Big brown bats will often roost in man-made structures as well, such as attics, barns, old buildings, under eaves and behind window shutters, particularly during the summer months. Like other bats, the big brown is nocturnal, becoming active just before dark. Bats navigate through the night skies by use of echolocation, a method by which ultrasonic sounds emitted through the mouth or nose of the bat are bounced off objects in the bat's flight path. By analyzing the returning echoes, a bat can tell the location, speed, and distance of nearby objects and determine whether the object is food or an obstacle to be avoided. Big brown bats are insectivorous, eating many kinds of night-flying insects including beetles and wasps which they capture on the wing.
Individual bats eat many insects and large colonies can consume countless quantities nightly.
Big brown bats hibernate during the winter months, often in different locations than their summer roosts. Some winter roosts tend to be in caves and underground mines where temperatures remain stable but 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, defecate, and even breed. Mating among big brown bats occurs sporadically from November through March. After the breeding season, pregnant females separate themselves into maternity colonies.