If you have a wind proof coop that has enough ventilation that their breath doesn't cause condensation, and enough chickens to produce body heat, they should be fine. Chickens can stand pretty cold temps if you have the right conditions (a heated coop is usually worse for them, not better). According to one website you have to watch that their crest feathers don't freeze. I assume this is because they aren't near the body where they have a heat source. I think they'll do fine if you have a good winter setup. Do some research on winter coops. Joel Salatin talks about it in his backyard chicken book and he lives in the North East so he gets pretty harsh winters.