Individuals of some species of bats roost in human-made structures despite the apparent availability of natural roosts. We compared patterns of thermoregulation in relation to microclimate and compared reproductive timing for maternity colonies of big brown bats (Eptesicus fuscus) roosting in natural and building roosts in the prairies of southeastern Alberta. During pregnancy, bats roosting in buildings used torpor less frequently than did rock-roosting bats, but achieved lower body temperatures when torpid. Less-frequent use of torpor leaves more active days for fetal development, and bats in building roosts gave birth earlier than those in rock roosts. We observed predators and predation in rock roosts, but not in building roosts, and suggest that bats roosting in rocks use shallower torpor to remain vigilant. Patterns of torpor use suggest that bats in buildings save more energy than rock-roosting individuals by roosting in the warmer microenvironments of buildings and by achieving lower body temperatures when ambient conditions are cold and foraging is not productive. The warmer building roosts are also conducive to juvenile growth, and young building-roosting bats fledged 1–2 weeks before rock bats. We propose that advantages for bats roosting in buildings (lower predation risk, earlier births, faster juvenile growth rates, and increased energy savings) lead to greater long-term reproductive success for building-roosting bats and make buildings preferred roosts.