About Bats

Bat on a tree © David McChesney
© David McChesney

Bats are truly remarkable animals as well as our only flying mammals. Unfortunately their unwarranted reputation has prevented many people from appreciating how beneficial and unique they are.

Bats belong to the order Chiroptera, which means "hand-wing." Their wings are composed of two thin layers of skin or membrane, attached to elongated finger bones. Each membrane has four fingers and a thumb, which control the wing's movement. The thumb, located at the top of the wing, acts as a hook with which the bat is able to crawl on flat surfaces. 

Bats in Massachusetts

The two most common bats found in Massachusetts are the Little Brown Bat (Myotis lucifugus) and the Big Brown Bat (Eptesicus fuscus). Both have short, soft fur covering their head and body and rich brown bodies with slightly darker brown wings.

The body of a Little Brown Bat measures 4.5 to 5.5 inches long, including the tail, and has an 8.5 to 10.5-inch wingspan. The Big Brown Bat's body ranges from 5.5 to 8 inches in length with a 12 to 13-inch wingspan.

In the spring and summer, female Little Brown Bats form colonies consisting of hundreds of individuals. Big Brown Bats, which prefer the more urban areas inside Route 495, are usually found in colonies of less than 200 bats.

More about bat species in Massachusetts >

Benefits of Bats

All bats found in Massachusetts are insectivores. They feed primarily at night, catching thousands of mosquitoes, moths, and other night-flying insects.

It is estimated that an individual bat can eat 600 insects per hour. Dr. Thomas Kunz, a bat researcher at Boston University, estimates that the bats living within Route 128 eat 13 tons of insects each summer.

Life Cycle

During the late spring and summer pregnant, female Little Brown and Big Brown bats form large nursery colonies and roost in dark, hot places such as, attics, barns, and other outbuildings to give birth and raise their young.

Males, often solitary or in groups less than a dozen in the summer, roost in cooler spots behind window shutters, and awnings and under the bark of trees. Bats rest in these protected roost sites during the day leaving at sunset to search for food.

Mating takes place in the fall before hibernation and the sperm is then stored in the female's uterus throughout the winter. The eggs become fertilized when hibernation ends in the spring and in June or early July the females gather into "nursery colonies" and give birth. The gestation period for both the Little Brown and the Big Brown bats is 50 to 60 days.

Little Brown Bats give birth to one offspring and the big brown delivers one or two. As soon as the baby is born it clings to its mother, attaching itself to a nipple and, for the first few days, is carried by her when she flies at night searching for food. At three or four weeks the young begin to take short flights and catch their own food. When the young are able to feed themselves the "nursery colony" disbands to begin preparing for hibernation.


The nursery colonies break-up in late summer and little brown bats migrate to hibernation caves and mines, mainly in western Massachusetts, upstate New York and Vermont. Big Brown Bats, which are more tolerant of cold temperatures, may migrate to caves or spend the winter in the same buildings they occupy the rest of the year.

Bats choose hibernacula with temperatures between 30 degrees and 40 degrees above zero. If the temperatures drop below freezing they will move to a location with more favorable temperatures. As hibernation begins, breathing slows to one breath every five minutes and the body temperature drops until it is only slightly higher than the air surrounding it.


Bats are not blind as many people believe. On the contrary, they have excellent eyesight but use echolocation in the dark to detect objects as minute as a human hair. When in flight they continuously emit high frequency sounds (unheard by humans) which bounce back to their ears enabling them to locate objects, as well as the flying insects they feed on.

The bat uses the increasing frequency of the echoes to zero-in on its target. During its search for food the bat may emit 10-20 pulses per second, increasing to 500 per second just before it attacks its prey.

Situations & Solutions

Since bats are so efficient at controlling bothersome insect, it is advantageous to leave them alone. That being said, there are instances where a bat is not welcome, such as inside your home. What to do if you encounter a bat >

White-Nose Syndrome

White-nose syndrome (WNS) is a disease that’s affecting bats in enormous numbers. More than a million of these mammals have died in the Northeast and Canada, and some hibernacula—sites where bats hibernate, like caves—have witnessed a 90-100% decimation in their population. More about WNS in Massachusetts >