They are waterproof, and made of an outer layer of interwoven twigs with a softer inner lining consisting of moss, bark, leaves, fur, feathers, lichen or other similar material. Summer dreys are less elaborate than winter dreys and may be no more than twig and leaf saucer shaped platforms on exposed branches. Tree dens are another type of nest used by eastern gray squirrels. These are holes or cavities in the main trunks of trees which are also lined with soft material. Formation of den cavities requires 8 - 30 years, and are more common in deciduous trees than in coniferous trees.
Squirrels often use dens in winter months and dreys in summer months.