If my memory serves me your chickens don't live much past 4 years old so I'm going to assume these are young birds (?)With the last cold snap, a lot of people posted "WHAT IS TOO COLD. A couple of points, cause we all tend to think of our own set up when we consider this.
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- SD is pretty arid
- I have a shed type coop
- I have created out door in the run wind breaks that act as sun porches with some old shower doors and plexiglass, it is often quite a bit warmer in there.
When I was in my twenties and traveling I would quite happily brave -15C and below if the air was dry and I was out of the wind even though the temperatures I normally lived in were in general above freezing. I was cold and my brain and body did all those things that it does when one is cold but I would go out in such temperatures.
Just because a creature goes out in such temperatures doesn't mean they are comfortable. I very much doubt that a middle aged chicken would leave the coop unless absolutely necessary in those temperatures. An elderly chicken would probably die.