How can I remove rust from a metal bird bath?

Coke will remove rust or if you plan to paint the surface Rustoleum

makes a rust reformer that prepares the surface for painting with minimum remover. It only requires a light sanding prior to application. It comes in a brush onor spray application.

vinegar and wire brush ( soak a cloth with vinegar to place on areas let it sit on rusty area 10 minutes or so then use steel wool or wire brush)or if it's really bad get a product called Naval Jelly

Tin foil and a lot of elbow grease.

Brillo pad and vinegar will work like a charm.