Bats found in Illinois are only 2 to 4 inches in length, with a wing span up to 12 inches. The most common bats here are the brown bats. Bats have to hibernate or migrate to warmer climates. They will gather in colonies and reside in hollow trees or attic spaces. Bats tend to hibernate in the same place year after year, causing roosting areas to become covered in droppings, known as guano. This needs to be removed from living quarters due to disease carried in bat guano.