@kurby22 found something that might help
https://www.wildliferemoval.com/ecoclear-and-rat-x-poison/
They have products for mice etc too. I might try the mice version. If you remove all food overnight and put that stuff out where the rats are used to feeding...might help. Maybe instead of the tray it comes in, you could drop it into something rats can enter but chickens can't get at it. Could also try adding a little bacon grease, peanut butter or such to increase them eating it. Have to read to see if adding something like that is ok though.
https://www.wildliferemoval.com/ecoclear-and-rat-x-poison/
They have products for mice etc too. I might try the mice version. If you remove all food overnight and put that stuff out where the rats are used to feeding...might help. Maybe instead of the tray it comes in, you could drop it into something rats can enter but chickens can't get at it. Could also try adding a little bacon grease, peanut butter or such to increase them eating it. Have to read to see if adding something like that is ok though.