One of the best ways you can support bat conservation is to put up an artificial roost, like a bat house. Since bat populations have decreased significantly, bat houses can be very useful in providing secure roost sites for bats. Bats provide a number of benefits to humans and the environment, and they need your help!
Why are bat houses important?
- Bat houses are particularly helpful in providing alternative roosting habitat for bats that are excluded from homes. In turn, this reduces the chance of human contact with bats.
- Installing a bat house on your property can provide a safe environment for bats, while protecting your yard from pest insects, like mosquitoes, moths, and beetles.
- Bat houses give females a safe, warm place to raise their young. Since most female bats only have one pup each year, bat populations grow very slowly. Additionally, due to habitat loss and degradation, it is becoming harder for bats to locate roost sites to raise their young. By installing a bat house, you can provide mothers and their pups with a safe home.
Building a bat house
Building a bat house is a great way to get involved in bat conservation and help these threatened animals. The style of your bat house depends on the tools, wood, and building materials you have available, as well as your skills and budget.
Since many bat species would typically roost under the bark of a dead tree, the goal is to make a bat house that mimics the narrow, rough space between the bark and trunk of a tree. Bats like tight spaces, so the inside of a bat house is very narrow. Additionally, bat houses are constructed using rough wood so the bats can hang on without slipping and falling. Bats also need warm places to raise their babies, which is why bat houses are painted dark colors and the sides are caulked to keep the heat in and water out.
Below are some key elements for a successful bat house:
- A combination of exterior grade plywood and cedar is best. Do not use pressure-treated wood.
- Bat houses should be at least 24 inches tall (with chambers being at least 20 inches tall), 14 or more inches wide, and open on the bottom to prevent buildup of guano. Multiple chambers should be ¾ - 2 inches apart. Taller and wider houses are better.
- Add a vent to each side to allow air flow and prevent the bats from overheating. Vents should be about 6 inches from the bottom of houses that are 24 - 36 inches tall. For houses that are more than 36 inches tall, vents should be 10 - 12 inches from the bottom. The front vents should be as long as the house is wide. Side vents should be 6 inches tall by ½ inch wide.
- Make sure the interior wood is roughened for the bats to cling to. Rough-cut wood will suffice, or surfaces can be grooved, roughened, or scored horizontally at ¼ - ½ inch intervals. Chamber surfaces can also be covered with a durable plastic, nylon, or fiberglass mesh, screening, or hardware cloth (⅛ - ¼ inch). Mesh can be stapled to the inside of the bat house every 2 inches (up, down, and across) so the mesh does not sag, buckle, or curl. Ensure the mesh hangs all the way down to the landing pad, and ensure that staple points do not protrude into the roost chambers. Metal mesh is not recommended as this material is abrasive and can be corrosive and conductive, which can cause injury to bats. Please note that if mesh is used, guano may collect requiring it to be gently hosed or cleaned out periodically.
- It is crucial to caulk all seams using latex caulk, especially around the roof, to keep the bats warm and dry. Covering the roof with shingles, galvanized metal, or tar paper provides extra protection and will allow the bat house to last longer.
- A 3 - 6 inch roughened landing pad extending from the bottom of a bat house below the entrance allows bats to land and climb up into the chamber(s).
- Single-chamber bat houses are a great starter box, but bats seem to prefer larger structures which offer more stable temperatures. Bats will use both single- and multi-chambered houses, but houses with more chambers give bats opportunities to find warmer or cooler walls within the house as the weather changes and, therefore, improves the ability of the bats to regulate their internal temperatures. The bigger, the better.
- Exterior grade (galvanized, coated, stainless, brass, etc.) screws, rather than nails, are recommended to increase longevity of the bat house.
Below is a simple bat house design:
There are many more elaborate and detailed designs available for a variety of different roost sizes and types. There are also step-by-step instructions and several videos available on the internet to help you build your own bat house.
Buying a bat house
Bat houses range in size from single-chamber to bat condos. There are easy-to-assemble bat house kits available from the Boy Scouts of America and many garden centers, as well as pre-assembled bat houses through a variety of online retailers and many home improvement stores.
Bat Conservation International (BCI) has a Bat Approved Certification Program with specific certification criteria for bat house design and construction. You can view a list of certified bat house vendors on the BCI website.
Bat Conservation & Management has bat house DIY kits and assembled bat houses in a variety of sizes available to purchase on their website.
Installing a bat house
Prior to installing a bat house on or near your home, first ensure your home is bat proofed by carefully examining for holes that may allow for bats to enter. Caulk any gaps, cracks, and holes larger than ½ inch, use window screens, chimney caps, draft guards beneath doors to attics, and screens on attic vents. Also fill electrical and plumbing holes and make sure flashing, eaves, and ridge vents are secure. Window screening, netting, caulking compounds, and expanding foam insulation work well for sealing openings and excluding bats from your home.
Installing a bat house can give bats a safe and comfortable place to live. The location, temperature, and design are key factors in bat house occupancy. Below are some installation recommendations for the best chance of bats occupying your bat house:
- Bat houses can be mounted on wooden posts, steel posts, or on the sides of buildings, but they should never be mounted on trees. Trees receive less sun, make the bats more vulnerable to predators, and have branches that make it difficult for bats to drop into flight. Bat houses mounted under eaves on buildings are ideal locations as they are still exposed to the sun while being protected from rain and predators.
- If mounting to a wooden pole, a metal predator guard may be helpful. Poles can be wrapped with an 18 inch piece of sheet metal 3 feet above the ground.
