo Pravettoni (2015). Nathusius' Pipistrelle distribution and migration. Living Planet: Connected Planet, Rapid Response Assessment. UNEP/GRID-Arendal.
- Theo Douma, Daniël Tuitert & André De Baerdemaeker (2019). Een tweede kraamkolonie van ruige dwergvleermuizen Pipistrellus nathusii voor Nederland. VLEN-Nieuwsbrief 80(1): 8-11. https://www.zoogdiervereniging.nl/sites/default/files/2022-09/vlen_nieuwsbrief_80.pdf
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