Thank you to all who responded on and off list with suggestions on how to 
seal my concrete birdbath.  Since a few others have expressed interest, I 
thought I'd let everyone know that I'm convinced by Bill Ellis's discussion 
(in a post on MD-Osprey), and have decided to try using a sodium silicate 
solution.  

That seems like the safest bet for the birds, since I'll know exactly what 
I'm dealing with chemically, it (hopefully) shouldn't be that hard to 
apply, and it's not too expensive for the small amount I need.  If all goes 
well, it'll arrive in time to do the job this weekend.

Jason Waanders
NW DC
jwaandersATstarpower.net