any pet store should carry aquarium heaters. Buy the lowest wattage you can find (probably 50 watts) and it should have no problems keeping a bird feeder warm. Just set the temperature to about 75 and leave it. That way it will not be too hot for the birds but hot enough to keep it from freezing. Also make sure that the bird bath is deep enough to cover the entire heater. If the heating coils rise above teh water it will burn out and possibly shatter.