Wildlife Damage Management February 04, 2008|Print

Bats | Bat Overview | Bat Damage Assessment | Bat Damage Management | Bat Resources | Bat Acknowledgments | ICWDM | Wildlife Species Information


Figure 1. Little brown bat, Myotis lucifugus


Conservation and Public Education.

Despite their ecological value, bats are relentlessly and unjustifiably persecuted. Bats are often killed because they live near people who needlessly fear them. These actions emphasize the need to educate the public on the reasons for bat conservation and why it is important to use safe, nondestructive methods to alleviate conflicts between people and bats. General sources of information on bats include state Cooperative Extension Services, universities, government natural resource and health departments, and Bat Conservation International (Austin, Texas). Except where control is necessary, bats should be appreciated from a distance, and not disturbed.

Identification and Range

Bats, the only mammals that truly fly, belong to the order Chiroptera. Their ability to fly, their secretiveness, and their nocturnal habits have contributed to bat folklore, superstition, and fear. They are worldwide in distribution and include about 900 species, second in number only to Rodentia (the rodents) among the mammals.

Among the 40 species of bats found north of Mexico, only a few cause problems for humans (note that vampire bats are not found in the United States and Canada). Bats congregating in groups are called colonial bats; those that live a lone existence are known as solitary bats.

The colonial species most often encountered in and around human buildings in the United States are the little brown bat, (Myotis lucifugus, Fig. 2), the big brown bat (Eptesicus fuscus, Fig. 3), the Mexican free-tailed bat (Tadarida brasiliensis, Fig. 4), the pallid bat (Antrozous pallidus), the Yuma myotis (Myotis yumanensis), and the evening bat (Nycticeius humeralis).

Figure 2. Little brown bat, Myotis lucifugus. Figure 3. Big brown bat, Eptesicus fuscus. Figure 4. Mexican free-tailed bat, Tadarida brasiliensis.

Figure 2. Little brown bat, Myotis lucifugus Figure 3. Big brown bat, Eptesicus fuscus Figure 4. Mexican free-tailed bat, Tadarida brasiliensis

Solitary bats typically roost in tree foliage or under bark, but occasionally are found associated with buildings, some only as transients during migration. These include Keen’s bat (Myotis keenii), the red bat (Lasiurus borealis), the silver-haired bat (Lasionycteris noctivagans), and the hoary bat (Lasiurus cinereus). Excellent illustrations of all bats discussed herein can be found in Barbour and Davis (1979), Tuttle (1988), Gelusoetal. (1987), and Harvey (1986).

Several species of bats have been included here, with significant interspecific differences that need to be clarified if well-planned, comprehensive management strategies are to be developed. Any problems caused by bats are limited to species distribution; thus nuisance wildlife control personnel need not be concerned with every species.

Colonial and solitary bats have obvious differences that serve to separate the species into groups (refer to Fig. 5). Much of the descriptive material that follows is adapted from Barbour and Davis (1979).

Figure 5. Anatomy of a typical bat.
Figure 5. Anatomy of a typical bat.

Colonial Bats

Little brown bat (Myotis lucifugus)

Recognition. forearm — 1.34 to 1.61 inches (3.4 to 4.1 cm); wingspan — 9.02 to 10.59 inches (22.9 to 26.9 cm); ears — 0.55 to 0.63 inches (1.4 to 1.6 cm); foot — approximately 0.39 inches (1.0 cm); long hairs on toes extend beyond claws.

Distribution. (Fig. 6a)

Color. Pale tan through reddish brown to dark brown, depending on geographic location. The species is a rich dark brown in the eastern United States and most of the west coast. Fur is glossy and sleek.

Confusion may occur with a few other “house” bat species. In the East, it may be confused with Keen’s bat (M. keenii), which has longer ears [0.69 to 0.75 inches (1.7 to 1.9 cm)] and a longer, more pointed tragus (the appendage at the base of the ear). In the West, it resembles the Yuma myotis (M. yumanensis), which has dull fur and is usually smaller. However, the Yuma myotis and little brown may be indistinguishable in some parts of the northwestern United States where they may hybridize.

