The Tinbergen building is one enormous bat enclosure

The signs are hard to spot during the day – an animal that fits into a matchbox could never leave large droppings. But the windows of the Tinbergen building are covered in tell-tale chocolate sprinkles: the droppings of the pipistrelle bat.

Although seventeen of the Tinbergen building’s nineteen storeys are off-limits to students, they are open to bats. All bats are protected in the Netherlands, including the not-particularly-rare pipistrelle bat. Tinbergen is their favourite building on Woudestein campus.

“Their droppings are the size of a chocolate sprinkle”, says ecologist Garry Bakker from the Rotterdam Urban Ecology Unit, with his nose pressed against the window of the abandoned Faculty Club on the seventeenth storey. “It’s a small, brittle dropping that sometimes glints with chitin from the wings of mosquitoes – a favourite morsel in the diet of the common pipistrelle bat.” Once you’ve spotted the first miniature dropping, the windows of the Tinbergen building suddenly prove to be full of them. The animals themselves are a little harder to find, though they can be seen flying over the lake by the pavilion at night. “They are easier to spot using a thermal imaging viewer. Pipistrelle bats swarm around all floors of the building. The whole building is one enormous bat enclosure.”

The bats are not new to the Tinbergen building,  and have probably been there since its completion in 1969. There are round openings in the concrete – purposefully included to allow the concrete to breathe – that offer the perfect hiding places for these tiny mammals. Not for nesting, mind you, but the climate is ideal when it isn’t mating season. “The concrete stores heat on cold days, and doesn’t get too hot on warmer days.”

There used to be moments when a bat would venture indoors and flutter around the heads of professors in the club – who would then promptly call security to come to the rescue.