The nipples will freeze up, but when the chickens tickle them the warm water thaws them almost immediately. We are at -30 in the winter here. We actually have a 130 gal spray tank in a insulated box I built. The spray tank has a stock tank heater and a submersible pump. The hose from the pump runs up to the ceiling and through the wall to the bucket in the coop. The hose self drains. We got broken 2" insulation sheets from the lumber yard for 5 bucks each to built the box the spray tank is in. The pump is on a 'light' switch. Flip it on and fill the bucket. Every month or so we fill the tank.
We have 26 birds, 2 bucket with 5 nipples each. In the winter we go to 1 bucket.