bees, in general are pretty cold hardy, and more so than we think and amazing. if you had a robust healthy hive and plenty of stores, don't worry too much about it. we lose them, there is nothing we can do in winter months, it's really all about what we do to get them ready for winter and prepare the hives for winter, wrapping, insulation, ventilation, wind break, etc.. with that said, we do all the right things and we still lose them. sometimes i will lose bees to 'cold starvation' in extended sub zero temps, where they won't move with feed so close to them. it's just part of beekeeping in a cold climate.