Figure 6. Distributions of selected bat species in North America: (a) little brown bat, (b) big brown bat, (c) Mexican free-tailed bat, (d) pallid bat, (e) Yuma myotis, (f) evening bat, (g) Keen’s bat, (h) red bat, (i) silver-haired bat, (j) hoary bat.
Figure 6. Distributions of selected bat species in North America: (a) little brown bat, (b) big brown bat, (c) Mexican free-tailed bat, (d) pallid bat, (e) Yuma myotis, (f) evening bat, (g) Keen’s bat, (h) red bat, (i) silver-haired bat, (j) hoary bat.

Habits. This is one of the most common bats found in and near buildings, often located near a body of water where they forage for insect prey. Summer colonies are very gregarious, commonly roosting in dark, hot attics and associated roof spaces where maternity colonies may include hundreds to a few thousand individuals. Colonies may also form beneath shingles and siding, in tree hollows, beneath bridges, and in caves. Litter size is 1 in the Northeast; twins occasionally occur in some other areas. The roost is often shared with the big brown bat (E. fuscus) though the latter is less tolerant of high temperatures. M. keenii may also share the same site. Separate groups of males tend to be smaller and choose cooler roosts within attics, behind shutters, under tree bark, in rock crevices, and within caves.

In the winter, little brown bats in the eastern part of their range abandon buildings to hibernate in caves and mines. Such hibernacula may be near summer roosts or up to a few hundred miles (km) away. Little is known of the winter habits of M. lucifugus in the western United States.

The life span of little brown bats has been established to be as great as 31 years. The average life expectancy, however, is probably limited to only a few years.

Big brown bat (Eptesicus fuscus)

Recognition. forearm — 1.65 to 2.01 inches (4.2 to 5.1 cm); wingspan — 12.80 to 13.78 inches (32.5 to 35.0 cm); ears — with rounded tragus.

Distribution. (Fig. 6b)

Color. From reddish brown, copper colored, to a dark brown depending on geographic location. This is a large bat without distinctive markings. Confusion may occur with the evening bat (Nycticeius humeralis) though the latter is much smaller.

Habits. This hardy, rather sedentary species appears to favor buildings for roosting. Summer maternity colonies may include a dozen or so, and up to a few hundred individuals, roosting behind chimneys, in enclosed eaves, in hollow walls, attics, barns, and behind shutters and unused sliding doors. They also form colonies in rock crevices, beneath bridges, in hollow trees, and under loose bark. Litter size is 2 in the East to the Great Plains; from the Rockies westward 1 young is born.

E. fuscus frequently shares roosts with M. lucifugus in the East, and with M. yumanensis, Taderida, and Antrozous in the West. Males typically roost in smaller groups or alone during the summer.

The big brown bat is one of the most widely distributed of bats in the United States, and is probably familiar to more people than any other species. This is partially due to its large, easy-to-observe size, but also to its ability to overwinter in buildings (attics, wall spaces, and basements). Its close proximity to humans, coupled with its tendency to move about when temperature shifts occur, often brings this bat into human living quarters and basements in summer and winter. Big browns also hibernate in caves, mines, storm sewers, burial vaults, and other underground harborage.

While E. fuscus will apparently travel as far as 150 miles (241 km) to hibernacula, the winter quarters of the bulk of this species are largely unknown. Big brown bats may live as long as 18 years.

Mexican free-tailed bat (Tadarida brasiliensis)

Recognition. forearm — 1.42 to 1.81 inches (3.6 to 4.6 cm); wingspan — 11.42 to 12.80 inches (29.0 to 32.5 cm); long narrow wings; tail (interfemoral) membrane — does not enclose the lower one-third to one-half of the tail, hence the name free-tailed; foot — long, stiff hairs as long as the foot protrude from the toes.

