One of our coops is a 10 x 12 shed that has been insulated for the bantys in the winter. Well, sure enough a couple rats made their way into the ceiling of the coop a few months ago. We knew they were in there, but they avoided the traps at all costs and obviously multipled a lot. Finally after surprising four of them in the coop the other day, DH came up with the bright idea to gas them. I hate the idea of killing anything, but realized that this is an out-of-control problem and it was necessary. I put all the chickens in the run and he hooked up a hose to the car, going into the coop. After running the car for two hours 12 rats total came out of the walls and went to a final sleep. He assures me it's a silent death and it was effective. Obviously this wouldn't work in a larger barn but it was quite effective in our smaller coop. Much cleaner than snap traps and no worries about secondary poisoning.
