We had a squirrel chew through perfectly good wood under our deck, penetrate a sill and take up residence inside a wall in our basement. We have a cat, and she couldn't get this one. What worked eventually was to dump mothballs into the access hole, block the hole, scrupulously watching for others to appear. When she came out to avoid the fumes kitty got her. Now your squirrels have everything they need in a well-vented area, so you're going to have to study them, or shoot them. They are not above stealing eggs or killing chicks, either. What kind of squirrel? Thirty you say! Oh they've got to go.