Hibernation allows animals to survive lengthy periods of energetic deficit, but is not without costs. Hypometabolism, low body-temperature, and inactivity are associated with a variety of costs such as immuno-incompetence, dehydration, and build up of harmful metabolites. Additionally, conditions within hibernacula have a profound influence on hibernation patterns and survival. Periodic arousals and site selection are thought to mitigate these costs, and often involve timing arousals to foraging opportunities and overwintering in locations with stable temperatures and high humidity. I studied prairie-living big brown bats (Eptesicus fuscus) that overwinter in rock crevices and take flight outside of the hibernacula despite a lack of foraging opportunity. My goal was to describe their winter ecology and behaviour, and investigate reasons for winter flight. I found that E. fuscus in my study area use relatively dry hibernacula compared to known cavernous sites and show fidelity to sites between and within years. I found that temperature and wind are important predictors of winter flight, and that arousals remain under diurnal influence. My data suggest that individuals from this particular population spend the majority of their winter energy-stores during steady-state torpor and have mechanisms to decrease evaporative water loss during hibernation. I found typical levels of dehydration as winter progressed and my data indicate no use by bats of a supplemental water source. My research elucidates novel behaviours and traits of this population of E. fuscus, and reduces the paucity of knowledge about winter bat-ecology in the prairies.