Bats of Colorado
The following species accounts are from
David M. Armstrong, Rick A. Adams, Kirk W. Navo, Jerry Freeman and Stephen
J. Bissell. 1994. Bats
of Colorado: Shadows of the Night. Colorado Division of Wildlife,
Denver, Colorado.
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CALIFORNIA MYOTIS Myotis californicus
Recognition: This is a small, pale-colored myotis with dark ears. The hairs
of the dorsum are yellowish brown and lack burnished tips. The braincase rises
abruptly above the rostrum. Total length of nine individuals from northwestern
Colorado averaged 83.4 mm, length of forearm averaged 33 mm and weight averaged
4.8 g. Wingspan is about 210 mm. The only other Colorado myotis as small as
the California myotis is the western small-footed myotis, which may be darker
brownish in color, has dorsal hairs with brassy, burnished tips, a smaller hind
foot and a skull with a flat profile, the forehead rising gradually from the
rostrum. Care is needed to distinguish the species in the field, and certain
identification requires comparative material in the museum.
Distribution: Like a number of other species of myotis, this is a bat of western
North America, ranging from central Mexico north to British Columbia and from
the Pacific Coast to Colorado, where it occurs at lower elevations in valleys
and canyons along the Western Slope.
Habitat and Habits: This is a species of semi-desert habitats, including pi-on-juniper
woodland and desert scrub. The animals roost by day in crevices, mines, caves,
buildings, beneath bridges or behind loose bark. Night roosts include trees,
shrubs and structures such as porches, eaves and outbuildings. Night roosts
are dark and sheltered from wind. There may be local movements to suitable hibernacula
in caves and mines, but long-distance migration is not known. The winter range
of Colorado’s population is unknown, but the animals probably hibernate
in the state.
Breeding: Copulation occurs in fall, and sperm are stored over winter by the
female. Fertilization and implantation take place in spring. Just when implantation
occurs is not known, so the gestation period has not been calculated, but probably
it is about six weeks. A single young is born to a female in a nursery colony
perhaps in May or June or even later. Lactating females have been captured in
Colorado as late as August.
Food: The California myotis emerges in early evening to feed, just after the
tiny western pipistrelle, which has a butterfly-like flight. It forages in arroyos,
gaps between trees and near boulders and cliffs. It is active until about midnight
and then again about dawn. Typical food includes flies, moths and spiders.
Remarks: This bat appears uncommon in Colorado except locally in the canyon
country at lowest elevations on the Western Slope. Its biology here is poorly
understood.
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WESTERN SMALL-FOOTED MYOTIS Myotis ciliolabrum
Recognition: A small bat with a very small (8 mm) foot and dull to burnished
brown pelage. The ears are dark, nearly black, and a distinct facial mask is
frequent. It is almost impossible to distinguish from the California myotis,
and only specialists can tell them apart with certainty. Total length, length
of forearm and weight are 80 mm, 30 mm and 4 g, respectively. The wingspan is
about 220 mm.
Distribution: This bat is widespread and common in the western United States.
In Colorado it occurs statewide in suitable habitat. It seems to be most common
in the canyon country of the Western Slope and in rocky areas of northeastern
and southeastern Colorado.
Habitat and Habits: Despite its wide occurrence, little is known of habitat
preferences of this species, although it is known to inhabit rocky areas and
is more common at lower elevations. Summer roosts are highly variable and include
buildings, mines, under bark on trees, beneath stones and a variety of other
sites. The small-footed myotis is a year-round resident of Colorado. It hibernates
in caves and mines alone or in small groups. Despite its small size, it is known
to hibernate in open tunnels at low temperatures and low humidity, a situation
one would assume is stressful. It sometimes hibernates near other bats, including
Townsend's big-eared bat, at elevations to 9,500 feet.
Breeding: Little is known about the reproduction of small-footed myotis. Small
nursery colonies of 10 to 15 are found occasionally in caves, mines or buildings.
One young per year is usual. Young are born in mid-June following a gestation
of about two months.
Food: The western small-footed myotis feeds early in the evening on small flying
insects such as flies, small beetles and winged ants. This species is highly
maneuverable in flight, often foraging among boulders, along cliffs or shrubs
and trees.
Remarks: Like most species of Myotis, the small-footed myotis is often misidentified.
It can be confused with the western pipistrelle or other mouse-eared bats, especially
the California myotis. Earlier information on this species in Colorado was published
under the names Myotis subulatus or Myotis leibii.
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LONG-EARED MYOTIS Myotis evotis
Recognition: This species has the longest ears of any myotis found in Colorado.
Black membranes contrast with the medium yellowish-brown fur. The belly is paler
than the back. Hairs are lead-gray at their bases. Length averaged 86.4 mm,
length of forearm averages 37.6 mm, and weights average 6.1 g. The wingspan
is about 275 mm. The only species with which the long-eared myotis might be
confused is the fringed myotis, which has shorter, narrower ears and a conspicuous
fringe of stiff hairs on the trailing edge of the uropatagium.
Distribution: Like the Yuma myotis, this species ranges from central Mexico
north to British Columbia, but it ranges farther east to the western edge of
the Great Plains, including the western Dakotas and Nebraska. This species occurs
at moderate elevations throughout the western three- fifths of Colorado, at
elevations from 5,000 to 9,800 feet.
