I set one up with a heater used to keep a pail of water available for livestock, basically a metal heating plate that you put at the bottom of your birdbath, they come in several sizes and I picked the smallest one, about the size of a bread plate. I did this because I'd read years earlier that liquid water was a major asset and attractant for birds in the winter and while the heater worked well, what I discovered was that birds that overwinter in my area (more than 45 degrees north latitude) had no interest in the warm water whereas every squirrel within probably a mile did. At first I was glad to be a nice guy for their benefit but it turned out that they were still coming in the warmer months, which became a little trying since they all seemed to try to outdo each other in destroying things like tomato plants and rose bushes, so I stopped the practice. For the record, it gets cold enough here that I didn't try to actually heat my whole birdbath; instead I found a suitable broad and shallow plastic food storage container and put that on top of the bird bath. That meant that I could dump out what water was left at the end of the day and put out fresh water in the morning, not only saving electricity but also a very long warm up time over thawing out the icy birdbath every day- if that had been possible at all. Canadian problem only, possibly.