17th October 2016

Parti-coloured bats (Vesperilio murinus) do it differently from other bats, at least as far as we know. We are talking here about song flighting! Parti-coloured bats performing the song flight are very rare in the Netherlands (and unknown in the UK), or at least seem to be very rare there. So far, there has only been one record of a song flighting male, but there could be many more (and they may be here in the UK as we have odd records of vagrant individuals). The autumn is the best time to go and look for them, and everybody can help with this. In this article we explain what to look for.

Song flights of bats

Several species of bat, such as common pipistrelle, Nathusius' pipistrelle, and the noctule, perform their song flights in August and September, and if the weather is good even into October. The males occupy territories which are defended and made obvious to females, by loud calling. In some ways this is similar to the territory of breeding birds. Pipistrelles defend their song flight territories against other males. Somewhere in the area their actual mating roost is situated and mating takes place. Nathusius' pipistrelles and noctules defend a specific roost, the mating roost, in a tree or a building. Sometimes several individuals may be found in different splits in one tree. In this case there seem to be two conflicting interests. On the one hand the males are concentrating themselves in certain traditional places, perhaps so that they are more noticeable or more well-known to females. On the other hand, they defend their area against competition with other males. However the system works, for those of us interested in surveying for bats it is of course useful that they make such a loud noise in the breeding season.

Song flights of parti-coloured bats

The parti-coloured bat is also a song flighting bat, which has its own version of the song flight. Sometimes the sound can be heard in August or September. The most intensive period however is much later in the year: as far as we know, from the end of October until into December. In towns such as Uppsala and Kiev, males can be found song flighting when the snow is 2 m thick and the temperature is -10°C. In Copenhagen you might be walking through the centre of the town, wrapped up warm, in a big crowd of people shopping for Christmas presents, while above you the air is full of song flighting parti-coloured bats. The peak frequency of the song flight call is approximately 14 kHz. This is only slightly higher than the song of a firecrest. Without a bat detector, the sound is a high "zing" which is repeated approximately 4 or five times per second in a continuous rhythm "....zing....zing....zing....zing...." If there are many males song flighting, the air is filled with a zooming sound, similar to the sound made by cicadas in southern Europe, but at a higher frequency. If you listen with your bat detector set to 14 kHz, it sounds like "pwooit...pwooit...pwooit". One individual call is actually made of several syllables. The "pwooit" is preceded by a series of very short pulses which sound like "trrrrrr" This trill can be heard in the background of the "pwooit" sounds.

Sonogram in Bat Sound of parti-coloured bat song

The males often fly in front of a flat object which reflects sound. In built-up areas, for example, they song flight in front of south-facing sides of tall buildings, such as blocks of flats, cathedrals, castles and churches. In nature they use south-facing cliffs. Even though they do song flight at temperatures below zero, they probably search for places where the midday sun has produced slightly higher temperatures. But sometimes you also find them song flighting above a small village in the mountains. The puzzle is far from complete.

Song-flighting parti-coloured bats are mostly found in northern and eastern Europe, but they also occur in central and southern Germany. In the Netherlands one song flighting animal was heard in 1992 near a lighthouse on the coast on the 'Maasvlakte'.

Since then no more records have been made. At the same time the number of records of parti-coloured bats found in or near buildings in the Netherlands is increasing. This, in combination with the finding of nursery colonies near Utrecht and Groningen, suggests that an increase may be occurring in the Netherlands. It is noticeable that the animals found near buildings are often found in late autumn, and near tall buildings. The question is: do they not song flight in the Netherlands, or do we stop listening and stop using our bat detectors from October?

Searching for song-flighting particoloured bats

Let's use the sound made by the males during song flighting in a positive way, and search for song flighting particoloured bats. Dust off your bat detector, and go out from November to the first half of December early in the evening, to listen on the southern side of tall buildings. Do it especially after relatively warm and dry afternoons. Stay at each building for about a quarter of an hour. The more buildings can be surveyed, the higher the chance that we will find a song flighting male particoloured bat. At buildings where you can listen easily, because they are close to you, or you go past them on your way to the pub, or whatever, it is certainly useful to listen repeatedly. If you think you have heard something, try to make a recording. Send your recording to us. We will then listen, and possibly send an experienced person and/or somebody with a time expansion detector along to get more certainty.

With or without detector

The calls are also audible without a detector. Because our ears have a large range, and compared to a detector are quite sensitive, sometimes it is easier to hear the sound without a detector. The problem is that you must be aware of what you hear. 14kHz is much higher than most of the sounds we hear, and many people do hear it, but don't perceive it. Being certain that it is a bat, and possibly a song flighting particoloured bat, is easier with the detector, particularly if you have never heard the sound before. Swap between listening with a detector and listening without detector. Once you have heard the sound once and you know it, you will use your ear to search more than you will use your detector.

We assume that the phenomenon of song flighting parti-coloured bats is definitely taking place in the Netherlands, but it may be very rare (and even rarer here in the UK where we only have vagrant records of the species). It is therefore important that many people try to hear these calls. Send this request to as many bat workers as possible in your environment, or tell them to read this article. Because you don't necessarily need a detector to join this hunt, people who don't own a detector can also be very useful. Birders and amphibian people can also use their ears to search.

Thanks to Ingemar Ahlén, we can share some examples of recordings made with a heterodyne bat detector tuned to 14kHz, some wav-files of time-expanded recordings, and some specrograms. Get in touch with Lisa Worledge if you would like these emailed to you.

A cake for the first positive recording!

Herman Limpens and Eric Jansen, Dutch Mammal Society.

(Translated by Nancy Jennings)