Beeswax is the safest (non-toxic) thing to use as a type of sealer for bird baths, although this need to be re-applied periodally, plus if you live in a freeze zone it's best to place under cover in a dry place, shed, garage etc. Wendy

I have made numerous birdbaths and have not applied a sealer or paint. On larger ones I opened up any pits or pores in the bowl and then applied two coats of one part cement to three parts sand by rubbing it into all pores and into the entire surface. This is not a perfect seal, but it works quite well, and not enough water is lost to be concerned about.
On smaller pieces I have inserted a large dinner plate into the mold to form the inside of the bowl. This is decorative and no water is lost due to porosity.

Is your bird bath made of 'tufa? If so, 'tufa is designed to seep water. Concrete bird baths are not recommended for use in areas with freezing temperatures. Store them inside or if kept outside, tipped up on edge, upside down and covered, anything to keep water from collecting.
Fredw10's idea of inserting a dinner plate is interesting. It would make it easier to keep the bath clean too. I think I need to try that one.........

Try it. You'll like it. I wish I could find a platter much larger than the eleven inch ones usually seen in for a dollar or two. Small BB's can be made with a groove in the bottom that fits over the top rail on a deck railing. My mix is always a tufa mix of some variety. What I meant was my large BB's haven't seeped enough to be of concern with a sand cement slurry rubbed into the bowl surface.

The house my family lived in when I was a little girl had a concrete bird bath in the garden. It was a squared deco column, as wide on the bottom middle and top except with a slight 1 or 2 inch lip onthe top where the water was. It was huge and heavy and definitely not tipped over in winter. Unfortunately it was too large to take along (it was in the garden when my parents bought the house) and I don't know what ever happened to it. I wonder how it survived our winters for so many years. We lived there for almost 10 years (until I was 10) and I believe it was old at the time we moved there.
kathy