Field experiment reveals that female Bechstein's bats (Myotis bechsteinii) select bat boxes based on the space available for roosting.

Roosts are a crucial resource for bats, which choose them based on many factors, including the surrounding habitat, microclimate, and space available for communal roosting. The latter is important because many bat species benefit from social thermoregulation by forming colonies in their roosts. However, it remains unclear whether bats can base their roost choice directly on the space available for roosting when other characteristics, such as roost microclimate do not vary. We present results from a field experiment in which RFID-tagged Bechstein's bats (Myotis bechsteinii) in a maternity colony were given paired bat boxes with identical external dimensions but differing internal roosting space. This allowed us to control for other factors that might influence roost choice and to record the bats' nightly visits to the boxes prior to their occupation as day roosts. To assess whether roost temperature influenced roost choice in our setup, we measured the internal temperature of the boxes. Female Bechstein's bats showed a clear preference for boxes with more roosting space, as evidenced by a higher frequency of nightly visits and more frequent use as day roosts. As there was no significant difference in temperature between boxes with different internal volumes, roost temperature cannot explain the bats' preference for spacious roosts in our experiment. Our data provide evidence that bats can directly assess the volume of their roosts. Our results are consistent with the argument that small volume roosts may limit roosting group size potentially reducing social thermoregulation benefits.