Here is your state thread. Oregon. I had an infestation of rats in one coop in particular. I tried about everything to no avail. When I renovated the coop dozens of rats of all sizes poured out. I did notice tunnels around the coops and barn. I finally resorted to poison, doing it responsibly. I bought some rat bait stations. I put the rat bait in the bait stations (they usually come with some bait already in them) and the bait stations into a pet carrier and a cage so that nothing but the rats were able to get to it and put the pet carries/cage on a shelf. Rats are good climbers. Eventually the baits were being eaten less and less. I didn't find any dead rats laying around so I assume they went into their tunnels and died. I still keep them baited but no activity in quite awhile. The bait stations have a little window above the bait so I can check it often. As I said I tried many other things first but this has worked. Good luck...
The green line represents a partition the rats have to go around to get to the bait so that the bait stays in the bait stations.