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House Bat Management

Nuisance Problems

Why Bats Become a Nuisance

Some bat species use man-made structures in preference to their natural roosts, whereas others are forced to roost in buildings when natural roosts, such as caves and hollow trees, are destroyed (Fig. 1). Some caves (Fig. 2) are ruined by flooding, dam construction, burning of debris (Jones 1971) or by pesticides (Mohr 1972). Cave roosts also are destroyed by explosives used in mining and quarrying, vandalism, and tourism (Mohr 1972). Bats hibernating in winter have been deliberately killed by vandals or unintentionally by speleologists and biologists because activity resulting from disturbance depletes the bats' limited fat reserves (Mohr 1972). Forest management and deforestation, particularly removal of diseased or old trees with hollows (Fig. 3), have also reduced the number of available natural bat roosts (Jones 1971).

JPG - Possible roosting sites for bats.
Fig. 1.  Possible roosting sites in a house. (From Greenhall and Paradiso 1968)

GIF -- Roosting sites in caves.
Fig. 2.  Possible roosting sites in a cave and under rocks. (From Greenhall and Paradiso 1968)

GIF -- Roosting sites in trees.
Fig. 3.  Possible roosting sites in trees. (From Greenhall and Paradiso 1968)

In recent times bat colonies in houses appear to be less tolerated. Entire local bat populations, whose maternity roosts were in buildings, have been purposely destroyed. Bats that have adapted from one roost type to another are now imperiled by changes in human tolerance of bats and by changes in building construction, i.e., cavity wall insulation, modern churches without belfries, flat gravel roofs, and less use of attics and shutters.

The general requirements for bat roosts in buildings are known. Colonial bats that live in houses usually occur in areas near water and at the edges of woods where insects are found in adequate numbers (Fenton 1970). Less well known is the importance of other factors that govern roost-site selection such as temperature, humidity, vegetation, and the physical characteristics of roost sites. Older structures are particularly attractive to bats.

When Bats Cause Problems

Bat colonies may cause a nuisance when they are located in buildings. The noise created by bats squeaking, scratching, scrambling, and crawling in attics, walls, and in chimneys can be objectionable if the roost site is close to human living quarters. Bats nearly always reveal their presence by their fecal droppings left beneath entrance holes and below roosts. Brown stains and odors from urine, feces, and glandular body secretions, found near the eaves of wooden buildings and barns, may often indicate the presence of bats inside the structures. In old, loosely constructed buildings where there is an attic roost or a space between the wall and a chimney, excreta may seep through cracks and stain ceilings and walls. In churches, bats frequently enter through unscreened belfry louvers and leave droppings that are plainly visible on the front door stoop (Stebbings 1976).

Types of Bat Problems

Bats Inside Buildings

The discovery of one or two bats in a house is probably the most frequent problem. The large brown bat accounts for most of these sudden appearances. Common in towns and cities, it often enters homes through open windows or ungrated fireplaces. These bats may occur singly, in pairs, or in small groups. If unused chimneys are utilized for summer roosts, the young may fall or crawl through the damper and into the house when they are learning to fly and their parents may follow (M. D. Tuttle, personal communication). Sometimes one or more bats may appear in a screened room, and then disappear by crawling under a door crack. In the latitude of New England, the big brown bat frequently hibernates in houses or public buildings (Godin 1977), gaining entrance through crevices between the outer wall and the chimney, by a crack around a window, or through holes between loose boards or bricks. These bats may suddenly appear in midwinter during a warm weather spell and fly about the house. Migratory bats occasionally enter buildings overnight during their spring and fall migrations. A bat will usually find the way out by detecting a fresh air movement; therefore, the simplest solution to rid the building of the bat is to open all windows and doors leading to the outside. If it is still present at nightfall, the lights should be turned off to help the bat find open windows or doors. If the lights are turned on, the bat may seek refuge behind drapes, curtains, and wall hangings. Bats usually will not attack a person even when chased. If the bat refuses to leave, it can be caught in a net, small box or can, or in a gloved hand and released outside. Alternatively, local health authorities can be called to collect the bat (Fig.4).

JPG -- Member of New York bat squad.
Fig. 4.  Member of New York City's Bat Squad (Photo by Stephanie Marcus)

Most bats are able to squeeze through surprisingly narrow slits and cracks; the smaller species require an opening no wider than 0.95 cm (3/8 in.) or a hole the diameter of a dime. The little brown bat can enter a space 1.6 by 2.2 cm (5/8 by 7/8 in.); the large brown bat can squeeze through an opening 3.2 by 1.3 cm (1-1/4 by 1/2 in.).

