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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

A Little Brown Bat, a common species in Wisconsin – and a great mosquito eater. © Don Blegen

August 2003

Respect for the night patrol

Beleaguered by bad press, bats are fragile mammals and important allies that deserve our help.

Karen Kvool


The Little Brown Bat, a common species in Wisconsin – and a great mosquito eater. © Don Blegen
Saving bat caves | Evicting flying "tenants"
Providing a home of their own | A park where bats patrol 24/7
Bat houses that work | The Great Eight

"Going batty" suggests going crazy, acting weird, or perhaps being of unsound mind. In describing bats themselves, the term refers to the erratic way these acrobatic mammals can fly, fluttering at dusk searching for food. Though bats are demonized in films, myths and stories, it's a bad rap. Bats are exceedingly helpful to humans and fascinating in their own right. Horror moviemakers don't do bats or our ecology any favors by making people fearful of these nighttime aerialists.

Though flying squirrels can swoop and glide, bats are the only mammals that can fly, and they are not blind. All bat species see, but some see poorly relying on a good sense of smell, keen hearing and echolocation to navigate and zero-in on food as small as a mosquito that they sense many yards away. The little brown bat, for instance, is such an accurate insect predator that the bat experts at Bat Conservation International (BCI) estimate one little brown can eat up to 1,200 mosquitoes an hour!

Bats are the number one predator of beetles, moths and other flying insects considered farm and forest pests. Their value to the food and tree economy is measured in billions of dollars per year. Where bats are encouraged and prolific, the need for chemical pesticides can be significantly reduced. Several important crops depend on bats to pollinate their flowers or devour the pests that plague them. Some of the products helped by bats include avocados, cashews, guavas, peaches, bananas, dates, figs, cloves and mangoes.

Despite these values, bats are endangered in many areas and bat populations are in serious decline worldwide. Some of that loss is due to habitat destruction, some due to their slow reproduction rates (only one pup a year), but even more is attributable to willful destruction by humans. Bats tend to nest in large colonies, so a single act of vandalism or bad luck can destroy them by the thousands. In Wisconsin some of the manmade bat roosts like old barns and silos have fallen down. Other natural roost sites, like abandoned mines need a little maintenance to stay safe.

Saving bat caves

For instance the Neda Mine Bat Sanctuary near Mayville in Dodge County is one of the three largest bat hibernation sites in North America. The old iron mine, abandoned in 1914, was donated to the University of Wisconsin-Milwaukee (UW-M) in 1975. More than four miles of tunnels in the mine maintain a constant 40F temperature year-round and it's home to more than 500,000 bats that migrate here from at least a four-state region.

According to BCI's website, the decaying supports in the abandoned mine make many portions too dangerous for people to enter, so efforts to stabilize the bat hibernacula (winter hibernating chamber) have been dangerous and tricky. A host of cavers, environmental engineers and biologists contributed talents to preserve this lifeboat for hibernating bats. Partners included the University of Wisconsin, Department of Natural Resources, BCI, U.S. Fish and Wildlife Service, Zoological Society of Milwaukee County, the Max McGraw Wildlife Foundation and the National Science Foundation.

Both the bat populations and conditions in the dilapidated mine were studied to determine roosting areas and air flows.

"Bats are full hibernators," explains Jim Reinartz, manager of the UW-M Field Station that oversees the mine. "Their body temperature drops to just enough to keep them alive and they may stay in a sleeping torpor for seven months in Wisconsin. If you wake them up too much, it will deplete the little energy they have saved to emerge from torpor and it will kill them."

Studies at the mine surveyed airflow patterns to decide which entrances were most important to keep open, according to BCI. Since cold air was entering through the lowest entrances and relatively warmer air was exiting through the highest, scientists and engineers carefully preserved the chimney-type air movement. Eighteen entrances had to be made safe, 13 were closed with large rocks that excluded people but permitted airflow. Five entrances were stabilized and gated so bats can get through, but people and animal predators can't.