Habitat and Habits: The long-eared myotis is a species of coniferous forest,
on both sides of the Continental Divide. Ponderosa pine woodland is the most
common habitat type, although the animals also range down into piñon-juniper
woodland, where they may be abundant. Roosts are in trees (often behind loose
bark), caves, abandoned mines and other such sheltered areas. It is possible
that the long-eared myotis hibernates in Colorado, as late fall activity has
been documented in mines and caves, but individual
s
never have been found in winter.
Breeding: Reproduction has not been studied in detail, and dates of breeding
are unknown. Males with scrotal testes have been captured in July, August and
September. Females form small nursery colonies of one to three dozen individuals.
In Colorado, pregnant females are most common in June and July. Lactating females
have been captured in June, July and August. A single young is born. Probably,
the gestation period is 50 to 60 days, as in other species of myotis of similar
size.
Food: The long-eared myotis emerges after dark to forage near trees or over
water. The animals are gleaners, hovering to take prey from leaves in forest
gaps and edges. Principal food items are moths, flies, spiders and beetles.
Remarks: The long-eared myotis is not uncommon in parts of Colorado, but little
is known of its biology here or elsewhere over its range.
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LITTLE BROWN BAT Myotis lucifugus
Recognition: This is a relatively nondescript, medium-sized, small-eared bat
best recognized by eliminating the several more distinctive species that might
be confused with it. The Yuma myotis is of similar size but is paler in color
and the hairs lack metallic, burnished tips. The California myotis may be as
dark but is considerably smaller; the long-legged myotis is a heavier bat with
a keeled calcar (a spur of bone that projects inward from the ankle) and a wing
that is furred beneath from body to elbow. The fringed myotis has longer ears
(as has the long-eared myotis) and a distinctive fringe of stiff hairs on the
trailing edge of the uropatagium. Average length is 97.9 mm; average length
of forearm is 39.3 mm. Weights average about 7 g, and the wingspan is about
220-270 mm.
Distribution: The little brown bat ranges across North America, from Alaska
across Canada to Newfoundland and south, mostly in forested areas, to central
Mexico. In Colorado the species may occur statewide in suitable habitat, ranging
as high as 11,000 feet in Lake County. However, in the eastern two-fifths of
the state, there are actual records only from Greeley and Pueblo.
Habitat and Habits: This is a species of wooded areas -- including riparian
woodland in the mountains and lower valleys -- urban areas, woodlots and shelterbelts.
The little brown bat is one of the most tolerant of bats in terms of roost selection.
Night roosts are located in tree hollows, beneath tree bark, in or under buildings,
bridges, crevices in rock, behind shutters or beneath eaves. They may share
roosts with other species of bats. Day roosts in attics may be used by large
concentrations of bats. Hibernation sites include caves, mines and buildings.
Some little brown bats hibernate in Colorado, but winter habits are poorly known
here and elsewhere in the West. In Ontario, hibernation lasts from September
to May. In some parts of the range, the animals may move several hundred kilometers
from nursery colonies to hibernacula, but such long-distance movements have
not been documented in the West. The animals can move 50 miles a night at speeds
up to 19 miles per hour. Maximum longevity in these bats may be remarkably long;
the current record is some 31 years. The average lifespan, however, is much
shorter, as over half the young die in their first year. Predators include raccoons,
mink, snakes and owls.
Breeding: A great deal is known about the breeding habits of the little brown
bat in the eastern part of its range, but the species has not been studied intensively
in the West. Breeding takes place in autumn or early winter. There are two phases,
an active phase in which males and females are alert and a passive phase in
which males mate with torpid females. Breeding of both sexes is promiscuous.
Sperm are stored by the female in the uterus until spring, when fertilization,
implantation and gestation take place. Gestation lasts 50 to 60 days, depending
on temperatures. The young are born almost always singly in nursery colonies
from late May to early June. At birth, little brown bats are blind, but their
eyes open in two days. The mother can fly with her offspring attached to a nipple,
but the young usually is left in the roost during the mother's foraging flights.
Young can fly on their own by 3 weeks and reach adult weight about a month after
their first flight. About half the females breed their first autumn. Males breed
first as yearlings. Nursery colonies of several hundred females are known. Non-breeding
females and males roost away from nursery colonies.
Food: Little brown bats emerge at dusk to feed, often following the same foraging
route repeatedly through the night and on successive nights. They forage over
water or among trees, 3 to 6 meters above ground. Foraging flight is erratic
for a bat of this size. The diet consists largely of aquatic insects, including
caddis flies and midges, but also includes moths, mosquitoes and other flies.
The prey is knocked from the air with a wingtip, captured in the membrane between
the legs and lifted to the mouth. Foraging behavior has to be learned, and adults
are much more efficient feeders than are young; adults fill their stomach in
as little as 15 minutes.
Remarks: This is perhaps the most common, widespread bat of temperate North
America, and it is one of the best studied. It deserves greater attention in
Colorado, however, particularly because former colonies have disappeared or
have been reduced in size.
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FRINGED MYOTIS Myotis thysanodes
Recognition: The fringed myotis is distinguished by its relatively long ears
and a conspicuous fringe of stiff hairs on the trailing edge of the uropatagium.
