I'm often asked how we keep our animals warm in the extreme cold. We only have rabbits and chickens than we keep year round. I covered a lot of this in a previous post here
When we have a good amount of snow, it's getting below freezing a lot and they are staying in the house more than out, and their water is freezing in the house, we close the house day and night, keeping the chickens in for the rest of winter. We have two windows that we can open if it gets too warm and leaved cracked for ventilation.
When laying starts to decrease and we are losing a lot of light, we add a regular 100w light during the day for 12 hours of light. When it's -10 or colder, we add a 100w red light during the night to help it stay warm enough without the extra light. Surprisingly this works well to keep the coop above freezing until around -20, after that we have to put in a heat lamp. We use the red heat lamp so they don't have 24 hours of light, and continue the regular 100w bulb for 12 hours during the day. We made that mistake once, and when the cold snap was over, we abruptly went back to the 12 hours of light and they completely quit laying for weeks.
The rabbits do well with the above plan too, unless we are having a litter. When we are getting close to having a litter, we put a reptile heater at the bottom of the nest box. It has a warm to hot temperature controller so I can't say exactly what it's set at, but it's warm to the touch without being hot at all. This has kept the bunnies nice and warm even during this -30 to -40 degree weather.