Adventures with Pacific Northwest Bat Houses

The Washington Department of Fish and Wildlife

11 min readOct 25, 2022

Habitat at Home Coordinator Niki Desautels on setting up the “perfect” bat house

Bat houses are a bit of an adventure, as I discovered through my attempts to provide shelter for bats in my area. Bat houses are a great way to help replace bat habitat lost to human development. They can provide shelter (called roosts) for maternity colonies, as well as foraging, migrating, and dispersing bats. Additionally, they provide us with an opportunity to observe bats and the chance to educate friends on how they can support their neighborhood bats.

What if you have a bat house but have never seen bats using it? At my last home, I had a bat house up for seven years and never saw a bat. So, when I put up a bat house on my new home this spring, I learned as much as I could to have the best chance at getting bats to use it. I found it helpful to set my expectations by learning more about how and why bats use bat houses.

Bats cuddled up in a multi-chamber bat house. Strength and warmth in numbers! Photo by Mylea Bayless/BCI

Bat Behavior

Bats use a variety of shelters (called roosts) throughout the year. Washington has 15 bat species, 13 of whom are most active in Washington from spring to fall and hibernate or migrate south for the winter, and two species who stay active in the winter. Some species gather to hibernate in winter roosts called hibernacula.

Bat Roost Uses

Hibernacula: Winter roosts where bats hibernate.
Maternity colony: Group of pregnant females who live together to give birth and raise their pups.
Day roost: Place where bats sleep during daylight hours in the spring, summer, and early fall.
Night roost: Place where bats stop to nap and digest food as they forage throughout the night.
Transition roost: As maternity colonies disperse, or in the spring and fall as bats migrate, bats will roost for a night or two along their routes in the safest spot they can find.
Time of Year: Bat houses are mostly used in the spring and summer months when bats are active raising young and hunting insects. Some species switch day roosts frequently, while many species are loyal to one roost, sometimes for years.

Checking Your Bat House for Use

Washington has many species of solitary bats, such as silver-haired and hoary bats that may use a bat house but can be harder to spot when just one or two bats are living there. You can check for bats in your bat houses by looking for the telltale sign of bats, guano. Guano is bat poop. Shining a flashlight up into your bat house disturbs the bats and only tells you if they are there, not if they have been using it. Guano observation is the way to go. To look for guano, place a white sheet, towel, or paper under the bat house for a few dry nights and check each morning. Guano is small, dark, and oblong shaped, like mouse droppings. To tell the difference, put on gloves and pick up a piece, then smush it between your fingers (keep it away from your face). Bat guano will turn into a shimmery powder due to all the insects they eat.

Bat guano found directly under the bat house in late August 2022. Photo by WDFW

Afterwards, gather the remaining guano and put it in your flower garden; guano is an amazing fertilizer! Check for guano throughout the spring, summer, and fall since bats could stop by at any time.

My bat house is placed over my driveway, and the light-colored cement makes it easy to see guano under the bat house. Bats showed up for four days over two weeks in late August. We saw them emerge once and found small amounts of guano right under the box on four mornings. Possibly mom and a pup leaving the maternity colony using my bat house like an Airbnb! The following spring, the house was just used at night, when bats are active but often pause in their feeding to nap in local roosts.

Multi chamber bat house painted black and mounted on the sunniest side of the house. Photo by WDFW.

If you do have evidence of bats, please report it to us at the Washington Department of Fish and Wildlife (WDFW). Knowing where and when colonies and individual bats are present helps us with important research and population tracking. More information means we can do more to help bats in Washington.

If you have checked and still aren’t seeing signs of bats in your bat house, it might be time to move or adjust the house to be more appealing to bats in our region.

Northwest Bat Houses

There are many resources about northwest specific bat house styles, colors, and placement. Our website has building plans for multi-chamber houses (flat backed that can mount on a building) and rocket boxes (tall boxes that mount onto free standing poles), and a Bat House Care and Installation Guide that I highly recommend!

Follow these basic steps for the best chance of having bats use your bat house:

Full sunlight: Bat houses need six or more hours of sunlight to keep bats warm. Place them in full sun, but monitor the heat exposure especially in extreme heat. If temps get extra high in your area, consider adding a sun shade that can be used when necessary.
Medium-to-dark paint: Paint bat houses dark colors to help absorb and hold additional heat. If you are in an area that gets extreme heat, choose a lighter tone. Houses in more shade should be painted darker.
High off ground: Bat houses should be high off the ground, at least 12 feet, and away from any ledge or branches that would give predators access to get in or perch above.
Multiple chambers: Flat bat houses should have multiple chambers so bats can move into and away from heat as needed.
Multiple bat houses: If possible, include a variety of bat house types in different locations around your property. You can work with neighbors in urban and suburban areas to vary the types of bat houses available.
Vents: All bat houses should include vents to increase airflow and prevent bat houses from overheating.
Large size: Bat houses should be large (at least 24”x24”) to fit many bats and allow for room to move.
Small openings: Openings and spaces in bat houses should be small (¾"-1 ½") so our small local bats feel safe.
Rough surfaces: Wood surfaces below and inside bat houses should be roughed up so tiny bat claws can get a grip.

Rough cedar houses like this one allow bats to climb easily. High placement allows bats to utilize gravity to drop out and begin flying. Photo by Bob Davies.

The bat house at my previous home met none of these conditions. I purchased it from a hardware store and it looked like a small bird house; the openings were too big, it was pine colored, and I put it on the shaded side of my house. Live and learn!

Use the tips above and the resources on our website to adjust your house or find a new location. Remember: Placing bat houses in trees puts them at high risk of predation. If you are able, try putting up multiple bat houses in different locations to give bats options. Finally, be sure to remove any screen material from bat houses as it can trap bat claws.

