Try a rat-version of a "weasel box". Build a simple wooden box with a hole about 2" in diameter in one end and place a simple rat trap behind the hole and bait the box. I put hardware cloth on the other end so the scent of the bait gets into the air. For weasel you use dead meat, but for rats you could use something less disgusting such as grain and peanut butter. Mr. rat gets the scent, can't get in any other way, and sticks his head into the box, steps on the striker and gets his neck broken for the effort. Be sure to use bait outside the box as well so he gets used to eating a free cheap meal. When the easy feed is gone he'll want to keep eating and be comfortable with the set-up, so he won't be too concerned with sticking his head into the hole. The only down-side to this is that you'll often have the trap fire from mice instead of rats. Just keep re-setting it and be patient. Chickens live when predators die. Happy hunting.