The coop appears to need more ventilation. Both based on visual reference, and the comment about an ammonia smell. Chicken's respiratory systems are surprisingly fragile - nature didn't build them to be in enclosed spaces. Even if the lungs weren't an issue, cool temps + moisture is a recipe for frostbite on the combs and feet, even if their down jackets generally do a very good job of protecting them - without heat - from temperatures we humans consider cold, intollerable, or even potentially fatal (to us). BYC has people in Canada and Alaska raising birds with no heat, ever.
Yes, temperature stress can be a problem for birds who haven't acclimated to broad temp swings (40+ degrees in a day or so - the daily 20-25 degree swing is normal for them) - but GA's heat and humidity (not as bad as my own) is more concerning than the occasional sub freezing night. Typically, a stressed bird drops from the upswing - heat - not the downswing to cool.
As Kiki observed above, plenty of "silent killers" associated with feed. Sadly, the local farm stores often give terrible advice, and some of the "old wisdom" is based on old conditions which are rarely present in a modern back yard flock situation.
Your birds need well ventilated, draft free housing. The general "thumb rule" is (per bird) 1 sq foot free ventilation 24/7/365, 1 linear foot of roost (below the level of the ventilation, so any winds blow above their heads), 4 sq ft of floor space in the house, 10 sq ft of floor space in the run. Seasonally, given your climate, you will want even more ventilation, or accept that your birds will sleep outside the hen house... Floor/Run space helps with social interactions - abundance is a social lubricant, more is better.
Hope that helps.
What DO you feed them? In what quantity? Scraps? Scratch? Treats? (again, quantities matter)