Big Brown Bat (Eptesicus fuscus)

One the eight species of bats that is native to the Chicago region is the big brown bat.

Bats are fascinating mammals that are misunderstood and undervalued. To help dispel myths and share why bats are a needed and important part of our ecosystem Friends of the Chicago River celebrates Bat Month in June.

One of the most versatile of all bats, the big brown bat is found in all 48 continental states though it is more abundant in hardwood forests than in coniferous forests. The big brown bat has a large body (two and one half to three inches long) and long forearms.

The bodies of big brown bats are covered with shiny brown fur. The face, ears, feet and wing membranes are dark brown or blackish in color. The average weight of these bats is around 23 grams.

According to the Illinois Department of Natural Resources, the big brown bat is one of 13 species that 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.

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.