Sardines are great, jack mackerel even better. You can find it in the grocery store, next to the canned tuna & salmon.
You WILL catch neighborhood cats, though. Try this method- it's a little time consuming, but it will reduce your cat catches after awhile.
Level a spot for your cage trap on the ground. Put the trap in place & mark where the pan (the part the animal steps on to fire the trap) is on the ground. Remove the cage & dig a small hole in the ground about where the pan will be when you set the trap. Hole should be about 2 or 3 inches around, 5 or 6 inches deep, and more or less straight down. Dump a couple of big chunks of fish in the hole, trying not to cover it with dirt. You want that smell as strong as possible. dribble some of the fish juice around the top of the hole, place your cage & set trap.
This works better than just throwing bait in the cage. If a coon can see the bait, he may well reach through the wire to get it, probably setting off your trap without getting caught. If your bait is in the hole, he can smell it but not see it. Chances are he will enter the cage to investigate.
Of course you're gonna catch cats at first. But if you let them go, you've educated them. If they can't eat the bait, most of them will avoid the cage in the future. Let them get a bellyful of fish, they'll come back time and again, and you'll keep catching cats and not many coon.
Another thing- as the weather gets warmer, and time goes by, the fish will REALLY smell bad. That's ok; the worse it smells, the more coon, possum & skunk you'll catch. As the bait rots down, just add more fish on top of the old. Cats prefer fresh bait- coon don't care so much, and the stronger smell will really pull them into your trap & away from your birds.
Good luck!