That's not a bird's nest in your tree, so what is it?

*Related video above about acorns.

CHICOPEE, Mass. (WWLP) -- As the leaves on the trees begin to fall, the branches and anything living on them become more visible. If you see a large, round collection of leaves in your tree, it may not be a bird's nest but, instead, a home for squirrels.

These large clumps of leaves are called dreys, according to MassAudubon. All different types of squirrels create these nests, including the eastern gray squirrel, red squirrel, and northern and southern flying squirrel.

A drey can be up to 1 to 2 feet wide and is typically located more than 20 feet up in the tree. They are often found in oak trees as the acorns there provide an easy source of food.

The drey can start as a bird nest. Once the squirrel moves in, the animal adds more leaves and branches as well as layers of moss or pine needles.

Dreys are fully surrounded by the material used to make them, and squirrels usually stay in the middle of the nest. You will see dreys built in your tree near a fork between two strong branches or inside a tree cavity.

Squirrels usually build the dreys during the summer or early fall. During the winter months, squirrels will make their homes in them if they cannot find a warmer place to stay (such as your attic), according to MassAudubon.

In case you were wondering, most squirrels don't hibernate, according to the journal Science. While some are out looking for food year-round, most gather and store food for the cold season before spending the winter in their nests.