- The side of a house, garage, or barn is best location for single-chamber bat houses. Buildings, such as those mentioned, eliminate drafts on one side, provide a buffer from cool nighttime temperatures, and help the bat house heat up more quickly than those installed on poles. Single-chamber bat houses should not be mounted to poles unless two are mounted back-to-back. If mounting two back-to-back, place the houses ¾ inch apart and cover both with a galvanized metal roof to protect the space in-between from rain.
- Bat houses should be 20 - 30 feet from tree branches, vegetation, and wires, and 12 - 20 feet above the ground for clearance when swooping into and out of the bat house.
- Bat houses located close to a source of water (i.e. along streams, creeks, ponds, rivers, lakes, etc.) are the most successful.
- Do not place a bat house on the top of a hill where it may be exposed to excessive winds.
- Install bat houses away from bright lights, and avoid shiny and reflective objects on the ground under the bat house as they may disorient bats.
- Bat houses can be installed at any time of the year, but they are more likely to be occupied during the first summer if they are installed before bats emerge from hibernation in the spring.
Warm temperatures are favorable to bats, so the temperature of your bat house will greatly affect whether bats take up residency. Bat house temperatures are influenced by a variety of factors, including exterior color, compass orientation, amount of sun exposure, how well the house is caulked and vented, and the mounting and construction materials. Below are a few recommendations to help maintain an ideal temperature inside your bat house:
- Bat houses should be mounted in areas that receive 6 - 8 hours of direct sunlight every day.
- Internal temperatures should be 80 - 100° F in the summer for mothers to raise their young.
- Bat houses should face south, east, or southeast to catch the heat of the sun as it rises.
- In the Northeastern United States, bat houses should be painted black, dark brown, or a similar dark color to absorb heat from the sun.
Attracting bats to a bat house
Bats have to find new roost sites on their own and investigate new roosting opportunities while foraging at night. Although there are no known methods for attracting bats, you can certainly encourage them by creating suitable roosting habitat. Learning how to encourage bats to choose your bat house as their new home takes some effort and patience, but here are a few tips:
- Be sure to select a bat house that has been designed properly, and choose a suitable location.
- Your bat house has a better chance of attracting bats if it is near a source of water and is away from bright lights.
- The existence of bat colonies in nearby buildings increases your chances of attracting bats to your bat house.
- There must be a good source of insects nearby. Since bats eat night-flying insects, you may want to plant flowers that bloom late in the day or are night-scented to attract insects.
- Bats prefer diverse habitats with a mixture of woodlands, natural vegetation, and agricultural land.
It can take up to 2 years for bats to move into a bat house, and even then, only 15% of bat houses will be occupied. If bats do not occupy your bat house after 2 years, try moving the bat house to a new location. Whether or not your bat house is inhabited, by installing one, you're doing your part to help bats.
For further detailed suggestions for attracting bats to bat houses, please see the Massachusetts Homeowner's Guide to Bats.
Bat house frequently asked questions
Q: Are there laws or restrictions for installing a bat house?
A: No, as long as you are putting the bat house on your own property. Permission must be obtained before installing a bat house on a utility pole.
Q: A bat fell out of the bat house. What should I do?
A: If the bat seems injured, contact a local licensed wildlife rehabilitator. If it is necessary to handle the bat, wear thick work gloves. Try to place the bat as close to the bat house as possible, or off the ground and in a tree. If it is a baby bat, the mother bat will return for her pup.
Q: How many bats will occupy my bat house?
A: Depending on the size of your bat house and the number of chambers it has, your bat house may have fewer than 50 to several hundred bats.
Q: How long will it take for bats to find my bat house?
A: Bats often require a year or two to find and occupy a new bat house. Chances of earlier occupancy are increased if you follow the above guidelines, and if you live in an area of mixed habitat (i.e. farmland, woods, and water). Bats are loyal to their summer roost sites and should return yearly once they take up residency.
Q: Will guano (bat poop) pile up under the bat house?
A: It is likely that guano will accumulate over time under your bat house. However, natural elements, such as rain, sun, and wind, will naturally break down the guano. Since guano is high in nitrogen, it makes a great fertilizer. A potted plant or shallow, non-reflective plant saucer can be placed under the bat house to collect the guano and be used as fertilizer or disposed of.
Q: Will guano pose a threat to my family?
A: No. Guano does not pose any more of a threat than bird or cat poop would. It is not known to be toxic or harmful to humans, dogs, cats, or other animals.
Q: What kind of paint and how many coats should I apply to my bat house?
A: For the exterior of a bat house, apply three coats of exterior grade, water-based paint or stain. For the interior, use two coats of exterior grade, water-based stain.
Q: Is there maintenance required for bat houses?
A: A bat house that is constructed properly requires very little maintenance, but should be checked yearly for any problems. Check for cracks, leaks, warping, or exposed unpainted wood that may need minor repairs. Check for unwanted guests, like wasps, and remove any nests in late winter or early spring using a stick. New caulk and paint may be required after 3 - 5 years to prevent leaks and drafts. Any maintenance should be done in the months when bats are not present (November to early March).
Q: I have bats living in my house. How do I get them into my bat house?
A: If you are excluding bats from your home, alternative roosts (like a bat house) should be provided several months or one full season before the exclusion. In Massachusetts, attempts to exclude bats from your home can only be made in the early spring (during the month of May) or in the late summer (August 1 to October 15) when flightless young bats are not present. Please refer to the Massachusetts Homeowner's Guide to Bats for further information and advice on how to evict and relocate bats.
Q: Which bat species will occupy my bat house?
A: Little brown bats and big brown bats are the most common species to occupy bat houses in Massachusetts.
Q: How do I know how many bats are occupying my bat house?
A: To estimate the number of bats, you can do a visual inspection by looking into the bat house, count bats as they emerge around sunset, or observe the amount of guano beneath the bat house.