Bats in Buildings
Having "bats in the belfry" usually means that a person is considered crazy or erratic. The phrase derives from observations of bat use of a tall structure, such as a watchtower, and the flurry of activity as they leave at dusk. In modern times, few people own actual belfries, but they may have bats in their attic, garage, or other building sites. This article provides an overview of bats in buildings and describes how to deal with unwanted roosts.
Timing is absolutely crucial when dealing with unwanted bat roosts. If you follow the guidelines outlined below at the wrong time of year, you may create new problems for the roost owner.
First, a bit about bats…Bats are not rodents. They do not make or enlarge holes in buildings, they do not chew wiring, and they do not build nests. They use a structure just as they find it, although they may cause damage. Over time, a large roost can damage a building because of accumulation of urine and droppings. Bat urine has a very pungent odor, particularly noticeable during warm weather. A homeowner needn’t worry about rodent-like building damage, although removing the urine odor can be difficult without replacing affected building materials.
The same bats will use the same roosts from year to year. If you have bats this year, they will likely return next year unless you exclude them from the building.
Exclusion is the best way to deal with unwanted bats in buildings. Exclusion is a process by which bats are sealed out of a structure. Exclusions must be performed at the appropriate time of year and follow certain guidelines.
Why not use poisons?
Poisoning bats is illegal and irresponsible. Poisoning attempts seldom kill all the bats, leaving sickened bats that emerge, become grounded, and may be retrieved by children and pets. Poisoning also fails to address the real issue – that bats are gaining access to your structure in the first place. As bats are mammals, anything harmful to them will also be harmful to you and your pets.
In South Dakota, groups of bats in buildings are typically members of maternity/nursery roosts. In general, maternity roosts may number from less than 10 to more than 1,000 bats. Typical roost size in South Dakota is 20-30 bats. Bats differ from most other small mammals in that they give birth only once a year and generally only to a single pup. Adult females come together in the early spring to give birth and rear their pups. These roosts can form from wider areas with reproductive females sometimes traveling great distances to benefit from collective roosting. Considering the large area from which females may gather and the low reproductive rate of bats, destroying such roosts can have serious impacts to a particular population or species.
Exclusions must be timed to be completed before the females have given birth or postponed until after the pups have learned to fly. Optimal exclusion dates vary with a year’s weather conditions, but in general, exclusions in South Dakota should not be done between mid-May and September 1.
Seasonal visitors
Most groups of bats in buildings are seasonal visitors, arriving in the spring and departing in the late summer or early fall. Bat species that roost collectively in buildings must hibernate during the winter, as there is no insect prey available. Buildings typically do not provide the necessary conditions for bats to survive the winter. As cold weather approaches, they begin to move out of their summer roost and head for their hibernation sites (or to warmer climates farther south). By the time cold weather arrives, nearly all the bats will have left buildings. The ideal time to do exclusions is either after the bats have left for the winter or early in the spring before their arrival.
NOTABLE EXCEPTION: One particularly robust species, the Big Brown Bat (Eptesicus fuscus) has been known to winter in buildings in South Dakota, but not much is known about the extent of their winter building use. Buildings most likely to be used for hibernation generally contain brick or block sections (including foundations) where the bats can find a more stable temperature. Bats often move from their entry point to a distant location in the structure via cavities in walls to find their optimal seasonally specific temperature. Bat exclusion in buildings with such features should employ a more conservative treatment plan to prevent entrapping over-wintering bats. (Entrapment frequently causes bats to seek alternative ways to exit the structure, which may lead to their inadvertently entering the living space and confronting human inhabitants. In general, neither party is amused by such encounters.)
Excluding unwanted bat roosts
If you read the important preceding information and have decided to evict your bat tenants, here is the method to use when bats are (or may be) in residence. The following should only be used in roosts of less than 100 bats. For larger roosts, contact an experienced bat exclusion specialist for advice.
1 - Identify the bats’ favorite point of entry (there may be more than one). If there is more than one point of access but you are certain they are connected within the structure (different openings leading into the same soffit, for example), you may proceed with the following directions. If you are uncertain the sites are connected or they are in different parts of the structure, each roost should be treated separately.
