Big Brown Bat

Big brown bats are one of our common bat species. Photo credit: Ann Froschauer, USFWS, public domain

The big brown bat (Eptesicus fuscus) is one of our largest bat species in Kentucky and the eastern U.S. It is slightly shorter than the length of a dollar bill. Its wingspan which is measured from the tip of one outstretched wing to the tip of the other outstretched wing is a little longer than the length of two dollar bills lying next to each other. Big brown bats have a wide range and can be found throughout the continental U.S., up into much of Canada, down through most of Central America, across the Caribbean islands, and even into parts of northern South America.

Like the rest of our bats in eastern North America, the big brown bat is insectivorous, which means it eats insects. Because big brown bats have such a wide range that includes so many different types of habitats and climates, its diet includes a wide variety of insects. However, they seem to have a particular fondness for beetles, especially in the eastern U.S. This can be helpful to humans, because many of our agricultural pests are either beetles or moths.

Big brown bats can have relatively long lifespans with an average of 6-7 years; however, based on banding data, it isn’t uncommon for them to live for almost 20 years. They tend to be loners and prefer to roost by themselves or in very small groups, except during the summer when mothers will form maternity roosts. Maternity roosts are where female bats will roost together with their young. For big brown bats, the maternity roost can range in size from a handful of individuals to over a hundred individuals.

Mating occurs in the fall, and occasionally when a bat wakes up from hibernation for short periods during the winter, but the females store the sperm and do not become pregnant until the spring. The females will give birth to one or two babies, called pups, in the late spring or early summer. Twins are common in the eastern U.S., while single pups are more common in the western part of the country.

The pups are blind, hairless, and unable to fly when they are born. The mothers will leave their pups in the maternity roost at night while they go out and forage for food. She will often return to the maternity roost periodically during the night to nurse her pups. During the day, she will roost with her pups and nurse them. If the mother decides to switch to a different roost, then she can carry her pups to the new site, but otherwise the pups hangs out in the maternity roost with the other young big brown bats until they are able to fly in mid to late summer.

In the wild, big brown bats can be found roosting in caves, in rock shelters, and in tree cavities. They especially like to scrunch up into rock crevices at cave entrances while hibernating during the winter, even though these can be some of the coldest and windiest parts of the cave. However, they also have no problem having human neighbors and are one of the most likely species in the eastern U.S. to be found roosting under bridges, in attics, or in the hollow space under your eaves.

Since big brown bats are so large and will frequently take up residence in man-made structures, they are also one of the easiest for most of us to observe. If you see a large, almost sparrow sized, bat flying over your yard at dusk or flying around a bright street lamp, then it is likely either a big brown bat or a red bat providing you with some free pest control.

We may not always think of bats as being part of our backyard ecosystem, but it isn’t uncommon for big brown bats to be found in suburban and even urban neighborhoods. If you want to try to encourage them to take up residence near you, check out Bat Conservation International’s information on how to build and properly install bat houses. Who knows, you might just get a new batty neighbor.

This article was part of Shannon’s original Kentucky Pollinators and Backyard Wildlife blog which evolved into the blog for Backyard Ecology.

Backyard Ecology: Exploring Nature in Your Backyard
Nature isn’t just “out there.” It’s all around us, including right outside our doors. Hi, my name is Shannon Trimboli, and I am the host of Backyard Ecology. I live in southcentral Kentucky and am a wildlife biologist, educator, author, beekeeper, and owner of a nursery specializing in plants for pollinators and wildlife conservation. I invite you to join me as we ignite our curiosity and natural wonder, explore our yards and communities, and improve our local pollinator and wildlife habitat. Learn more or subscribe to my email list at

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