Adventures with Pacific Northwest Bat Houses

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 housing 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.

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 13 bat species who 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.

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

Northwest Bat Houses

There are many resources about northwest specific bat house styles, colors, and placement. Our website has resources for building plans for 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!

  • Dark paint: Paint bat houses dark colors to help absorb and hold additional heat.
  • 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.
  • 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.
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.

  • 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.

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.

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 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 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!

Photos by WDFW.

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.

Other resources

Download our care and installation infographic

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The Washington Department of Fish and Wildlife

The Washington Department of Fish and Wildlife is dedicated to preserving, protecting and perpetuating the state’s fish and wildlife resources.