Bats About Our Town


Excluding Bats

The following information is provided courtesy of the Washington Department of Fish and Wildlife.

Preventing Conflicts

For some people bats don't present a problem. For others, bats can be a worry, especially when they become unwanted guests in an attic, inside a wall of a home, or inside the home itself.

Unlike rodents, bats only have small teeth for eating insects, so they do not gnaw holes in walls, shred material for nests, chew electrical wiring, or cause structural damage to buildings. Damage caused by bats is usually minimal, but they can be noisy and alarming, and the smell of bats and their droppings can be offensive. It is possible to learn to coexist with bats, and to benefit from their presence.

If a conflict arises, first make sure bats are the cause by observing the following:

Bat droppings: Bats defecate before entering buildings and places where they roost. In buildings where there is an attic roost or a roost in a wall, an accumulation of droppings may fall through cracks and stain ceilings and walls. Insects associated with bat droppings rarely bother humans.

Droppings are usually the size of a grain of rice, crumble easily between the fingers, and contain shiny, undigested bits of insects. The droppings of mice are much harder and more fibrous.

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Excluding Bats

The best way to get rid of bats is also the safest—both for the bats and the humans involved. This is to humanely exclude them. However, because old buildings offer many points of entry it may be impossible to completely exclude bats from them, or from those with shake or cedar shingle roofs that have no underlayment.

A wildlife damage control company experienced in excluding bats can be hired, or you can do the exclusion work yourself. In attics and areas where large numbers of bats have been roosting for years, it is safer for you to hire a professional to do the work, including the cleanup of accumulated droppings.

Note: Never trap flightless young or adult bats inside a structure; this is needlessly cruel to the bats inside and can create a serious odor problem.

When Not to Exclude Bats

Bats Roosting in Groups: It is important not to disturb roosting bats at any time of the year. In the spring, disturbing a maternity colony when flightless young are present may cause young bats to be dropped to their deaths, or abandoned, by panicked females. Because some bats hibernate in buildings during the winter months, batproof a building only when you are sure no bats are hibernating in it. If bats are found hibernating inside after October 15, they should be left alone until early spring (prior to the birthing period in May) after the weather has warmed enough for insects to be out regularly. Meanwhile, seal all potential entry points into human living spaces, and develop a plan so the exclusion process can be accomplished effectively in spring.

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Why Winter Hibernation Is Important: With few flying insects available to them during winter in Washington, bats survive by hibernating, migrating to regions where insects are available, or a combination of these strategies. It is important not to disturb hibernating bats. If a bat rouses early from hibernation, it must use its fat reserves to increase its body temperature. A single disturbance probably costs a bat as much energy as it would normally expend in two to three weeks hibernating. Thus, if disturbed multiple times, hibernating bats may starve to death before spring.

Do Not Trap and Relocate Bats

Trapping and relocating bats is not recommended. Traps can be fatal to bats if left unattended and can quickly become overcrowded. In addition, bats have excellent homing instincts and, when released, they may simply return to the capture area. Yuma myotis bats released 240 miles from their roost have found their way back.

Partitioning Bats Off

Prior to excluding bats, consider partitioning bats off from the area where they are in conflict with humans, and allowing them to roost elsewhere in the structure. An effective partition can be made from construction grade plastic sheeting and wooden battens. Another consideration is to provide an alternate roost site, such as a properly designed and installed bat house mounted close to one of their exits. Install the bat house before excluding the bats as described below.

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The following will work to exclude bats from most structures:

Option A – Build bats out: From mid-October to mid-March, when bats should still be hibernating, or after you have made sure no bats are roosting in the attic or other area, seal all potential entry holes (see Fig. 5). Entering the attic during the day may reveal light shining through otherwise unnoticed cracks and holes. Insert pieces of fiberglass insulation or bits of stick in these holes to mark them for repair from the outside.

The advantage of caulk over foam is that it comes in a variety of colors and it is easier to apply. Before purchasing, check the label to make sure the caulk can be painted.

Insulation blown into wall spaces may be an effective barrier, but it must be done when bats are absent to avoid trapping them in the fill.

If bats are present, holes can also be blocked over a period of days early in the evening after the bats have left the structure to feed. Do this only from mid-August to mid-October (after the young bats have learned to fly and before cold weather arrives). Another window of opportunity occurs in early spring, before the birthing period in May.

For several days, bat counts should be made as holes are closed, leaving the main exit open. On the night of the final count after the bats have left, the main hole should be plugged to prevent their reentry. The following evening, the plugging should be removed to allow any remaining bats to leave before the exit is sealed.

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Option B – Harassment: If bats are present and have to be excluded, persuade them to move to one of their alternate roost sites by creating an undesirable atmosphere. The time to do this is from mid-August to mid-October, after the young bats have learned to fly and before cold weather arrives. Another window of opportunity occurs in early spring, before the birthing period in May.

Bats don't like to roost under bright, windy, or noisy conditions. Therefore, locate the area where bats are roosting and light the area with a bright light, such as a mechanic's drop-light or trouble light, located away from burnable objects. (Use a fluorescent light to save on electricity and keep the heat level down.) In addition, aim a fan and a loud radio at the bats. Begin the harassment process shortly before dark and keep it in place day and night.