Attractive openings are found in old frame structures where boards shrink, warp, or become loosened. Bats commonly enter buildings through the overhang of the roof made by overlapping sheeting or drop siding. They are most often found in attics, between roofs and ceilings or roof spaces, in cornices, fascias, or other crevices around the roof, in walls, in chimneys, around drainpipes, behind rafters and sheathing in open barns, between a window and screen, and occasionally in crawl spaces. Depending on the size of the space and on the species, bats will be found singly, congregated in groups of a few individuals, or in colonies of hundreds and occasionally thousands.

Guano, Urine, Odor, and Ectoparasites

Bat guano and urine accumulating in attics and wall spaces attract arthropods such as roaches and mites (Constantine 1970). The accompanying odor can be pungent but not dangerous. Bat ectoparasites, such as ticks, mites, fleas, and bugs, rarely attack humans (Scott 1963). They are most likely to cause a nuisance after a house has been batproofed, thereby ridding the edifice of bats but leaving arthropods. Arthropod problems are unlikely except in large, well-established bat colonies where fumigation may be appropriate (Pratt 1958). Ectoparasites quickly die in the absence of bats.

Bats Outside Buildings

Some bats temporarily roost behind shutters, under wood shingle siding and roofing, roof gutters, awnings, trim with overhang, under flashing around chimneys which has separated or loosened from the solid structure, open garages, patios, porches, breezeways, open livestock shelters, and under sheets of tarpaper. Shutters on brick houses are especially attractive as day roosts for transient bats in migration and for males that frequently take refuge behind shutters during the nursing season. In exceptionally hot weather, individuals may abandon an attic and reside behind shutters. Big brown bats are partial to roosting behind the trim below roofs of houses. Unusual roosting areas include sewers, wells, and graveyard crypts. Bats also will fly around swimming pools from which they may drink or catch insects that are attracted to water. Street and porch lights attract flying insects which in turn attract bats.

Distribution of Problems

House bat complaints come from throughout the United States (National Research Council 1980) and southern Canada. The greatest number originate from the Northeastern States: New England and adjoining Canada, New York, New Jersey, Pennsylvania, and Maryland. This distribution of complaints appears to be related to the attention given by news media concerning rabies (D. G. Constantine, personal communication). Additional bat complaint "hot spots" exist in Wisconsin, Illinois, and Indiana. Numerous complaints also originate in Texas and California.

Human reactions to the presence of bats in buildings vary from place to place. Often bats that annoy one resident may be encouraged by a neighbor to coexist. House bat colonies are common in Florida but very few complaints (of bats in buildings) come from that State. This may be due to the abundance and variety of Florida wildlife living in proximity to people. In sharp contrast to Florida is New York City where bats roosting in buildings are rare and a single bat flying into a private apartment or office may generate instant panic. The solitary migratory red (Lasiurus borealis) and hoary bats (Lasiurus cinereus) constitute the major problem in New York City, especially Manhattan, by flying into office buildings where they have been observed as high as the 27th floor (A. Beck, personal communication). They are particularly abundant during the 3-month period beginning with August (Anonymous 1977).

Confusion of Bat Problems with Those of Other Animals

It is essential to verify that a nuisance is caused by bats. Twittering and rustling sounds in old chimneys, often attributed to bats, may be caused by chimney swifts (Chaetura pelagica). Scrambling, scratching, and thumping sounds coming from attics and walls may be caused by rats (Rattus rattus), mice (Mus musculus), or flying squirrels (Glaucomys volans, G. sabrinus). Bats often become noisy before leaving their roosts at sunset and may chatter on hot days when they move into walls to seek refuge from heat. Thus, an increase in noises about dusk probably indicates bats.

Droppings from insectivorous bats are easily distinguished from those of small rodents because of their friability. They are easily crushed by rubbing between the fingers which reveal shiny bits of undigested insect chitin (the exoskeleton of the insect). In contrast, rodent droppings are unsegmented, harder, and more fibrous (Greenhall and Paradiso 1968).

Occasionally the droppings of birds and lizards that feed on insects may be found along with bat droppings. Bat droppings never contain the white chalky (uric acid) material characteristic of the feces of these other animals.

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