Birdbaths are usually fine to stay outside but there are a few things you need to check. Keep in mind that when water freezes, it expands. If your birdbath holds water that freezes, is there a rim or overhang that won’t let the ice expand? If so, your birdbath will be destroyed by the ice. If you know who made the bird bath, check to make sure they used clay and glaze that were fired to at least cone 6.