I thought that if they were under a certain weight (not sure what, though) you must not allow them to hibernate, as they will run out of fuel whilst asleep, and wake up dead, IYSWIM. We overwintered a pair for a local rescue place many years ago, and we didn't twig that one of them was eating all the food, and the other went without, and died. I hated the other one after that, and was glad when spring came.