The pelage varies in color from pale to rich brownish, individual hairs being
lead-gray at their bases. The membranes are dark. Total length of five individuals
from northwestern Colorado averaged 86.8 mm; length of forearm averaged 40.1
mm: and weight averaged 7.5 g. The wingspan is 265-300 mm. The only other medium-sized
myotis with long ears is the long-eared myotis, a species in which the ears
are 21-24 mm long (by contrast with ears 17-20 mm long on the fringed myotis).
Distribution: This is a western species, ranging from the Isthmus of Tehuantepec
in Mexico north to British Columbia, Montana and Wyoming. The animals apparently
occur as scattered populations at moderate elevations on the Western Slope,
along the foothills of the Front Range and the mesas of southeastern Colorado.
Maximum elevation is 7,500 feet.
Habitat and Habits: The fringed myotis is a species of coniferous forest and
woodland at moderate elevations in Colorado. Records of occurrence are few,
and the species isn't common in the state, but perhaps it is simply widely distributed.
Typical vegetation of the habitat includes ponderosa pine, pi-on, juniper, greasewood,
saltbush and scrub oak. The animals roost in rock crevices, caves, mines, buildings
and trees. They are known to hibernate in caves and buildings. Where this species
has been studied well, migration seems not to be extensive.
Breeding: Breeding takes place in fall; ovulation, fertilization, implantation
and gestation occur in spring, as sperm are stored over the winter in the female's
uterus. Up to several hundred females congregate in nursery colonies. Males
are solitary while the young are reared. In a typical season, all mature females
breed. The single young is produced after a gestation of 50 to 60 days. Newborn
young are hairless and pinkish, and their eyes are closed. Growth is rapid,
however, and they can fly by 20 days old. Maximum known longevity is 11 years.
Although some individuals may live even longer, the average lifespan is much
shorter than that.
Food: The fringed myotis feeds on such arthropods as moths, daddy longlegs and
beetles. They forage along water, above shrubs and woodlands or low over meadows
(apparently in more open areas than the long-eared myotis), emerging to feed
about two hours after sunset. Flight speeds average about 9 miles per hour.
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LONG-LEGGED MYOTIS Myotis volans
Recognition: This is Colorado's only medium-sized, short-eared myotis with a
keeled calcar (the spur of bone that projects inward from the ankle and helps
to support the uropatagium). It is a fairly heavy-bodied bat, and the medium
to dark brown fur on the back extends to the tail membrane. The short ears appear
more round, less pointed than those of other species. Hairs of the venter are
paler than those of the back and may even be buffy. The underside of the wing
usually is well furred to the elbow. Tips of individual hairs may appear slightly
burnished. Mean measurements of six males and nine females from Park County
were: length, 97.3 and 102 mm; length of forearm, 37.8 and 38.2 mm; weight,
8.7 and 11.2 g. The wingspan is 250-270 mm. The species most readily confused
with the long-legged myotis is the little brown bat, which differs in having
relatively longer ears, lacking the distinct keel on the calcar and frequently
having paler pelage.
Distribution: This species occurs throughout much of western North America from
central Mexico to extreme northwestern British Columbia and from the Pacific
Coast to the western margins of the Great Plains. The animals occur across the
western three-fifths of Colorado and the wooded mesas of southeastern Colorado
at elevations ranging from about 5,000 to 11,500 feet. This is the most common
myotis at higher elevations in the state.
Habits and Habitats: The long-legged myotis is a species of wooded areas in
foothills, mountains and high plateaus. Typical habitat is montane or subalpine
forest, ponderosa pine woodland, piñon-juniper woodland and montane shrubs
with willows or well-watered stands of sagebrush. The animals roost by day in
buildings, mines, fissures in rocks or beneath loose bark on trees. Usually
they roost singly or in small groups. Night roosts are in dark places like caves
or mines. Winter haunts of Colorado individuals are unknown, but they may hibernate
locally, with only short migrations, as late fall activity has been observed
at mines and caves. Elsewhere, they are known to hibernate in mines or caves.
Breeding: Pregnant females form small nurseries of up to several dozen individuals
in buildings, crevices or trees. Males are segregated and solitary during the
maternity season. In Colorado, males with scrotal testes have been captured
in July and August. It has been noted that the long-legged myotis has a particularly
long parturition season. Pregnant females have been noted from June to early
August in Colorado and elsewhere, and births are spread throughout the summer.
Lactating females are most abundant in samples beginning in August. Females
apparently breed their first winter. Reproduction in this species has not been
studied in detail.
Food: The long-legged myotis mostly eats moths. They emerge to feed early (while
it is light enough to observe them distinctly) and forage at heights of 3 to
4 meters over ponds, streams, open meadows or forest clearings, cruising a repetitive
circuit through the evening. Flight is more relaxed and leisurely than that
of other medium-sized species of Myotis, and experienced observers can distinguish
it by the flight pattern.
Remarks: This is one of our most common bats, and it deserves intensive study
in Colorado. Of particular interest would be some knowledge of its breeding
biology, population trends, and winter distribution.
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YUMA MYOTIS Myotis yumanensis
Recognition: This is a medium-sized, small-eared, pale grayish to reddish or
yellowish bat, its dark membranes contrasting fairly strongly with the fur.
