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big brown bat

big brown bat (Eptesicus fuscus)
Photo © Michael Durham

Features and Behaviors

The big brown bat has a large body (two and one-half to three inches long) and long forearms. The body fur is brown, and it has brown-black wings and ears. A broad, round tragus is present in the ear.

The big brown bat may be found statewide in Illinois. This bat lives in attics, barns, caves, mines, bridges and trees in the summer. In the winter, it can be found in caves, mines, attics and buildings. It eats insects. It forages anywhere that insects are present. It is commonly seen by humans since they often use the same buildings as the bat, and it may hunt in their yards. Summer and winter sites are not more than a few miles away from each other. The big brown bat withstands cold better than other bats, so it can hibernate in more places. Mating occurs in the fall, winter or spring. Those females mating in fall or winter store the sperm in the uterus until spring, when it is used to fertilize the eggs. Females form maternity colonies after leaving the wintering site. Young are born in May and June. Two young are born per female. The young bat attains adult size in about two months, but it can fly about four weeks after birth.

Illinois Range


​Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Chiroptera
Family: Vespertilionidae

Illinois Status: common, native