I goes to -35 here and I don't provide my chickens with heat, just shelter thay can go into to warm up. They have the whole horse barn to be in for the winter so lots of room to run about a peck at stuff. For the nights they can go into small areas like the standing stalls and I provide them with cardbord boxes and other disposable type shelters I exchange when they get poopy. I have stuck with the hardy winter breeds due to the temperatures here but really if you give them room just enough to cuddle in for the night 2 or 3 in groups they will keep themselves warm and be healthy for it. Beware the heat lamps we have had a few members here have winter fires and loose everything. Also beware the molds and mildews that grow and make your chickens very sick if you close your coop up too much, chickens have lots of water vapor they breath out and the coop needs lots of ventilation to not be moist especially in the cold.
I have never had a chicken freeze a foot that had a barn or coop to go into. I did have 2 get frost bite on a wattle. Mostly when there is snow on the ground the chickens don't stay outside very long. The one chicken that froze his feet got tangled in some kite string that blew into the run so he could not choose to stay inside. Sadly I did not notice before I went to work that day.
