I'm wondering how big your coop is. How many birds? Your window is closed, so, what openings do you have in that coop for ventilation? That's the biggest need. Otherwise all of the moisture from their respiration and poop will make a humid environment that puts them at huge risk of frost bite. Also, what are you using for a perch? Northern chickens do best on a wide perch that allows them to keep their feet flat so their belly feathers cover their toes. We've had 2 severe winters with day time temps that didn't even get up to 0*F for days at a time. During that time, I let the chicken's behavior tell me if they needed any extra heat. When they become more lethargic... standing around with feathers puffed, not scratching around and acting "chickeny", when their feed consumption goes down. I know it's time to give a bit of heat. Yes, "they say" that chickens do not need supplemental heat, and they've been surviving for years without it, and Grandma never had heat for her chickens. BUT, Grandma's flock was most likely bigger, and kept in a barn full of hay, with plenty of 4 footed animals, and manure... all of which are wonderful heat generators. Today's flock may be kept in a tiny little coop, consist of just a few birds, and have very little litter to snuggle down in. I'm not advocating for providing heat. I'm just saying that you need to be able to listen to what your birds are telling you.
