Bats are loosely divided into two groups – tree bats and cavity-roosting bats. Cavity-roosting species generally live in groups and enjoy huddling together in warm, dark, and predator-free spaces. This is especially important for females, who congregate together in the spring and summer in "maternity colonies," where they give birth to and raise their pups. Bats prefer roosting in dead trees, but forestry practices have largely eliminated these "hazard" trees, forcing bats to find other alternatives. Some species have adapted to roosting in man-made structures, particularly in barns lofts, attics, and eaves, which stay warm enough at night to protect fur-less pups from the cold spring nights. Some bats may also hibernate in buildings. The most common species found in man-made structures are big brown bats (Eptesicus fuscus).
A small number of bats roosting in your house or barn are not likely to cause any problem or health concern for you or your family. In fact, they are conveniently removing insect pests from your backyard every night in the spring and summer!
Where large quantities of bat droppings (called guano) have piled up for several years and are located in a confined space, there is a potential human risk of Histoplasmosis infection. Histoplasmosis is caused by inhaling the spores of the Histoplasma fungus, which is found in soil and then enriched by bird or bat droppings. The infection may be asymptomatic or cause flu-like symptoms, but it rarely requires treatment. Histoplasmosis is only a threat when the guano is disturbed and spores are released into the air. These conditions are usually only found in caves or abandoned buildings. Bats roosting inside a home typically do not occur in numbers great enough to produce such quantities of guano. Scattered or small piles of droppings pose no harm and can be swept up with no risk to human health.
For information concerning rabies, see Bats and Rabies.
The decision to evict bats that are roosting in a manmade structure should be an informed one. In some cases, bats are solely using the exterior of the structure and cannot access the inside. Bats may be roosting beneath the flashing of the chimney, in between the slats of shutters, or inside the soffit or overhang. Without access to the interior of the house, bats cannot enter the living space. Bats that do access the interior of the structure usually roost in the attic or chimney and may occasionally enter the living space.
Evicting bats from your home involves three general steps, inspecting the exterior of the house for all crevices that may be used as access points; sealing all holes except the main access point used by the bats, and installing a one-way door, which allows the bats to exit but prevents them from returning. Once all the bats have left the roost (usually after a week), the main access point can be sealed. Bat exclusions can be done yourself, but they are more commonly performed by professional bat excluders.
All bats in NJ are protected under state law, and it is illegal to harm, harass, or kill them. Therefore, all bat exclusions must be conducted following the Nuisance Wildlife Control Guidelines for Bats (48k PDF). Most importantly, bat exclusions may only be performed during April 1 – April 30 and after August 1. If bats are known to be hibernating in a building, then an exclusion can only occur during the month of April. Exclusion guidelines are in place to prevent bats, particularly non-flying pups, from being trapped inside structures.
Bats evicted from homes (even during the "safe dates" described above) face some challenges. Nightly April temperatures are typically <50°F with little or no insect food items. The stress caused by a lack of food, coupled with the need to search for a new roost site and the absence of a warm refuge, may cause death to adult bats or unborn fetuses. In addition, they may attempt to access a neighboring home, displacing (not resolving) the problem. Finally, bats are extremely site-fidelic and will return to former roost sites annually. Because of this, professional bat excluders may not guarantee the work for more than a couple years.
Installing a bat house near the original roost prior to eviction can provide immediate refuge to evicted bats, reducing stress and increasing survival. If evicted bats adopt the bat house, the bat-human conflict may be resolved for the long term.
If you have bats and are considering a bat exclusion, the Rutgers Wildlife Conservation and Management Program can provide and install a bat house on your property through our Bat House Distribution Program. In exchange for this service, we ask that you enroll in our Project: Batwatch program so that we can evaluate the success of this initiative to provide long-term solutions to bat-human conflicts.
For further guidance on bats in buildings or to report a bat colony on your property, please fill out our online reporting form.