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The placing of bat boxes has become a very popular method of monitoring bats and also an educational tool used by conservation groups and government departments alike to explain the value of bats to human societies. As a result, thousands of bat boxes have been sold or made by bat enthusiasts throughout Europe in past decades. However, very few papers have ever analyzed the importance of temperatures inside bat boxes in light of the expected effects of climate change. This paper is a first attempt to explore the possible effects of extreme heat on bats roosting in boxes.

Based on our experience in monitoring bat boxes since 1999, our research team decided to place 10 data loggers in new bat boxes randomly distributed amongst our established box stations to monitor the maximum recorded temperatures in boxes. We established that there was a risk of overheating when temperatures rose above 40ºC. Throughout the whole summer we recorded box temperatures every hour (17,560 readings). Overheating episodes occurred in six of ten boxes and in 2% of all samples (429 hours). Furthermore, in July 2013 we observed an overheating event in situ when 22 bats fell from a bat box while they were trying to emerge in the evening. Most European bat boxes models are small, have no ventilation and are usually located in places that are exposed to the sun for several hours a day. However, it has been assumed that bats avoid overheating in bat boxes by roost-switching. Nevertheless, in July 2013 we observed bats suffering from overheating in a wetlands where there were many alternative roosting sites (trees and other boxes).