Distribution. (Fig. 6c)

Color. Dark brown or dark gray. Fur of some individuals may have been bleached to a pale brown due to ammonia fumes from urine and decomposing guano. Confusion is not likely to occur with other species that commonly inhabit human buildings.

Habits. T. brasiliensis forms the largest colonies of any warm-blooded animal, establishing sizable colonies in buildings, particularly on the West Coast and in the Gulf states from Texas east. Hundreds to thousands may be found in buildings or under bridges. It is primarily a cave bat in Arizona, New Mexico, Oklahoma, and Texas. Buildings are used as temporary roosts during migrations. Litter size is 1.

Taderida often share roosts with other species. In the West, for example, they may be found in buildings with A. pallidus, M. yumanensis, and E. fuscus. Some males are always present in the large maternity colonies, but they tend to segregate in separate caves.

A few Taderida may overwinter in buildings as far north as South Carolina in the East, and Oregon in the West. Most of this species migrate hundreds of miles to warmer climes (largely to Mexico) for the winter.

Pallid bat (Antrozous pallidus)

Recognition. forearm — 1.89 to 2.36 inches (4.8 to 6.0 cm); wingspan — 14.17 to 15.35 inches (36.0 to 39.0 cm); ears — large, widely separated and more than half as broad as long. The ears are nearly half as long as the combined length of the bat’s head and body; eyes — large.

Distribution. (Fig. 6d)

Color. pale, upper parts are light yellow, the hairs tipped with brown or gray. Under parts are pale creamy, almost white. This large, light-colored bat is relatively easy to recognize. Confusion with other species that commonly inhabit human buildings is not likely to occur.

Habits. Maternity colony size ranges from about 12 to 100 individuals. Roost sites include buildings, bridges, and rock crevices; less frequently, tree cavities, caves, and mines. Litter size is most commonly 2. The roost is frequently shared with T. brasiliensis and E. fuscus in the West. While groups of males tend to segregate during the nursery period (sometimes in the same building), other males are found within the maternity colony.

An interesting feature of pallid bats is that they fly close to the ground, may hover, and take most prey on the ground, not in flight. Prey includes crickets, grasshoppers, beetles, and scorpions. They will also forage among tree foliage.

Pallid bats are not known to make long migrations, though little is known of their winter habits.

Yuma myotis (Myotis yumanensis)

Recognition. forearm — 1.26 to 1.50 inches (3.2 to 3.8 cm); wingspan — about 9.25 inches (23.5 cm); ears — 0.55 to 0.59 inches (1.4 to 1.5 cm); foot — 0.39 inches (1.0 cm).

Distribution. (Fig. 6e)

Color. Light tan to dark brown; underside is whitish to buffy. Confusion may occur in the West with M. lucifugus, though the latter tends to have longer, glossier fur, and is larger. In the Northwest, hybridization occurs with M. lucifugus, making the species indistinguishable.

Habits. Maternity colonies, up to several thousand individuals, form in the summer in attics, belfries, under bridges, and in caves and mines. Litter size is 1. Males typically segregate during the nursery period and roost as solitary individuals in buildings and other suitable harborage.

M. yumanensis is more closely associated with water than is any other North American bat species. Nearly all roosts have open water nearby. This species is not as tolerant as M. lucifugus of high roost temperatures and will move to cooler niches within a building when temperatures raise much above 100o F (37.8o C). M. yumanensis abandons maternity colonies in the fall, but its winter habitat is not known.

Evening bat (Nycticeius humeralis)

Recognition. forearm — 1.30 to 1.54 inches (3.3 to 3.9 cm); wingspan — 10.24 to 11.02 inches (26.0 to 28.0 cm); ears — with short, curved, and rounded tragus.

Confusion may occur with the big brown bat (E. fuscus), which can be readily distinguished by its larger size. It bears some resemblance to the somewhat smaller little brown bat (M. lucifugus) but can be identified by its characteristic blunt tragus.

Distribution. (Fig. 6f)

Color. Medium brown with some variation to yellow-brown in subtropical Florida. No distinctive markings.

