Here how I am dealing with it so far (not in a full winter yet). The nipple waterer is screwed into 2.5gal plastic container that sits in the coop window facing south-east. I am also planning to paint it black or put a black garbage bag over it to accumulate more sun heat. If by 10am when I open the coop the water in it is still frozen, I will bring 2L pop bottle with the nipple waterer from the house. Same in the evening if the main 2.5gal waterer is still frozen.