Linda M, is the top of birdbath large enough to hold a small/medium pot? If so you could put plants in a pot above what sounds like a very nice base with a squirrel.
If you need a purpose for the stand it depends on if it is flat or has a raised center. If it is flat you could use it as a table/stand on the patio by adding a wood or glass top. Keep in proportion to avoid tipping. If it is raised a potted plant perhaps or a plant saucer filled with some succulents or water if you still wanted to use it for the birds.
Indeed they do, Kathy R., but (speaking from personal experience) the way they fit together is so different from one to another, that making a mismatch match is much more of a challenge.
We have had glass tables flip in the wind and shatter into a million pieces. I would look for something unbreakable if you want a table. Glass and the outdoors don't seem to mix unless you don't get wind gusts.
1) You could paint the concrete base with latex stain and then attach (with HD construction adhesive) a painted round wooden top to it. 2) You could attach an interesting concrete, ceramic or resin statue to it to make it a garden sculpture 3) You could make a glass tower sculpture on top if it out of old plates, bowls and old glassware.
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Just a couple of thoughts...