This article is from the Problem Behaviors in Cats FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
Locate your birdfeeder in an area where the ground is clear, affording cats no cover. At the same time, try to locate it under something, like a tree, to provide refuge from attack by other birds.
To reduce the problem of birds on the ground (after dropped seeds) getting picked up by cats, use suet feeders instead of seed feeders.