Bat spotted! Five years of monitoring on the campus

On a beautiful evening in May, I go out with Vita Hommersen and Eric Jansen of the Nederlandse Zoogdierenvereniging (Dutch mammal society) to spot bats. Vita and Eric have been mapping the bats on our campus for the past five years.

This monitoring was because of all the building activities on the southern part of the campus and the exemption from the Nature Protection Act that was necessary for this. Based on their information, the university could determine what so-called mitigating measures were necessary. For example, where and how many bat boxes had to be hung to compensate. Now that this monitoring period is over, Vita and Eric are happy to share their findings. Later, we walk around the Berchmanianum, each with a bat detector in our hands. Will we be spotting bats tonight?

Vita Hommersen en Eric Jansen bij vijver Berchmanianum
Vita Hommersen en Eric Jansen bij vijver Berchmanianum

Natural transition area

"The great thing about monitoring over a number of years is that you gradually get a good picture of the species of bat that fly around, the places where they stay and what the influence is of specific changes in their foraging behaviour, for example," says Eric. Part of the forest near the Berchmanianum was badly hit by a storm in 2018. After this clearing, the forest floor appeared to be breaking down very quickly, which attracted all kinds of beetles. Large insects are on the menu of the serotine bat, a large bat species, so they suddenly had food in abundance at this location. Eric: "After the disappearance of Thomas van Aquinostraat, the associated greenery and the construction of the cycle path, a hard border with the forest was created. Behind it, bats had no place to go. Now many trees have been planted in the vicinity of the Maria Montessori building and wadis have been constructed. The expectation is that a natural transition area will gradually emerge where the bats can also hunt for insects. Vita and Eric pose for the photo by the Berchmanianum's large pond. They have noticed that the construction of this pond is a fantastic asset in terms of moisture and food for the bats.

Picky residents

"Bats are flexible when it comes to finding food, but less so when it comes to their housing," explains Vita. "They are quite fussy about that. They also live in different locations throughout the year. Sometimes they go to a warmer or cooler place. And there are separate maternity chambers, where bat mothers stay with their young." Eric has another nice anecdote about the old Thomas van Aquino street. "On warm days, the caretakers would sometimes open a window. A curious bat would enter through the window. But because of the glass, it could not find its way back. (Bats find their way by echolocation.) The result was that the bat kept flying back and forth and more bats were attracted by the noise. In no time at all there was a whole group of them inside, which had to be removed. Knowledge about the behaviour of bats is therefore important. Nowadays, there are enough suitable roosts on the campus. There are bats living under the gutters of the Berchmanianum, in the cavity wall of the Thomas van Aquino Street 1 building and sometimes in the bat boxes in the woods. Most of the bats that fly around here have their homes in the neighbouring districts and in buildings of the university hospital.

Tap dancer

When evening falls, it is time to spot bats. We use a bat scanner or bat detector for this. This is a handy device that converts the ultrasonic sounds of bats into sounds audible to humans. The sounds are between 15 and 120 kHz. On the basis of frequency, rhythm and timbre, you can determine which species of bat is flying around. "If you hear a sound that sounds like a tap dancer, for example, you know that it is a serotine bat," says Vita. "But the frequencies can also vary from species to species. A bat makes elongated sounds when it flies over an open field and shorter and shorter sounds when it approaches an insect." This evening we are lucky. We have spotted at least a few serotine bats, the common pipistrelle and the Nathusius' pipistrelle, which are already three of the total of 12 species found in the area. Among them are the brown long-eared bat, which looks very funny with its huge ears, and the Leisler's bat, a rare species.

Continued monitoring

Eric and Vita look back on the past period with pleasure. Not only has their knowledge of the area increased, the cooperation with the university has also been very pleasant. Eric: "The university has also done more than was required by 'the law'. I have noticed that my recommendations were really listened to. Bat-friendly lighting has been installed and less mowing has been done, which is good for the insect population. Both hope that the monitoring of bats will be continued, so that the knowledge is not lost. There is a good chance of that, considering all the building activities mentioned in the Campus Plan.

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