Plenty of acorns, the fruit of the oak tree, has led to rising numbers of chipmunks in the Northeast United States. A large supply of acorns on the ground last winter provided food for chipmunks across New England as spring returned. Acorns and other nuts are a main part of the animal’s diet. They also eat insects, berries, and other kinds of fruit. The plentiful food supply kept the chipmunks well fed as they got busy reproducing and having families this spring. Now, the growth in the chipmunk population is causing problems in some areas, with people saying the animals are driving them nuts.