We have had temps of -20C over night and daytime highs of -9C for over a week now (it is cold - I've had no running water in the hose since last Saturday as the pipes are frozen)
My hens haven't been treated any differently to the middle of summer. they can, and do cope well with the cold (even my naked neck)
The only thing I do, is make sure they have drinking water, so I bring in their drinkers over night, so that they don't freeze, and top them up with warm water on a mrrning.