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House Bat Management

Review of Biology and Ecology of Insectivorous Bats

Bats are the only truly flying mammals and belong to the order Chiroptera - "handwing." Their ability to fly, their secretiveness, and their nocturnal habits have undoubtedly contributed to bat folklore, superstition, and fear.

Bats are worldwide in distribution but mainly tropical. There are 18 families of bats and about 900 species-only the rodents exceed them in number of species within the class Mammalia (Tuttle 1979b). Bats are common in all the United States. About 40 species are found north of the Mexican border occurring from coast to coast and into the mountains, although they are most abundant in the Southwest (Barbour and Davis 1969).

The natural habitat of most North American bats is caves and trees. Many bats are found in mines and some in buildings. Day roosts are dark and secluded. Foraging areas are around water, forests, ravines, and buildings. With the advent of cold weather bats migrate or hibernate in caves, mines, and sometimes in houses. These hibernacula usually have high humidity and above freezing temperatures. Active, non-hibernating bats spend the day hanging in secluded retreats and become restless as evening approaches. Upon leaving their roosts to feed, bats usually first fly to a pond or other water source to drink. A second feeding period may occur just before daylight.

Most bats in the United States and Canada are insectivorous, catching small flying insects, many harmful, by sonar or echolocation. Some bats may consume up to one-half their weight of insects in a night. The little brown bat (Myotis lucifugus) commonly found in buildings, feeds on midges, mosquitoes, caddis flies, moths, and beetles. One study found 140 mosquitoes and other insects in the stomach of a single little brown bat (Bellwood and Fenton 1976; Anthony and Kunz 1977). Tuttle (1979b) stated that 500 bats can easily capture 500,000 insects a night.

The big brown bat (Eptesicus fuscus), also found in buildings, may fill its stomach in 1 h. Gould (1955) found that an adult accumulated food at a rate of 2.7 g/h. Seven orders of insects, including beetles and stinkbugs, are eaten (Phillips 1966). Many of the insects reported are harmful. The Mexican free-tailed bat (Tadarida brasiliensis) forms the largest colonies of any mammal. Some Texas cave colonies contain as many as 20 million individuals and could consume more than 45,000 kg (100,000 lb) of insects nightly (Tuttle 1979b).

Many animals use sonar to navigate, locate, and avoid obstacles. Bats use sonar for those purposes and to capture flying insects. High-frequency sounds, inaudible to humans, produce echos permitting bats to measure distance. Bats also make audible sounds.

In the United States bats mate in the fall and winter but the female may retain sperm in the uterus until spring when ovulation and fertilization take place. Pregnant females congregate in nursery colonies in caves, mines, buildings, or other dark retreats. No nests are built. Birth occurs from April through July and most species produce a single young, although some have twins and a few have litters of three or four. Young bats grow rapidly and are able to fly within 3 weeks. When adept at flying and catching food, juveniles become less dependent on their mothers and the maternity colonies disperse after weaning in July and August.

Around first frost bats prepare for winter. Some species migrate relatively short distances, whereas certain populations of the Mexican free-tailed bat may require migratory flights up to 1,600 km (1,000 miles). Bats in the northern United States and Canada may hibernate from September through May; hibernation for the same species in the southern part of their range may be shorter or even sporadic, and some fly during warm spells in winter (Davis 1970).

Unlike many small mammals whose average life-spans may be less than a year, bats often live 10 years or more. Two little brown bats were recaptured 29 and 30 years after being banded (Keen and Hitchcock 1980).

Almost all bats are of some economic importance and those of the United States and Canada are beneficial because of their insectivorous diet. The guano (accumulated bat droppings) is rich in nitrogen and at one time was commercially mined in the Southwest (Gile and Carrero 1918; Nelson 1926) and is still mined in Mexico as nitrogen-rich fertilizer. The importance of bat guano has declined because of reduced populations of guano-producing bats and the development of inorganic fertilizers.

Several bat species in the United States have declined in numbers during the past 20 years (Barbour and Davis 1969; Tuttle 1979a, 1979b). Jones (1971) suggested that the widespread use of pesticides has been a major, but not the sole, reason for these reductions. Chemical pesticides have decreased the food supply for insectivorous bats, and ingestion of contaminated insects has resulted in reduced bat populations (Geluso et al. 1976). Thousands of bats die annually when people disturb hibernating or maternity roosts. Bat species in the northeastern United States have gradually declined where insecticides and rodenticides have been widely used in an attempt to solve house bat problems.

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