In summer reproducing female bats prefer roosts with temperatures in their thermo neutral zone, but in northern Europe, such roosts may be difficult to find. In this study, summer roost site selection of pregnant and lactating soprano pipistrelles were investigated using radio telemetry (2004–2006) in a fiord landscape with complex topography at the species northern extreme. The soprano pipistrelles' common roost sites were in hollow trees (n = 14), but they also used crevices in rock walls (n = 2) and the attic of one building. The bats exclusively sought out roosts on the north side of the fiord with the slope facing south, where solar irradiation yielded the warmest micro climate. From 2005 to 2014, bat boxes were erected at three sites in the study area. From 2012 to 2014 we only found maternity roosts in such structures, thus there was a clear shift in roost site selection by the soprano pipistrelle in the study area. Man-made structures, such as buildings and bat boxes, provided the warmest cavities, whereas natural cavities did not differ significantly from air temperatures. The bats did not find thermo neutral roosts most of the time, and this should affect the energy budgets, and thus the bats' behaviour. Direct observation of predation at roosts in build-up areas suggests that living close to humans could be costly to bats emerging in daylight conditions. No predation attempts were recorded at roosts in woodland habitats.
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