It would have to be a freeze harder than we even get, here, to be of concern.So, no problem keeping livestock water flowing in the event of a hard freeze, then?
In this event, I would thaw one waterer in the house and put the other one out. Then, switch them out. Fortunately, there are only 20 birds(soon to be 16).
I have considered this possibility, but it is sort of far down on my list of issues. I live in what is considered a 'temperate rain forest'. If the climate changes to the point it starts freezing hard here on a regular (or irregular basis) we are going to have some other muuuuch bigger problems than how to keep chook water liquid.