Don’t place bat houses in trees! Predators have easy access to pups by climbing bark and waiting on upper limbs. Photo by WDFW

Bat House Maintenance

Yes, bat houses need maintenance. Aside from checking it for occupants in the summer, winter is the best time to give your bat house a little love.

Check for occupants: Make sure the bat house is unoccupied (a flashlight will work).
Clean out: Clear out the inside of the house of any insects, webbing, or casing.
Weatherproof: Touch up faded paint and caulk any gaps to keep it weatherproof.
Maintain surroundings: Cut back brambles and branches moving into space so bats can access it and predators can’t.

A single chamber flat bat house on a post in a local city park. Brambles have surrounded this bat and insect casing have filled the inside. Late fall and winter are the perfect time to clear the brush and clean the house.

For more details on cleaning and maintaining your bat house, use this video from Vermont Fish and Wildlife Department.

The Importance of Trees

In Washington, we have several species of tree dwelling bats, such as the long-legged and California myotis. If you have a lot of bat activity in your area, but no bats in your bat house, that’s OK! Your bats are active and roosting in the area, so the best thing you can do is protect those trees. Keep in mind that your bat house can still be helpful. It might be the perfect refuge if habitat loss occurs in your area in the future. It is also a great option for a future maternity colony. Be patient, protect your trees, and keep your bat house maintained in case local bats need it.

A bat’s favorite tree is known as a “wildlife tree”, or snag. Snags are standing dead trees in the early stages of decay. Decaying trees let off heat and create crevices bats (and other wildlife) love. Use our snag guidelines to safely trim and preserve these important trees.

Moving Bats from a Building to a Bat House

If you put up a bat house to move bats away from or out of a building or structure, you were probably disappointed. Bats are loyal to their roosts. It’s hard to find the perfect home, and no one wants to give it up once they find it! With some patience and planning we can make the bat houses more attractive than buildings, but it takes some work.

Step 1: Prepare your bat house. Use the resources above to make your bat house as northwest bat specific as possible. Build it, paint it, move it, clean it, and make it as bat ready as you can. Get the bat house up in the winter (ideally before the end of February to give it time to become part of the habitat and be available for bats as soon as they arrive in the spring).
Step 2: Create a plan to exclude bats. This must happen over the winter when bats are not present. Once they return in the spring, maternity colonies form and kicking them out means the loss of baby bats. Bats have just one pup a year and bat populations need our help, so if you need to block off entrances and remove access for bats, wait until late October or November and complete the work as soon as you can.

Step 3: Seal up entrances. When bats return, you need to make sure the places they used to live in are no longer an option. Use the resources on our website to seal cracks and crevices, replace rotting wood, close entrances, and set up a one-way door to allow bats who are still inside to escape but not re-enter.
Step 4: Be mindful of spring returns. Once the weather starts to get warmer and bats become active again, be extra vigilant to keep doors and windows closed around the former roost. Bats will look for another way back into their homes before giving up and finding a new roost. Hopefully, soon enough, they will spot the excellent bat house you set up for them.

Beyond Bat Houses to Building Bat Habitat at Home

Bats need all the elements of good habitat; food, water, shelter, and space to live. Only providing shelter won’t bring in bats who were not there before. If bats are not common in your area, it could be that you are missing one of these other critical pieces. You can create habitat that can bring bats back!

Food: Washington bats all eat insects so you can support their food supply by planting native plants that support night-flying insects. Flowers with white and light blooms, or that produce fragrance, attract insects that bats eat like moths, beetles, flies, gnats, mosquitos, and more. Avoiding pesticides supports native insects and keeps bats healthy.

Water: Water is essential for wildlife and a big need for bats. Bats drink “on the wing” and rarely land to drink. This means they need large freshwater sources that are free of pollution to give them the space to swoop to reach water. Water troughs and ponds can provide water for bats but if you can’t install such a large water feature, you can protect local water sources by keeping pollutants out of the drains. When streams and creeks dry up, or reservoirs are covered over, bats often leave the area.

Space: Some Washington bat species are highly adaptable and live in rural, suburban, and urban areas, but they still need space to thrive and stay protected. Put your bat house as far as you can from disturbances like walkways, loud garage doors, places with many people, or shelter set up for predators of bats like owl boxes, squirrel feeders, or bird perches.

Is my bat house perfect? No. Does it meet everything we talked about? No. But I learned these best practices, did the best I could, and I got some bat visitors this year! Try your best, adjust along the way, and keep improving the bat habitat all around you. Our insect-eating friends will thank you!

If you want further guidance in your bat house adventures, please reach out to HabitatAtHome@dfw.wa.gov

Resources:

Build a bat house

Single-chamber flat bat house: Best for placement on a building or post in a place that does not have a current maternity colony. Smaller, simplified bat house for solitary bats or non-maternity colonies.

Single chamber Rocket box: Best for placement on standalone post in a place that does not have a current maternity colony. Smaller, simplified bat houses for solitary bats or non-maternity colonies.

Multi-chamber George Jr house: Best for providing a roost for small maternity colonies. Mounts well on buildings or on a post.

Four-chamber nursery house: Best when needing to move a large maternity colony. Recommended for all of Washington and is especially ideal species who aggregate in large numbers like little brown or Yuma myotis. Can be mounted on a building or post.

Dual-chamber Rocket box: Best when needing to move a large maternity colony. Recommended for all of Washington and is especially ideal for who aggregate in large numbers like little brown or Yuma myotis. Best mounted on a standalone post in a clearing 20 feet away from cover such as a tree line.