Here in Georgia fire ants can threaten birds that nest in nesting boxes. These pesky ants will enter nesting boxes and actually kill hatchlings.
Several decades ago Jackson, Mississippi resident R.B. Layton came up with a novel way to keep these dreaded imported insects from reaching his nesting boxes. Layton soaked either wood thread spools or sweetgum balls with the oil additive STP and placed them between the boxes and the poles that held them aloft. Supposedly, this formed a barrier over which the ants would not pass.
If you decide to try this technique, since thread now comes on plastic spools, you will have to find them at a craft store. They are available in a variety of sizes. I would imagine that you need to buy spools that are unpainted; an unpainted spool would probably retain more STP than those that are painted. As for sweetgum balls, they can be located beneath sweetgum trees across the state.
Since I have never had a problem with fire ants entering my nest boxes, I have never tested this technique. However, if you try it, I would love to know if it worked for you.