According to “What’s That Clump of Leaves?” by Art Shomo, there are two types of nests: tree cavity dens and leaf nests. The nest is at least 20 feet up to the tree, and during winter, the squirrels move the nests from the outer branch to the main trunk to get more stable structures, by weaving twigs and compacted leaves together. Dens are good for keep squirrels’ body temperature and protect them better from extreme weather conditions, which are usually with 6 to 8 inches diameter. Oaks, beeches, elms and red maples are ideal trees for squirrels to build dens or nests. In the author’s study area West Virginia, gray squirrels prefer dens more than leaf nests.
On our site, there are many trees with possible cavities, especially the area where the red circle is at on the “Squirrels’ Cavity Location and Active Area” map. However, it is very hard to find a nest on our trees, which seems to be consistent with the result by other researchers as mentioned above. We remember during the winter time, there was one nest possibly on C3, as it is getting warm, the nest is gone. But the cavity is definitely a proof of how the squirrels are playing around and actually try to live in these oak trees, just like this photo is showing here on our tree C3:
The One Tree Project 