We live in South Central Pennsylvania and the winters can get cold (although not as cold as they did when I was a kid).
Last winter I had 15 chickens in our coop and I was scared they would freeze through the winter since the house is made from scrap and not insulated.
I told my husband of my fear and he said have you ever stuck your hand under a chicken's wing...it gets pretty warm.
All of the girls survived the winter without a heater.
We had a couple of nights were it was down as low as 5 degrees.
The one thing I did do was to pull the water each night and place it in the heated storeroom.
If I forgot to do this I was out with boiling water and a hammer trying to get the ice out.
My neighbor's one chicken had 5 chicks during this past winter.
Everyone survived.They settled in under thier mom's wings and kept warm.
We have a solar light set up that we use when we are closing the girls up for the night but otherwise we do not use any type of electric lights to keep them laying.
We just have less eggs in the winter and add what we need to the food budget.
I did loose a hen this summer because of the heat.
There were no signs of anything getting in the pen and she was pretty high in the pecking order plus I do not have any roosters.
My neighbor said sometimes the heat just gets them.