<--- rant... OK, please stop thinking about the chickens as if they are human babies. They are outdoors animals who are acclimated over time to their location. They wear a down coat year round. Really, cold isn't what you need to worry about so much as high heat. Heat can kill your birds. Cold rarely does. 

Since you've started them this winter with heat, it would probably not be a good idea to remove it at this point as they've become used to it and it will be quite a shock. However, for future winters, a flock of chickens on roost do quite well to temps of -20 to -30f with NO ADDED HEAT WHATSOEVER. Caveat they can't have drafts or breezes blowing right over them while roosting but DO require excellent ventilation.
The temp outside has no bearing on whether a chicken wants to go out. Generally it has to do with wind or moisture or a combination of both. As I said, they can handle the cold just fine unless there's a wind lifting their feathers or moisture soaking them.
All that being said, they are your birds, to do with as you see fit.

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