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I've found rat holes around the pens and shoved a poison bait cylinder down the hole as far as I could. Then covered the hole with a paver. A few days later you can smell the dead rats.
Additionally, ammonia works great in deterring rats. After birds go to roost and are locked up for the night, hang soaked ammonia rags knee high going around the chicken pen. Just hang them through the chicken wire. Bye bye rats. Remove the rags the next morning before letting your chickens out...