These bats emit volleys of calls through their open mouths. The duration of each call and interval between calls varies depending on whether the bat is in search, approach, attack, or terminal phase. Typically, insectivorous bats will increase the rate of echolocation calls as they close in on prey. Some data suggest big brown bats forage orient toward the loudest natural sound fields. This tends to correlate with high densities of insect prey.
Some bats require stable, highly insulated environments in order to hibernate.
Eptesicus fuscus has a more tolerant constitution so it can winter in less substantial structures. Besides human dwellings, it has been found to take up residence in barns, silos, and churches. In presettlement times it is presumed the big brown bat roosted in tree hollows, natural caves, or openings in rock ledges. All this bat needs is a small hole or warped, loose siding to gain entry into a home. Once inside, it prefers to roost in double walls or boxed-in eaves rather than attics.