Typically, conflicts between humans and bats only occur with a few species, including little brown, big brown, evening, pallid, and free-tailed bats. These species often roost in houses (usually attics), sometimes for years, without being noticed by humans. Occasionally, an individual bat is found inside a house, flying around and landing on curtains or furniture.
The rule with any bat encounter is to remain calm and keep pets and children away. Keep as near to a wall as possible when moving around the room. Close interior doors and provide the bat with an exit by opening an outside door or window. Never handle a bat with bare hands; leather work gloves offer adequate protection from a bat's teeth and will allow a person to safely and gently pick up a bat and release him outside, out of harm's way. Be prepared for the bat to vocalize loudly when he is picked up.
After the bat has been set free, it's important to find out how he entered the house. If open doors and windows can be ruled out, then it's likely the bat has been roosting within the outer walls of the house and has found a route to the living space. Common entry points include gaps around air conditioners, chimneys, and openings in interior walls that lead to attics or cellars. Inspect thoroughly, as bats can fit through openings as small as half an inch.
The key to excluding a bat colony from a building is to find any and all openings that the animals are using. A "bat watch" at dusk can help you find the entrances. Watch closely from before sunset until about 30 minutes after sunset; it only takes a second or so for a bat to exit and take flight. The best strategy is to let the bats leave on their own, then deny them reentry. However, bats should only be evicted when it is known that there are no young present. With that in mind, it is best not to solve bat colony problems from May through August. Waiting until the bats have left for winter hibernation also allows the exclusion to be done carefully and deliberately.
Bat Exclusion
A simple device, called a "check valve," lets bats exit the house but not return. For most openings, including openings on smooth exterior surfaces or through louvers, netting draped over an entrance—but open at the bottom—allows bats to crawl down and leave, but they will not be able to crawl back up under the netting. Securely attach plastic or other lightweight, flexible netting with 1/6 inch or smaller mesh (staples work well for this) above and on each side of the opening. Allow the netting to hang 1 ½ to 2 feet below the bottom of the opening.
For buildings with rough exterior walls (such as brick or stone) and for holes at corners and in horizontal surfaces, another type of checkvalve works well. Tubes with lightweight plastic sleeves that collapse prevent bats from returning once they crawl out. Tape lightweight plastic securely around the end of a PVC pipe or flexible plastic tubing 2-inches in diameter and 10 inches long. The pipe or tubing end of the checkvalve can be squeezed into narrow crevices or cut into flaps that can be opened up and securely attached to the checkvalve with staples, nails, or strong tape.
Evict only when no young bats are present; they raise their offspring from May through August. By about the first week of September, bats leave nursery colonies for winter hibernation sites. Bats occasionally hibernate in buildings during the winter. Early autumn, before cold weather sets in and after nursery colonies leave and before hibernation starts, is the best time to evict bats. If you find hibernating bats during the winter, postpone action until spring when the bats will be able to fend for themselves.
o Find all openings the bats are using to enter; holes as small as ½ inch or narrow gaps as small as ¼ inch wide. Carefully check the upper levels of your home’s exterior, around chimneys, at the union of dormers with roofs, at loose siding and loose-fitting doors, around windows, and gaps around conduits and utility vents.
o Well-used openings can be discolored from body oils that rub off the bats. But don’t count on finding all entrances by focusing only on this clue.
o Do not simply seal up openings at night. Not all the bats leave at the same time or even all every night. Install checkvalves on all entrances and leave in place several nights.
o Then, if possible, check the attic for bats. Watch the outside of the house in the evening to make sure the bats have not found another way in. If they have found a way in, add a checkvalve to the new entrance.
o After the bats are gone, remove checkvalves and permanently repairs openings.
If you must exclude bats from a building, consider putting up bat houses first so you can continue to enjoy their insect control activities.
