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.