Squirrel nests are messy, cluttered, and usually dirty. The animals are very persistent, invasive, and destructive. They expect others to tolerate and even welcome their presence. They can be aggressive if threatened and are flexible opportunists. If a squirrel has taken up residence in your home or attic, the only practical way to remove them is by trapping or professional removal.
Pack rats collect, claim and hoard materials in order to decorate their nests. A peculiar characteristic of a pack rat: if they find something they want, they will drop what they are currently carrying, and "trade" it for the new item. (They take your stuff and claim it as their own.) They are particularly fond of shiny objects and old junk. The critters consistently move their treasures around, rearranging and perhaps hiding them within their nest, but they never really get rid of anything.