If they do take up residence under a porch or garden shed, wait until late June, then sprinkle a thin layer of flour around the hole and examine for tracks soon after dark. When you discover tracks leading out, but not back in, seal the hole with lumber, wire fencing or concrete. Skunks are diggers, so you will have to extend your barrier 12-18 inches below the surface. Remember, skunks are peaceful animals that will usually walk calmly away when disturbed - provided that you don't overreact. Spraying, is used only as a last resort when the animal feels seriously threatened.
Squirrels sometimes seek out an attic as a safe location to have their young.
But after about six weeks, when the young are able to leave the nest, the entire family will move to a summer nest in the trees. This is the time to repair attic openings to prevent a recurrence, since squirrels usually have two litters a year. Bloodmeal fertilizer mixed with topsoil should put an end to garden excavations, and baffles added to your birdfeeder will prevent pilfering. Opossums are the least troublesome mammals, but should they decide to den under a porch, they may make nocturnal raids on the compost heap or your domestic pet dishes. They are, however, very wary and can be scared off quite easily by opening the nearest door.