I have them everywhere outside my enclosure. For a period I had 3 inside. I was lucky to catch it before they bred and got out of hand!
To get rid of the ones in the run I diligently picked up all traces of food when the chickens went up to roost. I set electronic traps inside the run along the perimeter. Rats will move close to a structure and rarely cross open ground. If you can see any tracks that's the place to set your traps but they're difficult to see on ground and bedding. MAKE SURE THE TRAPS ARE REMOVED BEFORE THE CHICKENS EXIT THE COOP IN THE MORNING.
I never caught any in my traps because my rats stayed up on the 2x4s at the roof line and it was too narrow there to set any traps. So, eventually, I set red lights out and went out at night when they were active with a pellet gun to get them.
I don't know how they got in in the first place but I packed any opening at all that I could find with steel wool. And then I started setting spring traps around the outside of my enclosure and the nearby fence at night. Every morning I trip them so the chickens can free range. Every night I set them. I move them every few days. Rats are smart and learn what's safe and what needs to be avoided. I catch a few a month and I haven't had any inside the enclosure again.
I really would discourage you from using poison. A poisoned rat takes a while to die. While they're still ambulatory they're still prey. A couple of our local mountain lions have died from ingesting poisoned rats. And domestic animals like cats and dogs can as well. I'm sure they do. It just doesn't make the news like a dead mountain lion.
It's hard getting rid of them. You have to stay at it. It's easier to make sure they don't get in in the first place by making sure your hardware cloth goes down 12" into the ground so they can't dig in. And keep your feed secured in metal containers. The less food they can get to, the less you're a target for them.