Bat populations in dozens of other abandoned mines in Wisconsin and Michigan have less certain fates. As supports fall in old mines, the potential to trap and kill hibernating bats is a real threat.

Evicting flying "tenants"

Bats tend to be quite timid around people. Other than occasionally seeing a bat screening the dusky air for bugs, you are most likely to see a bat if it gets into your house or finds a way into your attic. The dry, warm spaces in attics make natural roosts for bats, nevertheless your house doesn't have to be their cave. Bat Conservation International offers easy, inexpensive ways to find out how the bats are getting in and how to evict them humanely. A UW-Extension Bulletin G3096, "Bats: Information for Wisconsin Homeowners," also offers tips. Basically, you just need to keep watch around sunset to determine where the bats are entering and leaving your attic. Then set up a square of plastic mesh netting just a few inches away from that entry and exit spot, hanging several feet down. The screening forms a sort of one-way door. Bats will crawl their way out, but will consider it too much of a hassle to keep dealing with the netting. Commercial bat excluders are also available.

Once the bats have left, you can seal up the space, but why not consider this an opportunity? If the bats are attracted to an area of your house, it might be an ideal spot to erect a bat house outside the home. That way you could get all the benefits of bats without the drawbacks of uninvited "tenants."

Providing a home of their own

Birds and butterflies are fun to look at, but attracting bats to your homestead has an added benefit, you get your own squadron of bug battlers. No bug zapper or chemical fog sprayer holds a candle to the bats' abilities and desires to eat skeeters. Since a well-located, properly built bat house could attract up to 200 bats, you've got a lot to gain by establishing a bat colony.

Installing a bat house. A taller, narrower bat house keeps the bats warm and toasty, and has the right amount of ventilation and space to attract bats, but not wasps. © Holly Bartholmai
Installing a bat house. A taller, narrower bat house keeps the bats warm and has the right amount of ventilation and space to attract bats, but not wasps.

© Holly Bartholmai
Bat houses that work
Here are some tips from BCI for building and locating bat houses that work. Get more details from "The Bat House Builder's Handbook," available from BCI for less than $8:

Design: Bat houses look like a divided rectangular box or cylinder with open bottoms. Box types should be at least two feet tall, 14 inches wide and have a landing area at least three to six inches below the open bottom. Most houses have one to four roosting chambers spaced about inch to an inch apart. The wood surfaces should be roughened up, scratched or grooved horizontally every half inch to provide the bats with gripping space. Quarter-inch to half-inch tough plastic netting also can be tacked on to provide gripping and landing space. The house interior can get darn hot, so designs include both front and side vents.

Materials: BCI recommends exterior plywood or cedar, but do not use pressure-treated wood that contains chemicals. All screws and hardware should be exterior grade or galvanized to prevent rusting. Screws hold up to sun and weather better than nails. Caulk all seams.

Waterproofing and warming the wood: Apply three coats of exterior grade water-based paint or stain. Paint the house black in northern Wisconsin where mid-July temperatures are less than 85F; dark brown or gray where July temps range 85-95°F. Interior surfaces should be coated with two coats of black exterior grade water-based stain.

Location: Bats would naturally choose areas within a quarter-mile of a stream, lake or river, but you can attract them to areas that are 10-30 yards from forest cover and near mixed agricultural fields and vegetation where insects swarm. Bat houses can be mounted to poles or buildings at least 12 feet off the ground; 15-20 feet off the ground is even better. Wood or stone buildings that get sufficient sun make excellent choices. You can locate them under eaves that provide rain protection. Pole-mounted bat houses can be placed back to back and covered with galvanized metal roofing to shed rain. When placing a new bat house, install it in the spring. If you are trying to exclude a bat colony from a home or building, place the bat house two to six weeks before you start evicting bats.

