Bat Exclusion (Source: USGS)
Throughout history bats have aroused the curiosity and interest of men. Bats of the United States feed primarily upon insects, many noxious. Natural bat roosts are caves and tree hollows. A few species have readily taken their abode in houses thus gaining for themselves the name of "house bats" (Allen 1962).
Unfortunately, most bat complaints arise from an exaggerated fear of bats, not from any actual damage; however, some form of management is justified and the type of management depends upon the problem. Fear of rabid bats, as well as sensational and inaccurate news coverage, has engendered the use of potentially dangerous chemicals to kill bats in buildings. This may create worse public health hazards by increasing contacts between humans and sick bats, in addition to exposing people to dangerous pesticides through contact, inhalation, or ingestion of contaminated food. The conspicuous decline of bat populations, the excessive use of toxicants to kill bats in buildings, and the need for effective methods of bat management have led to the preparation of this manual. The purpose of this publication is to provide a better understanding of the biology and ecological role of insectivorous bats and to describe their occasional conflicts with people and how these may be alleviated. The present methods and practices in house bat management are reviewed and promising areas for further investigation are suggested. Special emphasis is placed on "batproofing" or exclusion as the soundest long-term solution for the management of house bats.
- We at Falcon Services, Inc. prefer the term "bat management" rather than "bat control". To some, control implies the reduction of bat populations without regard for the welfare of the target species, whereas management is directed at resolving the conflict without long-term adverse effects on bat populations.
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 (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. Common in towns and cities, bats often enter homes through open windows or doors. These bats may occur singly, in pairs, or in small groups. 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, you may call Falcon Services, Inc. to collect the bat.
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.).
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 (Source: USGS).
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). 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 (Source: USGS).
Confusion of Bat Problems with Those of Other Animals
It is essential to verify that a nuisance is caused by bats. 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 (Source: USGS).
Species of Bats Causing Problems
Among the 23 species of bats occurring in California, only a few cause problems in buildings. The most common house bats congregating in groups or colonies are called colonial bats. Other species live a secluded or solitary existence and are known as solitary bats.
The three species most often encountered by humans are the little brown bat, the big brown bat and the Mexican free-tailed bat.
The little brown bat is one of the most abundant species, often forming nursery colonies in buildings during the summer. Adults and young vacate the buildings in the fall to hibernate in caves and mines. Colonies may be as large as 2,000 (Humphrey and Cope 1976).
The big brown bat is undoubtedly the most familiar to humans and the only species for which buildings are ideal for both raising young and hibernating. Colonies are small, ranging from 12 to 200 (Barbour and Davis 1969).
The Mexican free-tailed bat is the most colonial of all bats. Its habits vary in different parts of the country. Primarily a cave dweller in the Southwest, a colony may include thousands of individuals. Some populations migrate 1,600 km (1,000 miles) to overwinter in Mexico whereas others are year-round residents (Davis et al. 1962).
Solitary bats live alone in tree foliage or under bark, but never in caves. The red bat, the hoary bat, and the silverhaired bat (Lasionycteris noctivagans) may occasionally enter buildings during spring and fall migrations as transients but do not permanently roost in buildings (Barbour and Davis 1969).
A number of other bat species are occasionally found in buildings but, because they infrequently cause problems, they are not discussed here.
You may read more about California bat species by clicking here.
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