To the OP, this thread should give you some insight into what you are up against. Good stuff starts on page 2 of the thread.
https://www.backyardchickens.com/threads/rats-when-the-army-went-to-war-on-them.1216226/
The historic videos......all of them.......are old, but every bit as valid today as back then. The first video explains it all. Only the types of poisons have changed.
Concepts remain the same. With a severe rat infestation, know they are there for the easy and abundant source of food.....chicken feed, eggs they steal, up to and even including attacking chicks and the birds themselves if things get out of control.
So step 1 is to eliminate the abundance of food. They may not take to the poison bait blocks until you do. So with hunger going for you, transition to Step 2, which is to offer up the bait blocks. If offered loose, they will steal and scatter them around where anyone can find them. So serve the poison blocks up from secure bait stations where you can control access and monitor the results. Keep going until the use has died off. Then switch to a different type and keep going until use dies off. Just know that if you havea colony of 100 rats, you might get 90% of them with the first type of poison, but that still leaves 10 to go. If you switch baits, you might get 90% of those remaining, but that still leaves 1. If that 1 is a fertile female, things will be back to where they were in no time at all. So goal is to get them all......100% if you can do it. At that point, your rat infestation should have abated.......but you are still not done.
Since at this point you know you live in an area with a rat infestation, you have not eliminated it, you have just greatly thinned the existing herd. So Step 3 is to confine birds and the feed you give them in a rat proof structure to protect them from future hordes. Building a true rat proof structure is no easy feat. Watch the video and you will see why. And do keep the bait blocks out in the secure bait stations to monitor things. Even if only one block at a time. Use means you have rodents. No use is the goal.