A heated watering bowl can prevent freezing.
For my chicken tractor I have a five gallon bucket full stacked on another five gallon bucket for height and then a PVC pipe with chicken nipples and that is connected by a piece of hose to the bottom of the top five gallon bucket. It doesn't really freeze where I am but I don't live in Boston or anything.
When it's time to move the chicken tractor, I just disconnect the hose from the bucket, move the tractor, move the buckets, reconnect. There is small problem of leaves falling in that I haven't had much trouble with but you could put a grate or something on top for that to prevent leaves but let rain still refill it.
Hope this helps!