Hi There,
I have a coop and run that are built on top of a 6 inch foundation of gravel, surrounded by a thick lumber frame.  I have hardware cloth around the coop, between the coop and the ground and covering any holes/gaps.  The run has 1/2 hardware cloth walls and a floor that is wood chips on top of the 6 inches of gravel below. 
I recently had put a few bales of hay in the coop for the chickens to hop around on.  But I noticed one of the strings holding the bale together had been chewed.  When I moved the hay bales, I was horrified to see that there were tunnels through the gravel underneath them.  I removed all the hay bales and sealed up the tunnels, but I am pretty positive that one or more rats are now living under the coop (the tunnels led back to under the coop)--my dog is sniffing like crazy around the edges.
I have grandpa's feeder, (which I think I need to do a better job of cleaning up around when I put the chickens up at night (they spill some around the edges), but I put all other food/scraps away in a metal bin.  For the last week, I've had up a motion-sensor camera and a trap with peanut butter.  I've not caught or seen anything.  I thought I was doing all the right things. Apparently not. 
Any suggestions?  I think the rats are burrowing under the coop and run, but but don't know how to get them out.  (The coop is not moveable). Also--who would think rats would tunnel through 6 inches of gravel???
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