I used loctite for my PVC bird perches. The connections lasts through a wash in the dishwasher. It is non toxic to birds. I dont have fish in the bird baths so I searched specifically for bird toxicity. Plus seeing how my birdbaths are made of cement, Loctite seems to be the best for this. It held the longest through cement contracting and expansion in weather changes. I've never used caulk and I don't recommend it unless it meets toxicity requirements. Or if your crack is large enough that you need a great amount and a caulking gun. I spread the loctite both on the top and bottom on the crack as it was near impossible to get it in between.
