Large Pack Rat Nests Pack rats, also known as woodrats, build massive nests. Up to four feet across, the large structures resemble beaver dams and are constructed of branches, twigs, and plant material. A midden is another part of a pack rat's nest. It's built from feces and urine and used to store the shiny objects the pests pick up in their travels. A Most Peculiar Coating Woodrat nests are coated with the animal's thick urine, which dries to create a hard shell. It is highly effective at protecting the interior. In fact, researchers have found some nests that are tens of thousands of years old. Nests in Inconvenient Places Although a pack rat nest is usually built in a tree or on a rocky ledge, the pests will also den in attics, sheds, and vehicles. Their foul nests follow soon after, creating problems for property owners. If homeowners spot a woodrat nest, the pests are nearby. Contact Critter Control for removal and advice on preventing future infestations.