I've had a bird fountain based on a heated bird bath for a couple years now, and last year I put a camera on it to watch the birds. I shut down the fountain for the winter, but since the bird bath is heated I kept it full of water, and was delighted to spot Bluebirds at it several times over the winter. A couple weeks ago I placed a small glass jar in it with mealworms in it, on the theory that the heated water ought to be enough to keep the worms alive, and it was, and the bluebirds have found the mealworms. They aren't nest building in my boxes yet, but they often don't here for a few weeks yet, but they're definitely hanging around. I can't wait!