Locations to avoid: Really shady areas will only attract intermittent use by bachelor bats and non-breeding females. Locations over bright or white substrate that reflect up into the box are poor spots. Keep the box well away from burn barrels as smoke disturbs bats. Keep the box away from brightly-lit areas, exposed hilltops and windswept areas. don't erect a bat house directly over highways as bats swarm the roost during their dawn returns and are vulnerable to auto traffic.

Sun exposure: In the northern two-thirds of Wisconsin, locate the bat house in an area receiving at least 10 hours of July sunlight. Areas with full all-day sunlight are preferable in Wisconsin. Avoid areas that are lit by bright night lights.

Predator protection: Bat houses on homes or metal poles provide the best protection from predators. The house should be at least 20-25 feet from the nearest tree branches where predators might roost.

Wasp protection: Wasps may try to set up shop in a bat house before the bats colonize a new house. Spacing the roosting areas about an inch apart may dissuade wasps. Clean out houses in late winter and early spring before the bats come back. Open bottomed houses will reduce use by mice and squirrels and will keep parasites from accumulating.

Bat houses are easy to build, but if you'd rather buy a prefab house, they are readily available. BCI not only offers house designs, but they test and rate commercial houses. Houses that make the grade carry a "bat approved" BCI certification seal. Recommended brands and models are listed at their website. Locally, the Beaver Dam Senior Center that makes birdhouses and butterfly houses we have noted in the past, also makes a line of bat houses. Contact them at Beaver Dam Senior Citizen Center, 114 E. Third Street, Beaver Dam, WI 53916, or call (920) 887-4639 for an order form.

A park where bats patrol 24/7

One of my family's favorite places to camp is Yellowstone Lake State Park near Blanchardville in northeastern Lafayette County. The park was built in 1954 following an extensive search for a place to create a park with a lake in the unglaciated hills and valleys of southwestern Wisconsin. The valley of the Yellowstone River was chosen, properties bought from local farmers and the extensive dike and dam project begun.

The resulting lake is 2 1/2 miles long, about a quarter-mile wide and covers 455 acres. It was stocked with fish and soon became a waterfowl refuge. Now it's surrounded by 5,100 acres of state land that offers swimming, fishing, water-skiing, sailing and boating. The property has campsite picnic areas, launches, a new shower and restroom building, and almost no mosquitoes. The reason for this wonderful camping perk I've learned is bat conservation, and the project is both the scientific interest and the passion of park volunteer Kent Borcherding of Hazel Green.

During our summer camping trip, my husband and I were walking down a park sidewalk toward the amphitheater when we saw several large wooden boxes atop long poles. They were too big to be bird houses, had many designs and each was marked with numbers and letters. "What's that?" I asked, and got my answer by reading the information posted nearby. They were bat houses, and they were darn effective. In fact, the bats that roost there seem to have nearly eliminated the mosquito population.

I remembered seeing bats on our farm as a child. They would fly fast and dive in the air around the farmyard at twilight.

Yellowstone Lake State Park is home to thousands of little brown bats (Myotis lucifugus), one of eight bat species in Wisconsin and about 900 species worldwide. The little brown is about thumb-sized with dark brown fur and black forearms and ears. Thirty-one hand-made bat houses placed throughout the park provide roosting places during daylight hours and nurseries where the bats raise their pups. Some North American bats hibernate in caves or rocky places, others migrate long distances in fall to warmer climates.

The bats are now a part of the evening entertainment at the park. Campers gather at the amphitheater to watch the bats flit about on their insect-hunting missions. Occasionally Borcherding gives educational presentations about the bats and his efforts.

Kent Borcherding keeps records on the performance of bat houses at Yellowstone Lake State Park. © Robert Queen
Kent Borcherding keeps records on the performance of bat houses at Yellowstone Lake State Park.

