Peppermint Oil!!!
Really, it works great for me, in two different locations I've lived.
Put a few drops of peppermint oil on steel wool and push that into any holes in/around the coop. If you don;t have steel wool, use a cotton ball. But the steel wool is more repulsive to mice, I don't know why. But it's the peppermint oil that keeps the mice away, and I even found a dead mouse by my cotton-ball soaked with peppermint oil. I don't remember any of the scientific reasoning, but i do know it repels and sometimes kills mice. Use only real essential oil when doing this. I had a mouse problem last year, until I put the steel wool and peppermint oil out there.
Good luck!