Chasing Bats in Utrecht

Imagine... a beautiful summer night in Utrecht's most beautiful park. The sunset paints the sky in wonderful tones of yellow and orange. Then, a group of people walk by, staring at the sky and pointing skywards with a mysterious, beeping device. To make the situation even more suspicious, they all wear a bright yellow vest... what are they looking for? Aliens? No. Spies? No. Pokemon? Almost right. They are searching for bats!

To be more specific, this group of biologists is searching for the serotine bat (Eptesicus serotinus), and I am beeping around with them. The Serotine Bat is one of the largest bat species in the Netherlands, with a wing span between 32 and 38 centimetres (figure 1). Like all bats, it hunts for insects during the night, using echolocation. Human beings cannot normally hear this, but we cheat, using our bat detector which modifies the sound, so that we can hear the echolocation. It resembles some sort of beeping noise (figure 2, a volunteer with a bat detector).

Bats mostly leave their roosts around sunset (in summer, the females and young stay in nursery roosts). It often takes only 10 minutes for almost the whole colony to leave the roost.(1) What we are trying to do, is track down nursery roosts in and around the city. This is done by the following approach: you wait in a place where bats have been spotted before. The bat detector is used to locate the bats. Then, if you hear or see a Serotine Bat flying around sunset, you know it has probably just left the roost. Therefore, you can run to the direction opposite the one the bat is flying in. Here, you wait until you see another bat flying and repeat this action. With this approach, we hope to locate the bats and eventually find the nursery roost, via tracking down the flightpaths the animals use.

Figure 1: A serotine bat. Source.

This project (part of BAT030) was set up by the municipality of Utrecht. Its goal is to find the nursery roosts in order to offer the bats protection. In The Netherlands, the Serotine Bat is a Red List species (source) and although accurate figures are still lacking, it is believed that its numbers have dropped over the last years. However, nursery roosts are difficult to find, as they are hidden in buildings, hollow walls, and sometimes in trees(2). Additionally, the animals leave their roosts very quickly and secretively. Between 10 and 50 bats usually inhabit a nursery roosts, sometimes together with other bat species, such as the common pipistrelle (Pipistrellus pipistrellus). Like all bats, Serotine Bats have separate winter and summer roosts, but the distance between summer and winter roosts is usually no farther than 50 km. However, during hibernation the bats are in torpor, so the chance of seeing them is even smaller (3).

Figure 2: A volunteer (Pascal Franssen) with his bat detector, on his bike, ready to chase after bats in Utrecht. Photo by courtesy of the author.

Originally, this bat probably did not breed in the Netherlands, as it roosted in crevices and natural caves (which are absent in the Netherlands). However, when humans started to build houses, castles and churches, they found 'caves' and 'crevices' in the form of slits in old walls, buildings, rooftops of churches, cavities between walls of houses, and spaces under roof tiles. Roosts are often hidden away, as these bats are vulnerable for predation when they are sleeping upside down during the day(4)(5).

We start our search for the Serotine Bat's roosts at parks, old forts and buildings along the edges of the city. We know most Serotine Bats fly out of the city at night, to forage in rural areas(4). The bat often commutes along hedges, parks, lines of trees or other green corridors(6). As the bats cannot make their own roosts and have particular demands (such as warmth, stable climate and good access) they do not move to new roosts unless they really have to. They are therefore philopatric, meaning that they are prone to use the same roosts for decades(7). Therefore, we can use old maps and observations of bats and try to find the Serotine Bat hotspots of a few decades ago, since we expect that they are still there. And they are! If you are around Kasteel Oud Zuilen, Fort de Klop, Vechtzoompark, Fort Blauwkapel or the Wilhelminapark (all in Utrecht), have a look for the largest bat you can see and you may greet the humble Serotine Bat!