MaryO,
If the birdbath is one of the ones that is similar to the ones that are very popular right now the pots are terracotta (low fire clay usually) and not really meant to remain out in much heat or cold weather. That said once it is glued together adn you cover it with a concrete mix you are basically making a new birdbath, however you are enclosing a potential problem dependent on what your original bath is made of and may create a situation where the inside cracks and moves and pops the outer layer. You may be better off to create a mound of sand and press your existing bowl on top of the sand, then remove it, score in the lines that are left in the sand and apply your concrete mix ontop of the sand (if you want a smooth surface lay a piece of drycleaner bag over the sand first).Then place your (reinforced) mix to a thickness that resembles your broken bath, besure to look at how the bottom of the bath fits onto the stand and recreate that as well. Make sure to mist and keep this damp and in a couple days you should be able to turn it over continue to mist and keep damp for several days before applying any paint or sealant to it. There is another discussion on this forum about sealants and safety.
Happy casting,
Dena

You convinced me. I'll use the bowl for a pattern on the sand and then scrap it. Thanks.