Over the years, people have tried a number of ways to keep the water in birdbaths from freezing.  One the such technique is to use glycerin as an antifreeze; this practice should be avoided.  If a bird drinks water treated with glycerin, the chemical can raise the amount of blood sugar in its body to dangerous levels.  In addition, glycerin can mat the birds’ feathers together.  This can be harmful as the matting can adversely reduce a bird’s feathers ability to insulate its body against the cold.