I have mice that show up in my coop now and then. They don't stay long however. They usually become the target in a rousing game of chicken "Keep Away". My birds have uncovered 2 nests of babies in the past 2 weeks. I will spare you the gory details and just say that my birds shouldn't be protein deficient for a while. Yuck!
I haven't seen a mouse/rat at my place in probably 10 yrs. But my DH helped his cousin clean out some chicken houses a few years ago and he said when they removed the pallettes under the chickens...there were millions and millions of rats! He said he had never seen anything like that in his whole life....yuckyyyyy I guess they don't bother the chickens all that much or the chicken house owners would do somethin....?
I used to have mice in my garden, in my coop, in my grain storage and hay sheds, etc. I don't know if the chickens kept it from getting worse, or the snake living rent free in the hay shed, but it was a mice fest for a while there.
I put out those black bait boxes with those blue blocks of poison and that helped tremendously. I hardly ever see one or hear one unless I get a new order of hay. In which case, I have to refill the bait boxes and in a few days, they're invisible again.
The hole is small enough for a younger chick to get into, but I my chicks don't free range with my chickens. They're in a separate enclosed area. If you have chicks running about with a mama chicken, you might want to pen them up somewhere if possible until they're a few weeks old (or big enough not to get into the hole). Chickens can't get into them. The holes are big enough for a mouse to get into.
I don't know but if someone has something foolproof I am dying to hear it, too. My standard egg layers will snatch any mouse they see, but the nasty little suckers have figured out that my bantams can't do anything to them, their pens are overrun with the evil things. Apparently chick starter is a favorite mouse snack. We were afraid that if we poisoned them, and they went somewhere and died where the birds could get to them, the birds would get sick, so we haven't put out poison yet. This isn't a problem?
Try setting some rat traps but be sure it is in a spot where nothing else can get in it. I don't like poison because when something eats the mouse/rat they can be poisoned too. There was just a thread a few days ago where a rooster I think it was died a pretty horrible death from the poison. So please be careful.