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Bats In Buildings

Information to help assess and address bat problems in the home.

Image of Little brown bats roosting between beams in a tractor barn.Little brown bats roosting between beams in a tractor barn. © Charles Hofer
House Bats

Of the nine different bat species that live in New Jersey, little brown bats (Myotis lucifugus) and big brown bats (Eptesicus fuscus) are the most likely to roost in man-made structures like attics, barns, eaves, and church belfries. Nicknamed "house bats," these adaptable critters have found that buildings can provide a warm, safe, virtually permanent source of shelter. Their natural roosts - mainly dead or dying trees - are less abundant than they used to be, and tree roosts are also more vulnerable to weather, predators, and simply falling over. So buildings have become quite important as bat roosts.

Image of Big brown bats in an attic space.Zoom+ Big brown bats in an attic space. © Phil Wooldridge

Bats are perfectly willing to share space with each other and often form large roosting groups during the spring and summer. These groups are called maternity colonies, and they consist only of mother bats and their young (adult male bats are more solitary). Females give birth to just one or two young each, in late May or early June. The mother bats must leave the roost to eat and drink every evening in order to produce enough nutritious milk for nursing. In fact, a nursing mother bat will eat her entire body weight in moths, beetles, mosquitoes, and other insects every night (be glad if you have bats in your backyard!). The youngsters are totally dependent on their mothers for about a month until they are strong enough to begin flying and foraging on their own.

By the end of July, all of the young bats are able to fly, and they leave the roost every evening with their mothers to practice hunting and swooping down for drinks of water. Between then and mid-October, the colonies disband and/or begin moving to their winter hibernation sites, which are mostly in underground caves and mines. However, we're learning that it may be quite common for big brown bats to stay all winter in an attic roost if the conditions are right.

How Do They Get In?
Image of A colony of 100 big brown bats roost in the attic of this building every summer. They enter and exit through a small gap where the eaves meet the chimney. Notice the dark "smudging" around the hole from the bats' body oils.Zoom+ A colony of 100 big brown bats roost in the attic of this building every summer. They enter and exit through a small gap where the eaves meet the chimney. Notice the dark "smudging" around the hole from the bats' body oils. © Phil Wooldridge

Unlike rodents, bats are not capable of clawing or chewing their way into places. But they can squeeze through openings as small as a half-inch to gain access to a suitable roost. Bats often find their way in and out of buildings through openings in the eaves, ridge vents into the attic, the space between soffit boards where they join at the peak, or gaps or cracks where the chimney meets the house's siding or eaves. The older the building, the more likely it is to have such entry points and that bats have discovered them.

Bats also sometimes roost behind shutters, under roof tiles, slate, or shingles, beneath clapboard siding, or inside defunct chimneys. Most homeowners do not view this as a problem, as long as the bats aren't getting inside human living spaces.

Once a maternity colony finds a good roost, the bats will return to it every spring. The colony may continue to grow each year as the previous season's female offspring join in as mothers, though it seems that most colonies reach a leveling point. That leveling point may have to do with the amount of preferred roosting space (like close parallel beams and other dark nooks) inside the roost.

Are There Bats in My House?

Bats are shy, cautious animals, and their main strategy in life is to go unnoticed. They're active at night to thwart predators, and even their daytime roosts are dark and snug. Many people coexist with bats without ever knowing it. But there are signs of occupancy to look & listen for.

Image of Guano (bat droppings) below a favorite roosting spot in a house attic. Droppings are less than a half-inch long and contain the shiny wing & exoskeletal remains of insects.Zoom+ Guano (bat droppings) below a favorite roosting spot in a house attic. Droppings are less than a half-inch long and contain the shiny wing & exoskeletal remains of insects. © MacKenzie Hall
  1. Guano (a.k.a. bat poo). Bats themselves are secretive, but wherever they hang, droppings will accumulate below as evidence. Bat guano looks a lot like mouse droppings, except that it concentrates in piles below a favored roosting spot and can be easily crushed to reveal shiny, indigestible insect parts (whereas mouse droppings contain plant fibers).
  2. Bat inside the roost. One of the easiest ways to look for roosting bats during the day is to find a guano pile and look straight up. You may need a flashlight to see inside possible roosting crevices. You may even see bat out in the open, like on the surface of a chimney. A frightened bat may take flight, but it will not land on you or dive-bomb you; it will just look for another (safer) place to land. Be careful not to disturb bats during the pup-rearing season (May 1-July 31), as it may spook them and cause pups to fall.
  3. Fallen pups. The occasional pup does lose its footing, and it's almost impossible for that fallen youngster to climb back up to the roost. Its mother cannot collect it from the floor, either. You may notice the tiny remains of one or more fallen pups among the droppings; this is normal and indicates that you have a maternity colony.
  4. Bats exiting the roost. From roughly May through July, adult bats exit their roosts like clockwork every night at dusk. They've gone the entire day without food or water and are eager for a meal. If you see a bat swoop out from an eave or shutter or other part of your home, keep watching the spot. There may be more. See our Summer Bat Count page for instructions and a reporting form.
  5. Bat sounds. Bats in flight are almost constantly echolocating, but those sounds are inaudible to us. Inside the roost, they do communicate with a squeaky chatter we can hear (though it's quiet). Bats often chatter when someone enters their roost. They also chatter at the roost exit before diving out at dusk. It can take a trained ear to hear, but some people can easily make out these chirpings. Bats inside the walls and eaves may make scratching sounds as they move around, but bats do NOT claw or chew through materials like rodents do.
  6. Bats inside the living quarters. Bats are more likely to end up in your living quarters if there's a colony "upstairs." Especially around mid-summer, young bats that are just learning to fly can make mistakes while trying to leave the roost and pop out near people by accident. If you get bats inside your house regularly, your house probably serves as a roost.

