If the bird feeder is in a tree or on a pole, put a metal cone around the tree or post, with the large part of the cone opening at the bottom. Like this:
l
^
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A bell on the cat would be nice, but it's a neighbor's cat. And I doubt if she wants to spend all day and night waiting for the cat to show up so it can be sprayed, although that would work.

Is this a workable solution?