Habits. Summer maternity colonies in buildings may consist of hundreds of individuals. Litter size is usually 2. Colonies also form in tree cavities and under loose tree bark. In the Southeast, T. brasiliensis commonly inhabits the same building with N. humeralis. This is one of the most common bats in towns throughout the southern coastal states. Very little is known about this species, and virtually nothing is known of its winter habitat except that it almost never enters caves.

Solitary Bats

Keen’s bat (Myotis keenii)

Recognition. forearm — 1.26 to 1.54 inches (3.2 to 3.9 cm); wingspan — 8.98 to 10.16 inches (22.8 to 25.8 cm); ears — 0.67 to 0.75 inches (1.7 to 1.9 cm); with a long, narrow, pointed tragus.

Distribution. (Fig. 6g)

Color. Brown, but not glossy; somewhat paler in the East. Confusion may occur with M. lucifugus, which has glossy fur, shorter ears, and does not have the long, pointed tragus.

Habits. Excluding small maternity colonies (up to 30 individuals are on record), M. keenii are generally found singly in the East. Roosting sites include: behind shutters, under wooden shingles, sheltered entryways of buildings, in roof spaces, in barns, and beneath tree bark. In the West, this bat is known as a solitary species, roosting in tree cavities and cliff crevices. Litter size is probably 1. The roost is sometimes shared with M. lucifugus. The sexes probably segregate during the nursery period. In winter, these bats hibernate in caves and mines.

Red Bat (Lasiurus borealis)

Recognition. forearm — 1.38 to 1.77 inches (3.5 to 4.5 cm); wingspan — 11.42 to 13.07 inches (29.0 to 33.2 cm); long, pointed wings; ears — short rounded; tail membrane — heavily furred on upper surface, with a distinctive long tail.

Distribution. (Fig. 6h)

Color. Bright orange to yellow-brown; usually with a distinctive white mark on the shoulders. Confusion may occur with the hoary bat (L. cinereus), which is frosted-gray in appearance and larger.

Habits. Red bats live solitary lives, coming together only to mate and migrate. Few people are familiar with this species. They typically spend summer days hidden in the foliage of deciduous trees. The number of young ranges from 1 to 4, averaging 2.3.

These bats often chase insects that are attracted to lights, such as street lamps. It is this behavior that most likely brings them in close proximity to people.

L. borealis is well-adapted for surviving drastic temperature fluctuations; it does not hibernate in caves, but apparently in trees. Some migrate long distances. During migration, red bats have been known to land on high-rise buildings and on ships at sea.

Silver-haired bat (Lasionycteris noctivagans)

Recognition. forearm — 1.46 to 1.73 inches (3.7 to 4.4 cm); wingspan — 10.63 to 12.20 inches (27.0 to 31.0 cm); ears — short, rounded, hairless; tail membrane — upper surface is sparsely furred on the anterior one-half.

Distribution. (Fig. 6i)

Color. Usually black with silver-tipped fur; some individuals with dark brown, yellowish-tipped fur. Confusion sometimes occurs with the larger hoary bat (Lasiurus cinereus), which has patches of hair on the ears and wings, heavy fur on the entire upper surface of the tail membrane, and has a distinctive throat “collar.”

Habits. The silver-haired bat roosts in a wide variety of harborages. A typical roost might be behind loose tree bark; other sites include tree hollows and bird nests. This species is solitary except when with young. Additionally, there are unconfirmed reports that it is sometimes colonial (Dalquest and Walton 1970) and may roost in and on buildings. The litter size is 2. The sexes segregate through much of the summer range.

L. noctivagans hibernates in tree crevices, under loose bark, in buildings (including churches, sky scrapers, and wharf houses), hulls of ships, rock crevices, silica mines, and non-limestone caves. It also may migrate, during which time it is encountered in buildings (they favor open sheds, garages, and outbuildings rather than enclosed attics), in lumber piles, and on ships at sea.