Eviction and Bat-Proofing:

Bats are important to our environment and our economy, and they are protected by law in New Jersey. Myths and misconceptions about bats are rampant, so be sure to evaluate your situation fairly while deciding whether a colony can stay or go.

You may feel that your "house bats" have become a nuisance and decide to evict them. Whether you consult a professional or attempt to bat-proof your own home, state guidelines should be followed to ensure that bats are not injured, killed, or trapped in the process. Physically removing bats or poisoning bats is NOT legal and NOT effective. Fly traps and glue traps should NOT be used in places where bats may come in contact with them. Any pest controller who suggests these methods is not doing credible work.

New Jersey's "safe dates" for bat eviction are April 1-30 and August 1-October 15.

More Information:

Download Nuisance Wildlife Control Guidelines for Bats

Nuisance Wildlife Control Guidelines for Bats - 152.3KB
"Safe dates," legal practices for bat eviction, and other relevant information for NJ homeowners and pest control professionals.

Download Professional Bat Excluders in NJ

Professional Bat Excluders in NJ - 107.3KB
The Professional Bat Excluders list is not comprehensive, but it includes companies that are known to us and that practice humane and effective bat eviction methods. Are you a pest controller who we could add to this list? Please contact us!

Download Three Chamber Bat Box

Three Chamber Bat Box - 256.8KB
Our favorite bat house design, created by the PA Game Commission.

Download Bat House Installation Guidelines

Bat House Installation Guidelines - 514.0KB
A "how-to" guide on installing bat houses.

Do It Yourself Bat Exclusion Tips:

A Homeowner's Guide to Northeastern Bats and Bat Problems (Penn State University)

Bat Houses: Offer an Alternative
Image of Mitchell Fechter (back row center) and his friends from Scout Troop 87 in Mercer County built and donated bat houses to the Conserve Wildlife Foundation. Mitch earned his Eagle rank for the project.Zoom+ Mitchell Fechter (back row center) and his friends from Scout Troop 87 in Mercer County built and donated bat houses to the Conserve Wildlife Foundation. Mitch earned his Eagle rank for the project.

When bats are evicted from a building, their first reaction is to try and find another way in to their familiar roost. If they aren't successful, they'll need to find another nearby nook to call home. Often times the colony just moves over to a neighbor's house, in effect just passing the nuisance on to someone else.

Bat houses can break the cycle. By putting up a bat house before bats are evicted from a building, the bats have a good chance of finding and settling into that new roost once their current one is blocked off. This solution keeps a roof over the bats' heads, allows the homeowner to continue enjoying a less buggy backyard, and prevents other cases of unwanted bats in the home.

Image of Installing a bat house. Zoom+ Installing a bat house. © MacKenzie Hall

Through ongoing partnerships with Scouts and service groups, we are able to offer FREE bat houses in cases where bats are being evicted from a building. Each of our bat houses can accommodate up to 80+ bats, so usually one is enough to house the whole colony.

The best place to mount a bat house is on the side of the same building where bats are currently roosting, on a sunny side as close to the bats' current entry/exit point as possible, and at least 12-15 feet off the ground. Alternatively, bat houses can be mounted on a pole or post in an undisturbed part of the backyard. Please contact us if you're interested in putting up a bat house.

Injured Bats and Fallen Pups:

Have you found a bat that needs care? Please contact a NJ-licensed Wildlife Rehabilitator near you. Currently, only six rehabilitation centers accept bats, and the NJ Bat Sanctuary is the only one that specializes in them. It is helpful if you can transport the animal, since these centers are very busy tending to the many animals in their care. They'll tell you how. Also please consider making a donation, if you can, for their painstaking, life-saving work!


Stephanie Feigin, Wildlife Ecologist:

Find Related Info: Bats

Thank you to our bat research and education sponsor:

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