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we built our coop out of an old victorian pig pen, unbeknown to us at the time there is a drainage system running right underneath it which makes it more or less hollow under neath, so the rat(s) find holes in the weakened concrete and keep getting in, no matter how many holes we fill in they always manage to make another one, its an on going battle, plus we live next to a corn field and when the corn is cut in the fall the critters head for somewhere else that is warm and dry, what do we do ?, i have laid traps and poison where the hens cant get, but the rats seem to be amune to it.
Get some dura bond. You can find it at any hardware or home improvement store. It is a power that is mixed with water to patch drywall. It will dry hard and fairly quick. Mix that with a little water, leave it kinda stiff, and break some glass up and add to it then put it down the hole. The rats will scratch at that and cut themselves then bleed to death. I have heard that their blood does not clot up. Of course you should add concrete on top so the chickens can't get to it. Good luck