Simply fill the moat with water and hang your feeder directly below, preventing ants (who can't swim) from getting to the feeder. Some people jerry-rig systems similar to this using wire, epoxy and old margarine containers; these moats are simpler and more aesthetically pleasing.
The decorative moat, shaped like an inverted umbrella, is made from black powder-coated steel (3-1/4" in diameter, holds 4 oz/118ml). The simpler version is made in the USA from clear polycarbonate (3-1/2" in diameter, holds 10 oz/295ml).