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Biologist removes a Townsend's long-eared bat from a mist net. Photo by USFWS; Ann Froschauer

Biologist removes a Townsend's big-eared bat from a mist net. Photo by USFWS; Ann Froschauer

Bat Facts Calendar



Sun Mon Tue Wed Thu Fri Sat
July is a busy month for Indiana bat researchers.  Mist netting is their most common method to capture Indiana bats for summer studies.   Mist nets have a very fine mesh and are set where bats are expected to be flying, often across a stream in a wooded area.  Researchers quickly and carefully retrieve any bats that entangle in the net. 
Researchers often radio-tag Indiana bats captured in mist nets.  They glue very small radio transmitters to the bat’s back so they can track the bat's movements.  Transmitter weight has to be minimal, ideally no more than 5% of body weight, or it will affect flight. For Indiana bats, transmitters weigh about as much as 20 grains of rice. 
Researchers attach radio transmitters to bats with glue made for use on human skin.  The glue is non-toxic and temporary. Both the glue and the transmitter batteries last about 2 weeks.  When the glue gives way, the transmitter drops off.
Happy 4th of July! Tonight will find many looking into the night sky enjoying fireworks. Another good way to enjoy a summer evening is bat watching. Bat watching is big business in Austin, Texas, where more than a million bats can be seen emerging from under the Congress Avenue Bridge. 
Thirty one years ago today, a maternity colony of the endangered gray bat (Myotis grisescens) was discovered in a flooded, inactive limestone quarry in Indiana.  This was one of the most northerly gray bat maternity colonies and the only one in Indiana.  Owners of this private site adopted voluntary measures to conserve this colony of over 6,000 bats.
Gray bats form maternity colonies in caves, unlike Indiana bats that form maternity colonies in trees.  These colonies are extremely susceptible to human disturbance. Young bats cling to their mothers and may lose their grip or be dropped if startled. Female gray bats may also abandon a site if they are disturbed.
Indiana bats use many types of forests during summer. This week we will focus on elements of forested habitats that are important for Indiana bats, with ideas on how landowners can improve their woodlands for bats.
For their primary roosts, female Indiana bats usually select large snags (dead trees) that retain some bark.  The bats form a colony under the bark and raise their young there.   Think about this the next time you see a dead tree; you may just be looking at an Indiana bat nursery!
In most of their range, Indiana bats prefer deciduous trees over coniferous trees for roosts.  The structure is more important than the tree species.  Trees like cottonwood, oak and ash, that retain thick pieces of peeling bark after they die, are most likely be used as a roost.
Indiana bats make their summer homes in large dead or dying trees.  This means that any given roost tree doesn’t last more than a few years because it will lose its bark or fall down.  So, an Indiana bat maternity colony uses a number of trees as roosts within its home range; when one roost falls down the bats have other options.
Indiana bat maternity colonies are very faithful to their summer maternity areas; they return to the same areas every summer.  For a colony to be able to use an area year after year, there must be a continuing supply of large dead or dying trees to provide roosts.
Indiana bat maternity colonies have been found in areas almost completely forested, but also been in areas where forests are interspersed with farm fields or other open areas. Where forests are fragmented by farms and other open areas, wooded fencerows or wooded stream banks provide critical connections to forested areas.
Indiana bats hunt for prey within forests, along forest and field edges, and along stream banks.  Bottomland forests and wooded wetlands are good foraging areas because they produce lots of insects.
Indiana bats do not fly across large open areas, instead, they often follow wooded fencerows or strips of trees along streams as travel corridors between wooded blocks.
Indiana bats forage “on the wing,” catching flying insects while in flight. Many types of nocturnal flying insects are part of their diet and their diet shifts over the summer as different types of insects are available at different times. Northern long-eared bats also "glean" insects off of vegetation.
Most of us have seen birds catch flying insects during the day, so we know that birds help control insect populations.  Just as birds are the main predator of diurnal insects, bats are the major predator of night-flying insects.  Bats can eat ½ their weight or more in insects every night.  That adds up to a lot of bugs!
The Mexican free-tailed bat provides an excellent example of the voracious appetite of bats.  It is estimated that 100-million Mexican free-tailed bats come to Central Texas each year to raise their young.  Every night these bats consume 1,000 tons of insects, many of which are agricultural pests.
Four orders of insects make up most of the diet of Indiana bats.  These include Coleoptera (beetles), Diptera (true flies, includes mosquitoes), Lepidoptera (includes moths), and Trichoptera (caddisflies).  These orders include many crop and forest pests. 
It is nearly impossible to tell what a bat is eating by direct observations. So how do scientists find out what bats eat?  Identifying insect parts in the stomach contents is one way, but this requires removing the bat’s stomach, so this technique is rarely used.
To study bat food habits, it is easier to study what comes out rather than what goes in.  Most studies of bat food habits  use an analysis of guano (aka bat poop).  Because insect exoskeletons are not easily digested, some parts remain in the guano and can be identified with a microscope, a lot of patience and practice.
A new way of studying bat food habits is to look for insect DNA in bat guano. This is more accurate than using sight alone because many fragments in the guano are too small to be identified to species.
Bats start foraging at dusk and may hunt for prey intermittently throughout the night until dawn.  There are advantages to this nocturnal lifestyle.  They avoid flying predators, like hawks, that are active in the daytime, and they take advantage of the abundance of insects active at night.
To find prey in the dark, insectivorous bats have developed a remarkable navigation system called echolocation.  It’s similar to hearing an echo when you shout in a canyon. You produce a sound wave that travels across the canyon, deflects off the opposite side, and moves back toward you. 
Bats make sounds the same way we do, by moving air past their vibrating vocal chords.  Some bats emit sounds from their mouth and others through their nose.  Scientists believe that the strange nose structures found in some bats, called nose leaves, focus the noise for more accurate pin-pointing of insects and other prey.
In most bats, the echolocation sound has an extremely high pitch that is beyond human hearing.  A bat emits a sound wave and the bat's brain processes the echo that returns.  Based on how long it takes a noise to return, the bat figures out how far away an object is.  Listen to the Echolocation Song for a fun explanation.
With echolocation, a bat can determine where an insect is, how big it is, and in what direction it is moving.  The bat can tell if an insect is to the right or left by comparing when the sound reaches its right ear in contrast to when the sound reaches its left ear.
Bat ears have special adaptations for echolocation , a complex collection of folds that help determine an insect's vertical position. Echoes coming from below will hit the folds of the outer ear at a different point than echoes coming from above, and so will sound different when they reach the bat's inner ear.  The study of bat ears is advancing science.
Echolocation allows a bat to determine the size of an insect.  A smaller insect reflects less of the sound wave and so produces a less intense echo.
Bats use echolocation to determine an insect’s flight direction.  The direction an insect is moving changes the pitch of the echo.  If an insect moves away from a bat, the return echo will have a lower pitch than the original sound, while the echo from an insect moving toward the bat will have a higher pitch.

A bat processes all of the echolocation information unconsciously, the same way we process the visual and aural information we gather with our eyes and ears.   Bats also process visual information and use echolocation in conjunction with vision, not instead of it.
Bat echolocation is refined enough to detect the vibrations from a moth’s beating wings.  Watch this amazing video of bat echolocation in action.
Last updated: July 20, 2016