brown bats

Big brown bat

The Big Brown Bat (Eptesicus fuscus) is larger in size than comparative species of bats, from about 4 to 5 inches (10 - 13 cm) in length and weighing 1/2 to 5/8 ounce. The fur is moderately long, and shiny brown. The wing membranes, ears, feet, and face are dark brown to blackish in color.

Big Brown Bats are nocturnal, roosting during the day in hollow trees, beneath loose tree bark, in the crevices of rocks or in man-made structures such as attics, barns, old buildings, eaves and window shutters. Big Brown Bats navigate through the night skies by use of echolocation, producing ultrasonic sounds through the mouth or nose. Together with the Hoary Bat (Lasurius cinereus), Big Brown Bats are the only vespertilionids to produce audible sound during flight. Its voice is a click or a sound like escaping steam.


Big Brown Bats are insectivorous, eating many kinds of night-flying insects including mosquitoes, moths, beetles, and wasps which they capture in flight. This causes the sudden, frequent changes in direction.


Big Brown Bats hibernate during the winter months, often in different locations than their summer roosts. Winter roosts tend to be natural subterranean locations such as caves and underground mines where temperatures remain stable; it is still unknown where a large majority of Big Brown Bats spend the winter. If the weather warms enough, they may awaken to seek water, and even breed.

Life Cycle

Big Brown Bats mate sporadically from November through March. After the breeding season, pregnant females separate themselves into maternity colonies.


The subspecies Eptesicus f. fuscus occurs in the entire eastern half of the U.S. except Florida. The subspecies Eptesicus f. pallidus occurs in Utah.


  • Reader's Digest North American Wildlife Mammals, Reptiles and Amphibians. 2nd Edition, Reader's Digest.

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