I agree with chickens being warm if they are dry. Get that moisture out of the coop. They each wear a wonderfully warm down jacket that does a great job in keeping the birds warm in winter. At -22 F my birds are out in their protected from the wind run all day. They hate being in the coop when there is daylight. I have a 6' x 8' coop with 10 square feet of vents that are never closed so that the moisture leaves the coop. The vents are placed so the birds do not have a breeze blowing on them which might ruffle their feathers and make them cold.
Think about it this way. You've seen those little sparrows and wrens outside in the winter. They are flitting all over the place. Do they look as if they are miserable and freezing to death? Nope, they have their own down jackets on and for the most part are perfectly suited to the temperatures they live in. My grandmother gave me all her books and articles on raising canaries. The books said to remove the ice from the water dishes first thing in the morning so the canaries can get a drink. The warning was to keep a draft off the birds so they won't get cold, not provide heat. So even canaries are hardier in the cold than most people imagine.
Chickens can handle cold weather. They have more difficulty dealing with the heat of summer. Yet no one has suggested an air conditioned coop for those hot muggy days of 90 degrees and 90% humidity.
