Just a thought, and Ive never tried this so I don't know how you will go but I think mixing the glitter with grout sealer and applying after you grout might give it a nice effect as the grout sealer is clear and drys a bit shiny like. You would have to use a fair bit of glitter though (if you wanted to cover the grout completely) And mix it up good.
As I said I haven't ever tried this, only imagined........ :)

Gretch, I've put glitter right into my grout when mixing it. For a birdbath, it will take a lot of glitter though. Others will have other techniques, I'm sure.

VV has an interesting idea there. Never thought of doing that.

When I experimented with glitter I tried it in the grout mix and was not successful. Mushing it in after it dries some, not successful either, made a big mess. You need really fine glitter first of all, but the amount you need is ridiculous to get a real glitter look, imho. So then I used glitter glue on the grout, after it dried. That was much more successful. I also tried the nail polish with glitter in it. It was also successful but it will color the grout a pinkish hue. I think there are several colors of nail polish glitter. Spectralock has a glitter product to use with their grout but I didn't like that either.

Experiment on a few small things and see what acheives the look you are going for before you go after that bird bath. Good luck and let us know if you experiment and what your results are.

I did a small mosaic plate, I grouted it first,wiped the grout, then while it was still wet I sprinkled some glitter on it and just rubbed it around. It worked out okay.

Yep, I used glitter glue too, and believe me, I waited too long to wipe it off the tesserae, and it was tough to get off, but it worked.