Among Colorado's species of Myotis, only the California myotis and the small-footed
myotis are as pale in color. The small-footed myotis is more yellowish, less
grayish and both it and the California myotis are smaller than the Yuma myotis.
Total length is 86-88 mm; length of forearm, 34-38 mm; and wingspan, about 235
mm. Weight ranges from 3 to 5 g. This bat may be confused with the little brown
bat, Myotis lucifugus in the field.
Distribution: The Yuma myotis occurs widely in western North America, from central
Mexico north to British Columbia, Montana and southeastern Colorado. In Colorado
these bats occur at moderate elevations in valleys on the Western Slope, in
the San Luis Valley and on the eastern slope north to the vicinity of Colorado
Springs. They don't seem common. Maximum reported elevation of occurrence in
Colorado is about 7,900 feet at Conejos in the San Luis Valley.
Habitat and Habits: This is a species of dry shrubby country, but it appears
to be tied more closely to water than any of Colorado's other bats. Typical
habitat is in piñon-juniper woodland and riparian woodland in semi desert
valleys. The animals roost in caves, crevices or abandoned buildings and other
structures. They forage over water, along streams, over springs, among riparian
or shoreline vegetation. The Yuma myotis apparently does not hibernate in Colorado,
but its winter haunts are unknown. They arrive in Colorado about April, and
they become scarce in September.
Breeding: Breeding has not been studied much in Colorado, although lactating
females have been captured and a nursery colony was discovered in 1990 in the
Colorado National Monument. Elsewhere, the animals are known to form nursery
colonies of upwards of several thousand individuals in caves or attics. Apparently
all adult females in a population breed. A single young is born late in the
spring or early in the summer. Small breeding colonies have been discovered
in the southeastern part of the state, but not elsewhere in Colorado. Males
are solitary as the young are reared.
Food: Food consists mostly of moths, flies and beetles, but also includes leafhoppers,
caddis flies, lacewings and crane flies. The animals forage near water and take
many aquatic insects. They are efficient feeders and can fill their stomachs
in 15 to 20 minutes. They forage in early evening, usually along the main channel
of a stream.
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RED BAT Lasiurus borealis
Recognition: As the name indicates, this is a colorful bat, varying from bright
reddish orange to yellow. It is a medium-sized bat with long (300 mm) pointed
wings and a distinctly long tail and uropatagium. Average measurements of 11
females from Kansas were: total length, 114 mm; forearm, 53 mm.
Distribution: Mostly a species of the eastern United States, the red bat also
occurs in the southwestern U.S. and Mexico. In Colorado, it apparently is rare,
and mostly is found in riparian woodlands (and artificial extensions such as
urban landscaping) along the South Platte and Arkansas rivers and major tributaries.
Habitat and Habits: The red bat is solitary and roosts by day in deciduous trees,
such as cottonwoods or fruit trees. It seems to prefer American elms where it
hangs among the leaves. Red bats usually are well hidden and only can be seen
from directly below. They usually roost 4-10 feet off the ground unless they
have young, in which case they will be found up to 20 feet high in the tree.
Red bats are highly migratory and migrate in groups. Males and females apparently
migrate separately, however. Hibernation occurs in the southern states (not
in Colorado) where these bats may arouse and forage throughout the winter. Predators
include opossums, domestic cats, hawks and owls.
Breeding: Breeding occurs in the fall. Sperm cells are stored by the female
until spring when ovulation and fertilization occur. Gestation takes 80 to 90
days. The red bat typically gives birth to twins (although four young are known)
in the early summer. The young are carried by the female from one roost to another
and occasionally on feeding flights. Offspring are cared for until they are
nearly fully grown. Some reports indicate that the mother carries the young
until their weight equals her own. Young red bats are weaned in 4 to 6 weeks
and can fly at 3 to 6 weeks of age.
Food: More than other bats, red bats seem to take advantage of insects attracted
to lights, and they often are found feeding near street lamps where they appear
to be territorial. They alight and feed on non-flying insects, including grasshoppers
and crickets. They feed near moth-infested areas, which is certainly a benefit
to humans. Foraging occurs in early evening when they leave their roosts and
fly in high, lazy patterns over the trees and along forest edges. When it is
dark, they descend and feed below the treetops to within a few feet of the ground.
Remarks: The unusual hibernating behavior of this species is of special interest
to science. Red bats survive at subfreezing temperatures and are able to arouse
readily from deep sleep.
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HOARY BAT Lasiurus cinereus
Recognition: The hoary bat cannot be mistaken for any other species. The
tips of the hairs are white or grayish with brown shafts giving a frosty or
hoary effect. The ears are short and round with a striking black border. This
is one of Colorado's largest bats with a wingspan of about 400 mm, a body and
tail about 140 mm long, and a forearm 50-58 mm long. Hoary bats weigh approximately
25 g.
Distribution: The hoary bat is widespread, ranging from northern Canada to Guatemala,
and probably occurring in all states except Alaska -- even in Hawaii, where
it is the only native land mammal. In Colorado, the hoary bat probably occurs
statewide from the plains to timberline.
