A small, heated roost facilitates nursery establishment and increases the size of a lesser horseshoe bat (Rhinolophus hipposideros) colony in the northern Swiss Alps.
Small thermal roofs in building attics might be a promising option to maintain and support nursery colonies of bats, but little empirical evidence is available. In a controlled study conducted from 2014 to 2021 on the northern side of the Swiss Alps, we investigated the effects of a thermal roof within an attic on the establishment and size of a nursery of lesser horseshoe bats Rhinolophus hipposideros. We installed a gable roof as a thermal roof (1.4 m2 of usable roosting area with two internal heating mats) in the attic of a building that previously did not host a bat colony, monitored the temperature and later counted the number of roosting bats. The building is near a power station that hosted lesser horseshoe bats but which was shut down and becoming too cool to support a nursery colony of this species. The ridge of the thermal roof had a temperature of about 33°C, while the temperature in the building's sub-roof was lower on average and subject to greater temporal fluctuations. Two years after installation, the bats started to use the roost consistently in summer and the numbers grew from 22 in 2014 to 239 in 2021. During 136 survey days we found that 85% of the bats were roosting in the small thermal roof, and only a minority were in the much larger sub-roof suggesting that the former was preferred. Our study provides empirical evidence that a thermal roof can initiate the colonisation of a replacement roost and support colony growth.