Try using an expanding cement-based material such as "Pour-Rock".  This is an ac
rylic-modified portland cement.  The crack in the bird bath should be enlarged t
o accept the filler.  If the birdbath is not cracked, only porous, an epoxy
paint might do the job safely.  Be sure to mix the components in absolutely corr
ect proportion and let the paint cure completely (at least one week @ 70 degrees
 F).  It would be a good idea to put it  in the sun for a week or two without
water to allow any residual volatiles to escape.