In America,” according to a common saying in the Netherlands, “everything is bigger.” The cars are bigger, the skyscrapers taller, even some American trees are among the largest in the world. And, as I discovered, that’s also the case with bat houses.
Bat houses in Europe grew out of the discovery that bats were using bird boxes placed in forests. Building on that idea, European bat houses almost never exceeded about 20 inches (50 centimeters) high by 12 inches (30 centimeters) wide. (This is a bit smaller than the bare minimum Bat Conservation International recommends.) Some people experimented with different models and materials, but bat houses were mostly small and located almost exclusively in forested areas.
Although some forest species, such as brown long-eared bats (Plecotus auritus), use these bat houses for nursery roosts, the European style simply does not provide enough space for large nursery colonies – not in forests or rural areas, and certainly not in urban areas.
But as more European bat workers encounter BCI’s Bat House Project, bat houses in Europe are changing. I was fortunate enough to be among the first to put BCI’s ideas to the test in Europe.
I discovered a roost of common pipistrelles (Pipistrellus pipistrellus) in a Tilburg Water Company (TWM) building just outside the city of Tilburg, the Netherlands. On that evening in May 2001, 46 bats emerged from a damaged ventilation joint in the building’s brick wall. A year later, I counted 104 bats emerging from the same joint.
TWM ecologist Jaap van Kemenade was delighted to have a colony of bats living in the building, but he was also concerned. The crack from which the bats were emerging was getting bigger every year, and reconstruction would soon be required. That, of course, could be a problem for the bats living in the cavity.
When Jaap asked me how we could make sure the bats were safe, I immediately thought about the big nursery houses I had seen in BCI’s Bat House Builder’s Handbook. I could think of no reason why a “large nursery house” would not meet the needs of the
pipistrelle bats in the TWM building.
The bat house, built by TWM, is 5 feet, 11 inches (1.8 meters) wide and 3 feet, 3 inches (1 meter) high. It has three chambers, each about three-quarters of an inch (2 centimeters) wide. The house is mounted on laths to the building’s wall, creating a fourth chamber between the bat house and the wall.
The top half of the third chamber is filled with insulation. Bats can move from one chamber to another through narrow slits. The interior of the house is lined with plastic mesh to provide footholds for the bats. The bat house exterior is painted black, and a ventilation slit was cut into the front panel.
The bat house – extremely unusual by European standards – was mounted in March 2004 just around the corner and about three feet (about one meter) from the crevice roost in the wall. We hoped the bats would discover and use the bat house voluntarily rather than force us to exclude them from the original roost.
Only two pipistrelles used the new bat house that first summer; the rest stuck with the old roost. But in July 2005, we counted 84 pipistrelles emerging from the bat house. And many of them were returning within an hour or so, possibly to feed their youngsters.
We examined the bat house in August and found at least 60 bats inside, with all the chambers occupied by at least some bats. Reconstruction work on the walls began a few days later. Although it was very noisy and taking place just outside the bat house, the bats did not leave. The workers managed to keep the old roost entrance open, so the bats could return to their old roost if they wished.
This past summer left no doubt about the value of the big bat house. We counted 111 bats emerging from it on June 8, while just three were still using the wall crevice. A close look on June 16 revealed many pipistrelle mums that were feeding their pups inside. We did not count the youngsters, but on July 8, we watched 235 common pipistrelles emerge from the bat house!
Our oversized bat house and its popularity with bats caught the attention of many other bat workers. At least five are now building multi-chambered bat houses. Mine remains the only one in the Netherlands housing a nursery roost, but I’m sure that many more will follow.