In this case, I am referring to ones with tails, beady little eyes, and four little paws. Our cat loves them, but we would prefer that he had a bit less entertainment mousing. We see the little pests in the house often enough (and not always caught by the mighty mouser) that we have been trying to figure out their point of entry.
My wife and I were on the back patio this evening, enjoying relatively warm weather and a gorgeously clear night, when I noticed something running across the concrete that runs completely around our house. Then something else. Sure enough, they were mice, running from the ground under the bird feeders to one of the foundation vents near the laundry room (where the cat sleeps at night). It appears that they are grabbing seed that has fallen to the ground, then back to the foundation vent, then back to get more seed. My guess is that somewhere inside that foundation vent, mice are creating the world's smallest granary.
But foundation vents have a screen in them, to keep small animals and large insects out. So where are they going? My guess is that they have cut a hole through the vent screen. (I just had two images flash across my brain: the first is a little tiny mouse, and then the guy in Animal House lighting up the blowtorch in preparation for converting the trashed Lincoln Continental into the Deathmobile. Then the images combined!)
You can replace the foundation vents, but it seems an excessive amount of work. You would think that someone would make a foundation vent screened cover--something that slides inside the house in the concrete, which has a screen. This lets you get the benefit of air flow, and yet keeps critters out. So far, I have not found such an item.
The foundation vent holes in the concrete, unfortunately, are not a simple rectangle. If so, I would just build an aluminum or acetal frame, and staple aluminum screening to it. These look like an stretched D--flat on one side, and rounded on two other sides. (But not, of course, a half-circle--that would be too easy.)
I suppose that I could build something that sat on top of the hole, but then these would have to be somewhat flexible, with weights to hold them down. Any ideas? (A neutron bomb to kill off all the mice would work, but might be considered excessive, as well as a bit out of my price range.)
UPDATE: The vent hole: