Rats are very smart creatures and establish territories for feeding and breeding where they feel relatively safe and well-fed. They tend to live & move in large groups, so when things get unfriendly, they all tend to move on, at least for a while. We have them moving in occasionally & always hit them full force when they try to take up residence. After trying all kinds of remedies, including live traps (which they were seldom dumb enough to go into), I use a combination of poison & the rat zapper ultra. They rat zapper electrocutes them instantly, but they tend to learn about this too, so it works at first and then it's hard to get them to go near it. If you get the rat zapper, do not get the original, because it will use way too many batteries. Get the new one that uses 4 D cell batteries. The rats at our place have tunnels on the perimeter of the barn & I place the poison deep into these tunnels where only something that is actually in the tunnels can get to it. The bad part is that they die under the barn and it stinks for a while, but at least the rats are dead. I usually only have to put out the poison once, and the rat problem is gone. It's usually several years before they show up again. As another poster mentioned, you must use poison very very carefully, but it IS effective. I personally don't like to kill anything, but I know what rats can do to chicks and the damage they will do to your coops and runs & if they show up, so it's definitely a war, take no prisoners.