In your yard: the Big Brown Bat


As bats go, Big Brown Bats are, well, big. But, as with any bat, don’t be afraid of them. They aren’t aggressive, won’t bite unless touched, and are a gardener’s friend because they patrol your yard nightly for flying insect pests.

They’re 4.5–5.0 inches (11.4–12.7 cm) long, with a wingspan of almost 14 inches (35.6 cm), which might sound alarming. But adults weigh next to nothing—about 0.5–0.75 ounces and aren’t at all interested in humans. They have a large head, large eyes, a broad nose, and ears that are medium-sized and rounded. Their coat is glossy and brown, with shades varying from brown to reddish-brown to chocolate. Their underside is slightly lighter. The face and ears are bare. Their tail extends slightly beyond the tail membrane.

Range and habitat

Big Brown Bats, Eptesicus fuscus, originally preferred deciduous forests but have proven themselves adaptable by now living in nearly every kind of habitat—from forests to meadows to deserts—with no strong preference for feeding near water as many other bat species do. They range from Alaska and central Canada throughout the United States and as far south as northern South America.


They hibernate about half the year, from late fall until spring, when insects are available for food. Often they change locations from their summer roost to a winter one.

In early summer, females separate from males and form maternity colonies of up to hundreds of individuals. The males roost alone or in separate groups during this period. After the young are weaned in late summer, both sexes roost together again. Big Brown bats are loyal to their places of birth and hibernation and return year after year.

Food sources

Big Brown Bats rest during daylight hours and take flight just before dusk to begin feeding. But they don’t say out long—by midnight, they’ve often taken their fill and are back in their roost. They’ll also come out in the daytime if they’re sorely in need of water or food. Like most other U.S. bats, they’re insect eaters and identify their prey by the sounds made by their wings—flying beetles are a favorite.


Big Brown Bats roost in such places as hollow trees, storm sewers, rock crevices, and under loose bark. Also, buildings: sheds, attics, chimneys, under eaves, or even behind shutters. The only invitation they need is a tiny opening and a promise of darkened, warm surroundings. For this reason, they’re the ones most likely to be noticed by humans.


The bats mate in the fall before hibernation (it may also occur during periodic awake periods in the winter). The young are born in mid-to-late June when plenty of insects are available as food. The gestation period is about 60 days.

Did you happen to notice that something seems weird about this time frame—like, how can a 60-day gestation deliver the young in June when mating occurred the previous fall? Here’s how that happens: The female stores the sperm in her body until she awakens from hibernation in the spring. Only then does her body ovulate, and fertilization takes place. (This phenomenon isn’t unique to bats—stoats, sea otters, and Roe Deer are among 47 other mammalian species that experience it.)

Mothers leave their babies (pups) together in the roost each night. She has to eat her weight in insects while she’s nursing to maintain her energy, and she’ll leave more than once, if necessary, to get her fill. Upon their return, the mothers call out to their pups, who’ll answer back. Each mother can identify her offspring’s “voice” and particular scent even in the cluster of dozens or hundreds of other noisy pups.

Life span and predators

Young bats, still ignorant of the demands of hibernation, have a 50 percent chance of dying in their first winter because they didn’t store enough body fat. They also face death by predation from climbing critters, such as snakes and raccoons, that might get into their roost. Once they begin flying, the little bats must learn to avoid owls and falcons. If they can make it through that perilous first year, they may live up to 20 years. 

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