© Robert Queen

"Everything in nature has its purpose," he said. Borcherding grew up on a farm and made birdhouses as a youngster. As an adult, he got interested in backyard birding and learned to make bat houses as well. "I did a lot of reading about bats and experimented with many designs," Borcherding said. He joined BCI, obtained some of their house designs and started working with the park rangers at Yellowstone Park.

He erected his first bat houses at the park in August of 1995 and has kept meticulous records of the designs, pole heights, baffles, angles to the sun and construction of each house. He regularly monitors how many bats use the different designs, and he experiments with different colored paints, heights and angles.

After talking with the park rangers, bat houses have been placed to provide maximum mosquito protection at high-use areas near the park office, boat landing, amphitheater, shower house and the maintenance shed. All the houses are located where there is lots of sunlight within 30-50 feet from trees. He is also careful to keep the houses away from wind and wind chimes. Since bats have such keen hearing, the regular tinkling sounds of chimes irritate them.

Borcherding also notes their aversion to smoke. If there is a prairie fire in the area, bats will move far away from it.

Bats like houses made of "old wood," Borcherding says. He collects his from dilapidated farm buildings. Obviously, his experiments are paying off. Little brown bats found and moved into his first houses within two weeks of their placement. The park bat population has grown to more than 4,000 in less than 10 years. Even with this concentration of animals, Borcherding notes there has never been one human/bat conflict in the park.

Borcherding also noted that bat guano is prized worldwide as a fertilizer and soil amendment by gardeners and organic farmers. "The giant pumpkins and other huge vegetables you sometimes see in pictures are often fertilized with bat guano," Borcherding said.

Bats may have another important role to play as West Nile virus continues spreading westward. Erecting bat houses in areas prone to mosquito infestations could be an important chemical-free strategy for controlling the mosquitoes that have proven to be instrumental in spreading this infection to bird, horse and human populations.

All the more reason to get excited about going a little batty, building a few houses and inviting these valuable, interesting animals to your neighborhood.

Karen Kvool writes from Hartland.

The Great Eight
A bit about Wisconsin's eight bat species:

Little brown bat (Myotis lucifugus) The little brown is about as long as your thumb. These bats are common throughout the state and most frequently reside in attics and buildings. They are colonial bats that hibernate.

Northern long-eared bat (M. septentrionalis) roosts by day in summer in buildings, under shutters and under tree bark. It commonly uses caves as night roosts. This bat is much more solitary than other members of the Myotis family and is found singly or in small groups statewide, most often in forested hillside country than along streams or lakeshores. They have a long hibernation period, beginning as early as August and lasting 8-9 months.

Indiana bat (M. sodalis) This endangered species is difficult to distinguish from the other Myotis species. It also hibernates in a colony.

Big brown bat (Eptesicus fuscus) This "larger" bat is about twice the size of a little brown and tips the scales at ounce. It is found throughout the state, but is more common in the southern half of Wisconsin. It roosts in buildings and frequently hibernates in colonies.

Silver-haired bat (Lasionycteris noctivagans) This bat is bigger than the little brown and smaller than the big brown. Its fur is dark, nearly black tipped with white. The bat is solitary in habit, comes to Wisconsin seasonally and migrates south in winter.

Eastern pipistrelle bat (Pipistrellus subflavus) Our smallest bat is yellowish brown in color and only three inches long. It prefers caves, abandoned mines and rock crevices. It hibernates in colonies.

Red bat (Lasiurus borealis) This bat is about the same size as the big brown. Its fur is a rusty red color that looks washed with white. The red bat takes daytime refuge in trees. It is a solitary species that migrates south.

Hoary bat (Lasiurus cinereus) This bat is larger than the big brown and weighs about an ounce. Its fur ranges from a grayish yellow-brown to gray with grayish white overtones. It is found statewide but seems to prefer northern forests. The hoary is a solitary species that migrates south.

Compiled from "Bats:Information for Wisconsin Homeowners," by Scott Craven and Frank Iwen, UW-Extension publication G3096