Habitat and Habits: The hoary bat is a solitary, tree-roosting species and can
be expected to live in any habitat with trees. It is migratory with the sexes
migrating north together in spring. During summer, the sexes separate, however,
perhaps to reduce competition for food while females give birth and rear the
young. Hoary bats arrive in Colorado in April and are gone by November. There
is no record of hibernation here.
Breeding: Most females give birth and raise their young in northern and eastern
North America, although a few females rear young in Colorado. Litter size is
generally two with a range of one to four. Birth occurs between May and July.
The mother can forage with the young clinging to her nipple, but generally the
young are left at the roost site. Young begin to fly at about 30 days of age.
Copulation usually occurs during the fall migration but may take place on wintering
grounds. Implantation of the new embryos is postponed until spring when growth
and development are completed.
Food: The hoary bat prefers moths but also feeds on beetles, wasps, grasshoppers
and even small bats. An open-air forager, the animals seem to approach moths
from the rear, severing the abdomen and thorax from the rest of the body. Flight
is swift and direct, and a keen observer can readily identify the hoary bat
by its flight pattern, body size and relatively long, narrow wings. It tends
to emerge later in the evening to forage than most other local bats. In Colorado
its foraging period seems to be varied, however, due to the vagaries of local
weather. The hoary bat can often be seen foraging with the big brown bat near
street lamps or other outdoor lights.
Remarks: Little is known about the natural history of this species in Colorado;
it deserves more study.
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SILVER-HAIRED BAT Lasionycteris noctivagans
Recognition: A medium-sized bat with black hair tipped in silver or yellow;
the silver-haired bat is one of Colorado's most distinctive and attractive mammals.
Wingspan ranges from 270 to 310 mm. Average measurements of 10 males from northwestern
Colorado were: total length, 104 mm; forearm, 39 mm; and weight, 12 g.
Distribution: This is a northern species that occurs throughout Alaska, Canada
and all of the contiguous United States except Florida. In Colorado it is found
statewide but is more common in the forested mountains at elevations up to 10,000
feet.
Habitat and Habits: The silver-haired bat is usually solitary, but it is occasionally
found in pairs. It roosts in trees, under bark, under rocks and infrequently
in open soft-walled caves or mines. Silver-haired bats also roost in open buildings,
such as sheds or garages. It is seldom found in closed structures, such as the
attics of houses. This bat occasionally has been found hibernating in mines
in Colorado but probably usually migrates elsewhere to spend the colder months.
Field data indicate the silver-haired bat is most abundant during late spring
and early fall when both sexes are found in the state. Predators include skunks,
owls and probably snakes.
Breeding: Little is known of the reproductive habits of this species. Two young
are produced in early summer. Mating probably occurs in autumn with the female
storing sperm during the winter. Ovulation occurs in April or May. Gestation
is 50-60 days. There is some evidence that the sexes are segregated during summer.
Silver-haired bats occasionally form fairly large nursery colonies of 200 or
more females and their young.
Food: Like all Colorado bats, the silver-haired bat is an insect-feeder, preferring
moths, flies, beetles and wasps. The silver-haired bat is a slow flyer, often
seen foraging near the ground. It commonly flies around ponds and woodland streams.
Remarks: The slow flight of this bat is unusual. The animals are hooked occasionally
by anglers fly-fishing. Because silver-haired bats roost in tree hollows and
behind loose bark, clear-cutting and removal of standing dead trees may affect
their distribution and abundance.
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WESTERN PIPISTRELLE Pipistrellus hesperus
Recognition: The western pipistrelle is the smallest of Colorado's bats. It
is easily recognized by its slow, erratic, butterfly-like flight. It has short,
black ears, grayish brown pelage and a distinctive black mask. It is one of
the most attractive bats in North America. It weighs less than 4 g, little more
than a penny. Other measurements are: wingspan, 190-215 mm; total length, 68-75
mm; and forearm, 29-31 mm.
Distribution: This species is found throughout the desert southwest. It ranges
as far north as Washington state and east to Oklahoma. In Colorado, it is found
at lower elevations on the Western Slope and in the extreme southeast.
Habitat and Habits: This is a bat of the desert. It lives in arid canyons or
dry shrub lands, near water. The western pipistrelle rarely roosts in human-made
structures, nor does it seem to use mines or caves much except as hibernacula.
It does roost in dense vegetation and beneath rocky slabs. Although the western
pipistrelle doesn't use the same roost day after day, it roosts in the same
general area. Day and night roosts are different. This bat is non-migratory
and sedentary. The only hibernaculum discovered to date in Colorado was in a
gold mine at 9,500 feet in the La Plata Mountains above Mancos. Predators are
probably snakes, birds of prey and other bats.
Breeding: One or two young are born in June each year. The western pipistrelle
was confirmed to breed in Colorado when an infant bat was discovered on the
floor of an abandoned building at Rio Blanco Reservoir, and a lactating female
and a juvenile were observed at the Colorado National Monument. These bats may
form small nursery colonies of 20-50 females and young. The young are very small
and exhibit the black mask at an early age. Males are segregated from females
during much of the summer. Copulation occurs in the hibernaculum, followed by
ovulation in spring.
Food: Western pipistrelles eat small moths, beetles, mosquitoes and other flies.
The western pipistrelle emerges very early to forage, sometimes before sundown,
and again in early morning. The animals forage near canyon walls and among scattered
boulders and shrubs. They remain active throughout the year. Foraging seems
to be limited by winds above 10 mph.
