Studying animals under natural or semi-natural conditions is essential to better understand the implications of a warming climate on species survival. Here, we provide evidence of the effects of increasing winter ambient temperature (Ta) on the thermal physiology, feeding behaviour and body mass (BM) of a small mammalian hibernator, the hazel dormouse Muscardinus avellanarius. We collected skin temperature, metabolic rate (MR) and BM data from captive, hibernating dormice exposed to two winters of differing intensity in outdoor aviaries. Additionally, we obtained skin temperature data from one free-ranging individual during the hibernation period. Statistical modelling showed that Ta and time since the start of hibernation both had significant non-linear effects on the probability of arousal from torpor. During the warmer winter (Ta<0°C on 26 days), foraging resulted in higher BMs for some individuals, despite more frequent arousals, than during the colder winter (Ta<0°C on 86 days). As expected, Ta had a non-linear effect on MR during torpor, with the lowest energy expenditure at intermediate Ta, and normothermic MR was minimal when Ta was warmer. The negative effect of increased arousal frequency on BM was counteracted by the positive effect of foraging on BM. Conforming with the accumulating body of evidence on high flexibility in physiological responses in many small mammals, we found that the thermoregulatory and behavioural responses of dormice during hibernation were more flexible than previously observed for this temperate, fat-storing species. The ability to respond to prevailing environmental conditions might help M. avellanarius cope with increasing winter Ta predicted for ongoing global warming.