If you can't get a long extension cord out to the coop for a heated dog bowl or chicken waterer, then a black rubber feed bowl might be the best bet. You can put warm water in it and some heated rocks to keep it from freezing for a longer period of time. If you can place it in a spot with some sun that's even better. It would freeze over night if it is cold, but it is easy to break the ice out of a rubber bowl. Maybe other folks will have other ideas.