Because bats may move to a dark, protected area, you may need to move the lights and other equipment, or install them in various areas. Putting up sheets of plastic to separate the bats from the rest of the area can be effective, but make sure you don't block the bats' exit or exits.

Commercially available ultrasonic devices may be effective if they are placed in a small, confined area with the roosting bats. Since bats can hear high frequency sounds, these devices, inaudible to humans, supposedly bombard the bat's range with jackhammer-like noise.

Naphthalene flakes or mothballs should not be used to exclude bats. These contain chemicals that can be toxic to humans and other life forms; poisoned bats may fall to the ground where they die slowly and are more likely to come into contact with children or pets.

If the exclusion process was successful, immediately seal up the exits to prevent bats from reentering. If necessary, install a chimney cover, available from home improvement centers.

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Option C – Install exclusion devices: Again, from mid-August to mid-October (after the young bats have learned to fly and before cold weather arrives), or in early spring (before the birthing period in May), identify the exit(s) bats are using. Have friends or family members stationed at the corners of the structure after sunset on a warm calm night. They need to be far enough away to see as much of the structure as possible without having to turn their heads; it takes only a second for a bat to exit and take flight. Note which side of the structure bats are seen from. On subsequent nights, focus your attention there to locate the exit hole. Remember this hole can be as small as ½-inch.

Bats often defecate when exiting and reentering a building, so look closely for rice-sized black droppings clinging to the side of the structure. If droppings are observed, the exit hole will be directly above it. (To make sure droppings are new, remove the existing droppings or lay down newspaper over them to see if more droppings appear.) Bat body oils may also discolor a well-used opening.

Seal all entry holes but one using the methods described in Option A.

Exclude bats by covering the one existing entry hole with a device that allows bats to exit the structure, but prevents them from reentering (see Figs. 6–10). Install the exclusion devices during the day and leave them in place for five to seven days (longer during particularly cool or rainy weather).

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Figure 6. A one-way door allows bats to exit a structure, but prevents them from reentering. Hang a sheet of construction grade plastic, screen-door material, or lightweight polypropylene netting (1/2 inch mesh) over the exit. Use staples or duct tape to attach the material to the building. The one-way door should extend 18 to 24 inches below the bottom edge of the opening. Leave the material loose enough to flop back after each bat exits.

(Bat Conservation International.)

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Figure 7. One-way tubes work where one-way doors won’t, such as on horizontal surfaces. A flexible pipe or cardboard tube is easy to fit into a crevice or cut to create flaps that can be fit over an opening and be stapled, nailed, or taped to a building (Fig. 8). Do not let the tube project more than ¼-inch into the opening to make sure that bats can easily enter the tube.

(Bat Conservation International.)

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Figure 8. One-way tubes should be at least 2-inches in diameter, 10 inches in length, and have a smooth interior so bats are unable to cling to the inside.

One-way tubes can be made from PVC pipe, flexible plastic tubing, empty caulking tubes, or dryer vent hose.
To reduce the likelihood of bats reentering, a piece of plastic sheeting can be taped around the exit end of the tube.

(Bat Conservation International)

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Figure 9. Some areas have lengthy crevices used by bats. Multiple exclusion tubes will need to be placed every few feet along the length of each crevice; spaces between the tubes should be closed with heavyweight netting or other material. The same procedure can be used in lengthy crevices created where flashing has pulled away from a wall.

(Bat Conservation International.)

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Figure 10a. One-way tubes for chimneys. If bats are roosting inside a chimney, construct a wire cage from ¼-inch mesh hardware cloth.

Fig 10b. Insert a modified section of 2 inch PVC pipe through holes cut in the sides of the wire cage.

Fig. 10c. To further reduce the likelihood of bats reentering, a piece of plastic sheeting can be taped around the exit ends of the tube.

(Bat Conservation International.)

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When bats are using multiple openings to exit and enter, exclusion devices should be placed on each opening, unless you can be sure that all roosting areas used by the bats are connected. If all the roosting areas are connected, all but one or two exit holes can be sealed as described below. Place exclusion devices over the one or two remaining exit holes.

However, if the colony contains a hundred bats or more, which is common, leaving only one exit point can create a "bat log jam." In these cases, some bats might start looking for alternative ways out of the roost area, leading to bats finding their way into human-occupied areas. So, always watch to make sure bats are able to exit freely. If they do not appear to be exiting, or appear to be having trouble doing so, open additional exits.

After all bats have been excluded, remove the exclusion devices and immediately seal up the exits to prevent bats from reentering. If necessary, install a chimney cover, available from home improvement centers.

Bats Roosting above Porches and Other Areas

Bats temporarily roost above porches or under overhangs at night to eat large prey, digest, rest, and socialize. In such cases, they may frighten humans, or their droppings may accumulate. Nontoxic aerosol sprays, designed to repel dogs and cats, can prevent bats from night-roosting in these areas.

The spray is applied by day when bats are not present, and is reported to be effective for several months. However, aerosol repellents are not an adequate substitute for excluding bats that are using the area as a day roost, and should never be applied when bats are in a roost.

Mylar balloons or strips of aluminum foil hung from the porch ceiling and allowed to move in the breeze may also discourage bats from roosting in that area.

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