Since the chooks sleep all night so aren't drinking anyway, I bring the water bowl into the house overnight and let the water warm up to room temperature (around 60). I purchased a thick rubber water bowl from TSC and that seems to retain heat quite well. As an example, yesterday we had a high of 22 but there was a little sun. I positioned the water bowl so it got as much sun as possible and the water was still unfrozen at 5:30pm, after being outside all day. On days where there is no sun, I sometimes put the water bowl in the coop, where the body heat of the birds seems to keep it from freezing. However it helps that I have a smallish coop for 7 birds (32 cubic feet of air). If the coop was bigger or I had fewer birds that might not work so well. Also, on really cold days I'll fill the water bowl with hot water from the tap so it is considerably warmer than room temp at the time I put it outside. I've had very few days where the water completely froze but was lucky that I was home on those days (weekends etc) and able to replace the ice with warm water halfway through the day.