Try Ortho Orthene. When you open the lid, leave the paper cover over the top, and just poke a few small holes in it. Think "salt shaker." Dust the area around where the fire ants are comming in and out of the ground. Since you're not spraying the entire yard, you're probably not going to poison the food supply. It's the most effective product I've found.
Another possiblity you might consider is to put a catch pan above the ground, but under the feeder. Something like a garbage can lid may work. Then you won't have to worry about the seeds and the birds getting on a treated lawn.
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