Identifying the entry point is usually not too difficult. Bat droppings may accumulate beneath the access points or a dark staining is sometimes visible around access points found in light-colored materials. If you have no idea where they are entering the structure, position yourself and some friends around the outside of the house at dusk to watch for the bats’ nightly emergence. Do this on a warm, calm evening, since cool temperatures or rain will usually delay or prevent bat emergence.
2 - Once you have identified the entry point, thoroughly examine the structure during daylight hours and identify any additional openings. This is probably the single-most important step in the entire procedure. Many openings may not be visible from a ground level vantagepoint, particularly for multi-story structures. Use binoculars or examine the structure from a ladder.
3 - All openings except the bat’s entry point should be sealed. Many people believe bats are larger than they actually are, requiring large holes to gain access. Some species are smaller than others, but you should fill any opening larger than ½ inch. (Smaller species can use the trough in the pointing between the ends of two bricks to pass under the fascia board and gain access to the soffit.) Use a good quality caulk to seal smaller holes or crevices and expanding aerosol foam for larger openings.
Remember: bats are not rodents; they do not make or enlarge holes.
It is absolutely essential to identify and seal all potential entry points, except for the known entrance. If the building has vents at the ends, check carefully to be sure they are screened. Even if vents appear to be screened, check for unfastened corners or holes that may be difficult to see with a cursory look. Also check around flashing for other easily missed access points. If possible, enter the attic with the lights off (during daylight hours) and note places where you can see daylight. The single most common bat entry points in western South Dakota are small gaps between an exterior chimney and where it adjoins the house. Fill all gaps on both sides of the chimney except for a single two-inch gap at the point where you’ve seen the bats enter or emerge. When you are finished sealing all potential entry points, the only entrance to the structure should be the identified bat access.
4 - Over this final opening, temporarily install what will be a one-way escape vent or chute. This vent is fashioned from heavy mil plastic or poly tarp material, sized approximately 24 inches in overall length, and shaped as a cone (similar to a windsock). Duct tape works well to hold the cone together (put the tape on the exterior of the cone so the interior remains smooth). The large end of the cone should be large enough to cover the opening the bats have been using plus a couple of inches surrounding the opening. The small end of the cone should have a hole with a diameter about the size of your thumb. Attach the large end of the cone over this last opening in the structure, making sure to completely seal all the way around the cone. Duct tape works well for attaching the vent. The bats must not be able to emerge anywhere except through the small end of the funnel. The cone should hang away from the side of the house and not lie flat against the structure (that's the reason for using a heavier material like the poly tarp).
5 - . Leave the vent in place for 5 to 7 nights of good weather to give all bats the opportunity to get out. They can emerge but cannot get back into the building. At the end of this time, remove the cone and immediately seal this final opening. If you have not missed any other openings, you have solved your bat problem.
REMEMBER: Simply blocking the bats’ access holes without first treating the entire structure will usually result in the bats finding an alternate entry point. If done at the wrong time of year (see above for acceptable dates), you may simply exclude the adult females and entrap juveniles not yet able to fly. This generally leads to more determination on the part of the adult females to find another way to get back to their pups, which can often result in bats gaining access to the human living space. In addition, when the mother does not return, the juveniles begin to search for mom and often end up crawling into the human living space.
A NOTE OF CAUTION: Be forewarned that any activity on a ladder at any openings in the structure may startle an unseen roosting bat into flight. Try not to work immediately in front of or directly beneath an access point. Such a surprise can easily cause a fall from a ladder. Where possible, place your ladder to one side of your work area.
Where do we go now? Consider erecting an alternative roost location for your newly evicted bats, such as a "bat box." Designs and tips are available at Bat Conservation International’s web-site: (http://www.batcon.org). Design and placement should be appropriate for the roost size and species. It is best to have this roosting alternative in place well before the exclusion is performed.
Properly timing exclusion of unwanted bats from roosts in combination with providing appropriate roosting alternatives (where applicable) is a workable, responsible method to deal with bat-related problems. Preventing contacts between bats and humans and bats and pets should be the goal of any control program, but this need not be done at the expense of the bats. The contribution of bats to a healthy ecosystem as the main predators of night-flying insects (many of which are forestry and agricultural pests) far outweighs any associated risks.
For more information about bats in South Dakota, consult the following web-site: http://nat_hist.sdstate.edu/SDBWG/SDBWG.html
Joel Tigner is a bat biologist and owner of Batworks, a consulting firm specializing in bat study and bat-friendly exclusion.