Remarks: The size of this bat can only be appreciated at close range. In the
air, it appears fragile. The slightest breeze blows it off course or causes
it to stall.
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EASTERN PIPISTRELLE Pipistrellus subflavus
Recognition: Slightly larger than the western pipistrelle, the eastern pipistrelle
is readily distinguished by its tri-colored dorsal hairs, dark brown at tips
and bases with a band of yellow between. Measurements include: total length,
70-90, forearm, 30-35, wingspan about 210; weight 5-8 g.
Distribution: This is a bat of eastern North America, ranging from Nova Scotia
and Minnesota south to Mexico, Guatemala and Belize. There are a couple records
of this bat from eastern Colorado.
Habitat and Habits: The eastern pipistrelle is an animal of open deciduous forests.
The animals are not strongly migratory, typically spending the summer in the
same general region as the hibernaculum.
Breeding: Adults in reproductive condition have been captured in the southeastern
U.S. in winter and spring. Twin births are the rule in late spring or early
summer.
Food: Eastern pipistrelles emerge early, while still light, to forage in clearings
or over still water for smaller insects, including moths, flies and beetles.
Remarks: The number of records of this species in Colorado suggest that until
information to the contrary develops, the conservative course is to consider
it an accidental occurrence.
BIG BROWN BAT Eptesicus fuscus
Recognition: This is a large bat with medium-sized, rounded ears. The wings
are long (325-350 mm) and powerful. Color is always some shade of brown, varying
from russet to almost black and chocolate. This is the most commonly seen bat
in Colorado. Average measurements of 10 females from Fort Collins were: total
length, 115 mm; forearm, 48 mm; and weight, 17 g.
Distribution: The big brown bat is found widely in the New World, from Alaska
to South America. It can be found throughout the United States in a variety
of habitats. In Colorado it can be encountered just about everywhere, especially
in towns and cities associated with buildings.
Habitat and Habits: The big brown bat is more closely associated with humans
and development than any bat in North America. It is often found in cities and
towns and around structures. The big brown bat congregates in large groups in
summer, and when these groups take up temporary residence in an attic, they
can be both an annoyance and an effective control on flying insects. They also
use rock crevices for roosting. Temperature seems to be a factor in roost selection,
females preferring higher temperatures than males. Big brown bats are year-round
residents of Colorado, hibernating in mines, caves, fissures, storm sewers and
a variety of other places. Prior to hibernation, they store up to 30 percent
of their body weight in fat reserves. This bat has been studied widely for its
remarkable homing ability. It can return to its roost site from distances of
over 400 miles in less than a month, and returns of up to 40 miles in a single
night are known. Predators include snakes, hawks, owls, weasels, house cats,
and raccoons.
Breeding: As with most Colorado bats, mating occurs in the fall, the sperm over
winter in the female's uterus and the young are born in midsummer, following
a gestation of approximately two months. In the West, a single young per year
is the rule; twins are common in the East. Females congregate in colonies of
up to 300 individuals. Mothers apparently can recognize their own offspring
even in masses of newborn bats. When young bats occasionally fall to the ground
or floor, there is evidence that mothers recover them and put them back in the
roost. Female big brown bats will move their young from one roost site to another
if disturbed.
Food: Big brown bats are generalist insect feeders. They eat a variety of larger,
flying insects and beetles, even some too large for these bats to fly away with.
Given their habit of living near humans, they are important in keeping urban
insect populations under some control. Studies indicate that big brown bats
eat many insects considered detrimental to humans. When big brown bats emerge
to forage in the evening, they make a strong, straight flight at a height of
20-30 feet above the ground. This flight is highly predictable, and the groups
can be seen at the same location for weeks at a time. Foraging is generally
in open areas, where they may be seen with the hoary bat. The species probably
do not compete, however, as their diets differ somewhat.
Remarks: The big brown bat is beneficial to humans, but it has suffered both
because and in spite of itself. The insecticide DDT, once used in this country
to control problem insects, is highly toxic to big brown bats. Ironically, a
natural insect control was destroyed by humans in their efforts to control insects
artificially.
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SPOTTED BAT Euderma maculatum
Recognition: Colorado's rarest bat is perhaps its most striking. The spotted
bat has enormous pink ears and three large white spots on the black dorsal surface.
Measurements of a Colorado specimen are: total length, 119 mm; length of forearm,
52 mm; weight, 13.5 g.
Distribution: This species is known from scattered localities in western North
America and apparently is one of the more difficult bats to capture, leading
to a perception that this bat is rare in the United States. In Colorado, the
spotted bat is known by published records from the vicinity of Dinosaur National
Monument, and from a number of places at lower elevations on the Western Slope,
including the Four Corners area where pregnant or lactating females have been
captured. Further, acoustic records show that distribution of this species in
Colorado is relatively widespread. Similarly, this species may be more widespread
across the United States than previously thought.
Habitat and Habits: The spotted bat has been found in a variety of habitats,
including ponderosa pine, piñon-juniper woodland and shrub desert. Early
researchers suggested that this bat favored ponderosa pine forests, but more
recent investigations suggest that the species may prefer areas with cliffs
and water. The species appears mostly solitary, forming small nursery colonies
or groups in hibernation. Little is known about hibernation or annual movement
patterns, although they have been found in buildings in Nevada in September.
