Hi Steve,
         I prefer the rubberised heaters that come in a flat 
horseshoe shape to the metal coil type.  I note, however, that the 
Bird Watcher's General Store (BWGS) 
http://www.birdwatchersgeneralstore.com/shop.htm doesn't carry that 
kind.  So it will be interesting to see what others here have to 
say.  BWGS (on Cape Cod) does carry several others kinds, two for $50 
and one small one (44watts) for $20.  I hope this helps!
                         :-)  June

At 04:47 PM 1/21/2008, you wrote:
>I'm new to birding and, in addition to feeding them in winter, I'm 
>considering
>placing a heater in the (cast concrete) bird bath.  I've noticed 
>there are many
>different sizes, both in shape and wattage.  I'd like to do this 
>right the first
>time.  Any advise would be appreciated.
>
>Thanks, Steve in Swanton