when choosing a good birdbath check the top for stability .
check that their is a way of securing the top to the base in case you have children or dogs as if it falls off it can injure someone.
check that the base can be secured ot the ground for stability and safety .
these days garden ornaments can disappear in the night and go walkabout so look for a method of chaining them down and bury the chain.
ask if the birdbath is reinforced or has stones in the mix.
find if the color on the concrete is toxic and if it sealed and what with.