Details of mortality are unknown, although known predators include kestrels
and owls.
Breeding: Little is known of the reproductive biology of this species. A single
young is born, probably in late May or mid-June. One newborn weighed 4 g, 25
percent of the weight of its mother.
Food: The preferred food of spotted bats is moths. Apparently only the abdomens
are eaten as the spotted bat has been observed to remove and discard wings and
heads of captured prey. Other food items include beetles, katydids and grasshoppers.
Foraging occurs throughout the night, in open habitat, 5-10 m above the ground.
The echolocatory calls are audible to humans.
Remarks: This species may be found in the major canyons along the western border
of the state and southeastern Colorado. Its unique appearance should enable
correct identification. Any sighting of the spotted bat should be reported to
the Colorado Division of Wildlife.
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TOWNSEND’S BIG-EARED BAT Corynorhinus townsendii
Recognition: This bat's ears are remarkable -- reaching a length of 38 mm. The
face is marked by a large lump on either side of the snout. Color ranges from
pale brown to slate gray. Females are larger than males. Measurements include:
total length, 90-112 mm; length of forearm, 39-48 mm; and weight, 9-14 g. Wingspan
is approximately 280 mm.
Distribution: This is a bat of western North America, ranging from southern
British Columbia to southern Mexico. Townsend's big-eared bat can be found throughout
Colorado. Its distribution seems to be determined by availability of roosts,
such as caves, mines, tunnels, crevices and masonry structures with suitable
temperatures, making the conservation of suitable roosts essential to the management
of this species.
Habitat and Habits: This bat is generally solitary or gathers in small groups,
although during summer females may form larger maternity colonies. Townsend's
big-eared bat can be found in mines, caves and structures in woodlands and forests
to elevations above 9,500 feet. They often hang near the entrances to roosts,
in the "twilight zone." The animals do not make major migrations and
appear relatively sedentary. Hibernacula have low and stable temperatures, sometimes
with moderate airflow, during late October to April. Hibernating, the bats hang
singly or in small clusters in the open, not in crevices, with pelage erect
to provide maximum insulation and the ears coiled back like a ram's horns, perhaps
to reduce heat loss. The bat is quite sensitive to changes in temperature and
humidity within the hibernaculum and may arouse to move to a more favorable
location. Care should be taken not to disturb them because paying the price
of accidental arousal in winter could deplete energy stores and prevent arousal
in spring. Populations may be principally limited by high winter mortality due
to the absence of roosts with stable temperatures. Most mortality occurs during
the first year of life. Predators have not been documented, but snakes, owls
and hawks probably take these bats.
Breeding: Copulation, which occurs in late fall, is preceded by ritualized courtship
behavior by the male. The female stores sperm in the reproductive tract until
spring when ovulation and fertilization happen. Gestation takes 50-60 days.
Young are born in mid-June, and strong bonds form between mother and offspring.
About 90 percent of all females in the nursery colonies produce young. Young
fly in 2 to 3 weeks and are weaned by 6 weeks. Only one young is born per female.
Food: This bat feeds mainly on small moths, but also eats beetles, flies and
wasps. Townsend's big-eared bat usually is a late flier (except for females
from maternity roosts) and forages along the edge of vegetation. The animals
sometimes glean insects from the vegetation. Like other species, this bat may
use a night roost and then feed a second time just before dawn and the return
to its day roost.
Remarks: Populations, especially in the nursery and hibernaculum, are highly
susceptible to disturbance and have been reported to be declining. Little is
known of the natural history of this species in Colorado, but the animals certainly
stand to benefit from the Bats/Inactive Mines Project of the Colorado Division
of Wildlife.
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PALLID BAT Antrozous pallidus
Recognition: The pallid bat is a large, pale, rather long-eared bat, not easily
confused with any other species. Their eyes are large, and their face is covered
with wart-like sebaceous glands. Color varies from pale creamy to light brown.
Average measurements of 12 individuals from northwestern Colorado were: total
length, 107 mm; forearm, 56 mm; and weight, 19 g. Wingspan is approximately
380 mm.
Distribution: The range of the pallid bat extends from southern British Columbia
to central Mexico and east to central Kansas and Oklahoma. In Colorado this
bat inhabits lower elevations of the Western Slope, broken rocky areas of the
southeastern part of the state and along the foothills to Colorado Springs.
A report of an isolated colony at Torrington, on the plains of eastern Wyoming,
suggests that the pallid bat could be found in rocky habitats or structures
on the plains of eastern Colorado.
Habitat and Habits: This is a species of deserts and grasslands near rocky outcrops.
The pallid bat's habit of using structures built by humans may allow it to extend
beyond what would otherwise be its natural range. In Colorado, the pallid bat
occupies semidesert scrub and piñon-juniper woodlands to about 7,000
feet. Pallid bats are gregarious, although males may separate from breeding
females in summer. Elsewhere, this species is not migratory and makes only short
movements to hibernation sites. Pallid bats probably hibernate in Colorado from
mid-October to April, although there is no direct evidence of their winter habits
here. Pallid bats form small colonies and may use rock crevices as day roosts.
