Is it safe to cover the bottom of a bird bath with Flex Seal spray rubber seal?

They might not like the smell of it because it is probably petroleum based, and it might be too slippery.  

I would suggest a water-based spray paint with a concrete texture to it if you need to seal it. Then wash it thoroughly. That gives a grip to the surface and does not smell.

Supposedly but there have been reports that the birds will no longer use it.

I would not.  Birds are very very sensitive and this may be toxic to them.

The rubber coatings are safe, but you will have to get the bird bath real clean. They are swimming pool paints.

I would think after it cures it would be okay. E-mail the company and ask them.

i would ask someone at a pet store about it