I had rat problems with my coop on the farm I grew up on, getting rid of them can be very difficult. I've had friends with rats from their coop getting into their house and causing lots of stink and damage. if they have an unlimited supply of food they will multiply faster than rabbits. consider using one of those feeders that the chicken has to step on to open, and/or remove the food at night. they have been known to even eat chicken droppings. the very best solution I've found is to line the coop with 1/2 inch galvanized hardware cloth, sides, ceiling and floor. you don't have to go under the coop, but if you don't , it's recommended to go two feet down and at a 45 degree angle away from the coop all around the perimieter, that way predators that dig will lose ground as they go down, often deterring them. the upside is that by making one's coop rat proof, you pretty well keep out all the other unwanted critters as well.