For those using nipples, we had great luck with heat tape last year. We have heat tape wrapped on both sides of the PVC, and the remaining length of the tape is wrapped around our water reservoir, which is a 30gal plastic container.
Highly recommend this method. The chickens didn't bother the tape or the electrical tape holding it on at all.
The pipe insulation was too interesting to pass up, though. They had a ball pecking that thing apart. This year, I'm leaving it off.