In much of the northern U.S. and Canada, this is the bat most often seen in winter, and the one usually found hibernating in buildings in urban areas. Very common throughout much of their range, Big Brown Bats spread out over the available area individually or in small groups. Those that reside in buildings, usually one to five per structure, hang from rafters, hibernate in cracks or under objects, or burrow in insulation. Some individuals retreat to caves or mines, where they hang near the entrance. This species often emerges from the hibernaculum and flies outside, but—at least in northern parts of its range—it does not feed in winter; it depends for energy on fat reserves, which constitute up to one-third of its body weight. A relatively fast flier, at speeds of 40 mph (65 km/h), the Big Brown Bat eats mostly beetles but also takes wasps, ants, plant hoppers, and leafhoppers. It rarely eats flies or moths. Particularly important foods in the Midwest are June bugs, green stinkbugs, and cucumber beetles; all are agricultural pests, especially the latter, whose larva, the corn rootworm, probably inflicts more crop damage than any other agricultural pest in the country. All farms benefit from the presence of Big Brown Bats. Maternity colonies, numbering up to about 600 individuals, are usually located in buildings, but sometimes under a bridge or in a hollow tree. Pups often fall to the floor or the ground, but those that are able to climb partially up a wall or other structure are often retrieved by their mothers, who find them by their continual sharp squeaking notes.

description A large bat. Brown above, varying from light (in deserts) to dark (in forests), usually glossy; belly paler, with hairs dark at base; wings and interfemoral membrane black. Calcar keeled. No reduced premolar behind canine. No fur on wings or interfemoral membrane. Tragus short, broad, and rounded. L 4 1/8–5" (106–127 mm); T 1 5/8–2" (42–52 mm); HF 3/8" (10–11.5 mm); FA 1 5/8–2" (42–51 mm); Wt 3/8–5/8 oz (13–18 g).

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