Something that absolutely fascinates me, is the Nathusius's 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.
You can read more about my categorization of unusual social calls of the Nathusius's pipistrelle in this blog post.
Can documenting these unusual calls give us more information?
In 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.
I have started to update the map each year with new recordings of unusual social calls in spring:
List of locations of the 2024 map:
Giessenburg (2021) - wavy-line
Alphen aan den Rijn (2024) - part E + wavy-line
Fijnaart (2024) - part D
Gouda (2024) - part D + trill
Grou (2024) - part E + trill
Hoofddorp (2024) - part E
Klundert (2024) - part D + part E + trill
Nieuwegein (2024) - 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
's-Graveland (2025) - wavy-line + part D + part E + trill
Zwarte Waal (2025) - part E (+ possible part D)
Vlaardingen (2025) - W-type call + part D + part E +trill
Zuidoostbeemster (2025) - W-type call + part E
Noordbeemster (2025) - wavy-line + part D + trill
Oudeschild (2025) - W-type call
Vollenhove (2025) - part D
Bussum (2025) - Part D
Updated map (latest update 2025):
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 location of interest would be De Alde Feanen National Park: a Natura 2000 area that contains swamps, 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 in 2024.
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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