Polyurethane comes in a spray
Here is a site where a lady used Rusteolum triple thick glaze in a spray can to seal her mini bird bath. Just scroll down to get to the bird bath.
Oil based poly does. Put on at least 3 coats.
Rather than take any chance that a sealer will be non-toxic to birds, I seal with whatever is appropriate for the material the bird bath is made of and then put a shallow glass/pyrex dish in the bowl of the bird bath. It's easy to take out & clean & no worry about toxicity. Be careful what kind of dish you use. Never use a clay flower pot saucer, & I'd be concerned about anything glazed, maybe even some plastics because of where they may gave been made. Right now, the only thing I could find to fit just right is a deviled egg serving dish. Besides the water, a little bird seed can be put in some of the egg sections around the outside.
Concrete birdbaths do not require a sealer.