The down side of using poison to kill rats is if you have cats or dogs that eat one of the poisoned rats - they will die too.I found this thread because I am having the same problem. When it first started I used snap traps under a milk crate, it worked pretty well until they got smart about it and then i was just finding empty, unsnapped traps. The problem died down for a little bit but it's back now. I walked in my coop tonight to get eggs and heard a noise before i opened the door but after i looked inside i saw the feeder(25lb) swaying side to side and a HUGE hole dug out under the grit box.
I have a mouse cat, she does catch rats sometimes but we have a large Vole population here too so they keep her busier, easier to get to. We also have feral cats around here whom I have literally caught sleeping in the nest boxes on cold nights. It's just not enough for the amount of rats though.
Tonight I reset the snap trap with peanut butter and placed it under the crate but tomorrow I am going to mix a poison block with peanut butter and put it around the Outside of the coop where there's obvious rat holes. Keeping my fingers crossed it'll work as well for me as it did for you, Kanga77510.