Hoary bat (Lasiurus cinereus)

Recognition. forearm — 1.81 to 2.28 inches (4.6 to 5.8 cm); wingspan — 14.96 to 16.14 inches (38.0 to 41.0 cm); ears — relatively short, rounded, edged with black, and with fur; tail membrane — completely furred on upper surface.

Distribution. (Fig. 6j)

Color. Dark, but many hairs are tipped in white, giving it a frosted appearance. This bat also has a yellowish or orangish throat “collar.” Confusion may sometimes occur with the much smaller silver-haired bat (Lasionycteris noctivagans), which lacks the fur patches and markings on the ears, markings on the throat, and has a tail membrane that is only lightly furred on the upper surface.

Habits. Hoary bats generally spend summer days concealed in tree foliage (often in evergreens), rarely enter houses, and are not commonly encountered by people. L. cinereus at their day roosts are usually solitary except when with young. The litter size is 2. The sexes segregate through most of the summer range.

This is one of the largest bats in North America, a powerful flier, and an accomplished migrant. Records indicate that some L. cinereus may hibernate in northern parts of their range.

Bats | Bat Overview | Bat Damage Assessment | Bat Damage Management | Bat Resources | Bat Acknowledgments | ICWDM | Wildlife Species Information

Food Habits

Bats in North America are virtually all insectivorous, feeding on a variety of flying insects (exceptions among house bats were noted previously). Many of the insects are harmful to humans. While there must be some limitations based on such factors as bats’ body size, flight capabilities, and jaw opening, insectivorous bats apparently consume a wide range of prey (Barbour and Davis 1979). The little brown bat’s diet includes mayflies, midges, mosquitoes, caddis flies, moths, and beetles. It can consume insects equal to one-third of its body weight in 1/2 hour of foraging. The big brown bat may fill its stomach in about 1 hour (roughly 0.1 ounce per hour [2.7 g/hr]) with prey including beetles, moths, flying ants, true bugs, mayflies, caddis flies, and other insects. The nightly consumption of insects by a colony of bats can be extremely large.

General Biology, Reproduction, and Behavior

Most North American bats emit high frequency sounds (ultrasound) inaudible to humans and similar to sonar, in order to avoid obstacles, locate and capture insect prey, and to communicate. Bats also emit audible sounds that may be used for communication between them.

Bats generally mate in the fall and winter, but the female retains the sperm in the uterus until spring, when ovulation and fertilization take place. Pregnant females may congregate in maternity colonies in buildings, behind chimneys, beneath bridges, in tree hollows, caves, mines, or other dark retreats. No nests are built. Births typically occur from May through July. Young bats grow rapidly and are able to fly within 3 weeks. Weaning occurs in July and August, after which the nursery colonies disperse.

Bats prepare for winter around the time of the first frost. Some species migrate relatively short distances, whereas certain populations of the Mexican free-tailed bat may migrate up to 1,000 miles (1,600 km). 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. Some may fly during warm winter spells (as big brown bats may in the northeastern part of the United States). Bats often live more than 10 years.

In response to a variety of human activities, direct and indirect, several bat species in the United States have declined in number during the past few decades. Chemical pesticides (particularly the use of persistent and bioaccumulating organic pesticides) have decreased the insect supply, and contaminated insects ingested by bats have reduced bat populations. Many bats die when people disturb summer maternity roosts and winter hibernacula. Vandals and other irresponsible individuals may deliberately kill bats in caves and other roosts. Even the activities of speleologists or biologists may unintentionally disturb hibernating bats, which depletes fat reserves needed for hibernation.

Modification and destruction of roost sites has also decreased bat numbers. Sealing and flooding of mineshafts and caves and general quarrying operations may inadvertently ruin bat harborages. Forestry practices have reduced the number of hollow trees available. Some of the elimination of natural bat habitat may contribute to bats roosting in buildings.

Arthur M. Greenhall. Research Associate. Department of Mammalogy. American Museum of Natural History New York, New York 10024

Stephen C. Frantz. Vertebrate Vector Specialist. Wadsworth Center for Laboratories and Research. New York State Department of Health. Albany, New York 12201-0509


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