Hey there @dima guy
! I live on the front range in Colorado. Last winter we had a 2 week period with night temps of 10-15 below zero. Couple of things you need to address in freezing weather, especially with large combed birds, but more or less with all of them:
1. Moisture is a killer! Combine cold with wet and it's a recipe for sick/dead birds. You need as much ventilation as you can get, without causing...
2. Drafts/cold wind directly on your birds... You NEED the ventilation, but it's best if you have incoming air down low away from where the birds roost, and out-flowing air high up above where the birds roost. Natural convection (heat rises) will carry moisture (bird sweat if you will) up, away, and out of the coop. The droppings (poop) in the litter are not a huge concern right now as they will be freezing solid and not be an issue (moisture) until they thaw, but then moisture won't be as much of an issue (no longer freezing). chickens will survive in 40 below temps outside as long as they aren't wet and there's little wind. They have a natural down/feather blanket covering them remember?
3. Even low amounts of moisture in very cold air can cause frostbite on your bird's combs, wattles and toes.... kinda just like on humans if you substitute ears for combs and noses/chins for wattles. Rub/wipe/smear/coat/cover the combs and wattles with a petroleum based/oil based substance like Vaseline/bag balm/Vitamin A&D/even triple antibiotic. They won't like it, trust me, but they'll benefit from it. Do NOT use water based products! We're trying to prevent moisture on the birds skin surface, which will freeze.
4. Look at your roosts... chicken paws are not really designed to grip/perch like a wild bird. Though they can, it's not best. If they have a flat(er) larger surface they can lower themselves down on, and cover their toes with feathers, then you don't have to worry so much about frostbitten/frozen toes.
5. Cold weather means heavier energy consumption to keep warm... just like for us. I tend to up the fat content in their food a little during winter to give them more to burn off and keep warm. Remember... it takes protein to burn fat, so upping that a bit is also a good thing.
Hope this helps! Good luck!