These roosts are selected for suitable temperature and protection from predators.
Night roosts are also selected on the basis of a temperature that minimizes
energy loss. Night roosts, such as crevices, shallow caves, overhangs, and man-made
structures, are usually near day roosts. Both day and night roosts change seasonally
as their thermal characteristics change. Mortality is highest when young bats
are first beginning to fly. Their habit of foraging on the ground makes pallid
bats susceptible to injury and predation. Predators include hawks, kestrels,
owls and snakes.
Breeding: Copulation occurs between October and December, and sperm are stored
by the females until spring, when ovulation, fertilization and implantation
happen. Gestation is about 9 weeks. Older females generally give birth to two
young, whereas younger females have only one. Young are born in June and July,
and the sex ratio of the newborn is 1 to 1. Young begin to fly between 28 and
35 days old and are completely weaned at 6 to 8 weeks. Mothers forage with the
young until the young become independent.
Food: The pallid bat feeds principally on flightless ground-dwelling arthropods,
such as crickets, grasshoppers, beetles, scorpions and spiders. Their presence
is revealed by audible clicks as they forage after full dark and again before
sunrise. Other foods include large moths, lizards, small bats and small rodents.
Remarks: Social behavior of pallid bats is quite interesting. There is a strong
bond between mother and offspring, and the association may continue year after
year. There is also strong group cohesion and roost site attachment. Young are
provided preferential positions in roosts, pregnant females are attentive to
other females in labor and females actually signal mothers whose young are in
distress.
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BRAZILIAN FREE-TAILED BAT Tadarida brasiliensis
Recognition: This species is small and gray-brown with long, narrow wings and
a tail that extends well beyond the membrane between the legs. Individuals weigh
8-12 g and have short ears that almost join at the midline of the forehead.
Wingspread is approximately 300 mm, total length is 90 to 105 mm and the forearm
is 36 to 46 mm long.
Distribution: Brazilian free-tailed bats range from southern Oregon and Nebraska
to South America. In Colorado, this species seems confined to the southern half
of the state. Previously the Brazilian free-tailed bat was considered only a
wanderer in Colorado, but it is now known to be a summer resident.
Habitat and Habits: The Brazilian free-tailed bat roosts in caves and mines
and is often found in man-made structures. The animals are highly social. In
Arizona, New Mexico and Texas, nursery colonies of 10 to 20 million individuals
have been reported. Males generally form small colonies farther north, although
a colony in Colorado has an estimated population of as many as 250,000 individuals.
This bat does not hibernate in Colorado. The species is highly migratory and
travels south to Mexico and Central America for winter. There may be distinct
migratory pathways. Some apparently live to be 15 years old, but most have a
considerably shorter life span. Predators include owls, kestrels, various hawks,
raccoons, skunks and snakes.
Breeding: Brazilian free-tailed bats breed on their winter ranges. Gestation
is 90 to 100 days, and a single young is born in mid-June or early July. Although
most young are born south of Colorado, a few are produced here. At birth, infants
weight 25 percent of the mother's weight, about 2.5 to 3 g. For many years it
was believed that mothers in colonies of hundreds of thousands or even several
million females probably did not nurse their own young, but simply fed the first
young to grasp the nipple. However, it has now been demonstrated that females
locate their offspring by auditory and olfactory cues. Young begin to fly at
about 5 weeks and are weaned shortly thereafter.
Food: This bat eats mostly small moths, although beetles, bugs, mosquitoes and
wasps are also taken. The large population of this species in south-central
Colorado consumes tons of insects each year. The bats forage high over large
open areas, leaving their roosts in a flight formation that resembles a long
plume of dark smoke. They may forage up to 40 miles from their day roosts.
Remarks: Populations of this species are declining across their wide range.
Reasons for the decline seem to be disease, pesticide poisoning and human disturbance
of nursery colonies. As other bats, this valuable species deserves continued
respect, study and protection.
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BIG FREE-TAILED BAT Nyctinomops macrotis
Recognition: Like the Brazilian free-tailed bat, these animals have a tail that
extends well beyond the uropatagium. This bat is much larger than the Brazilian
free-tailed bat, and its ears join at the midline of the forehead. Color varies
from pale brown to black. Wingspan is approximately 425 mm. Average measurements
are: total length, 140 mm; length of ear, 25 mm; length of forearm, 60 mm.
Distribution: The big free- tailed bat occurs mainly in southern California,
Arizona, New Mexico, Texas and Mexico. Long thought to be an accidental wanderer
in Colorado, recent data show the presence of breeding colonies in Colorado.
Indeed, audible echolocation calls and recordings provide evidence of widespread
occurrence across Colorado.
Habitat and Habits: This bat frequents rocky or canyon country
where it roosts in crevices. This migratory species is a swift, powerful flier,
and occasional individuals wander as far north as Canada. Little is known of
mortality and longevity.
Breeding: Breeding probably occurs in midwinter while the species is in warmer
latitudes. A single young is born in mid-June to early July. Females form small
nursery colonies, and the young do not leave the nursery until they are almost
full grown.
Food: Moths seem to be the mainstay of their diet, although little data has
been collected. This bat emerges late in the evening and forages at high altitudes.
The echolocatory calls are audible to humans.
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