Could be chipmunks. We have them by the millions. A pellet gun works very well. We sit out in our backyard and have contests to see who can kill the most. They just keep coming....and coming..... and coming. One day we killed 17 of them and the next day we killed 9. You would think they would get the hint. We see a few mice but the chickens get them. I guess that's what happens when you live in the northwoods.