No mammal in Illinois’s atlas is better documented than the eastern gray squirrel, with 15,729 records spread across 92 counties. Cook County contributes nearly 6,000 on its own, and the named clusters reach from Chain O’ Lakes State Park in the north to Shawnee National Forest and Cache River State Natural Area in the south.
A life built around mast
Gray squirrels live on tree seeds, acorns, hickory nuts, and walnuts above all, and that diet ties them to mature deciduous woods, wooded river corridors, and the old street trees of Illinois towns. The Illinois record splits neatly between the two: heavy counts in the oak and hickory country around Starved Rock State Park and Shawnee, and heavier ones still in the forest preserves and neighborhoods of the Chicago region, where big trees and bird feeders substitute for forest.
Two busy seasons
The record stays full in every month but swells twice a year, in April and again from September through November. The fall rise is the mast season, when squirrels spend their days cutting nuts and burying them one at a time across the ground, a scatter-hoarding habit that plants a fair share of the next generation of trees. They do not hibernate, riding out cold snaps in leaf nests or tree cavities and foraging on mild days.
Status in Illinois
NatureServe ranks the eastern gray squirrel S5, Secure, in Illinois, and the species is native to the state. USGS NAS does not list it as nonindigenous here.
Full species profile
