Kathy, I don't know what your set-up is there, but are you using bowls? This won't stop the birds eating the cat food, but it might cut the problem down quite a bit.... depending upon how many bowls you're putting out, take one or two of those cheap plastic laundry baskets, cut a hole in them large enough for the cats to get into, and put them upside down over the bowls.
We solved the problem by putting the cats' feeder up in the woods, and putting out bird feeders (great cat TV for the inside kitties!), and scattering birdfood on the lawn. Now the birds eat the bird food, and the cats eat the cat food.