Many bat houses sit empty for a season or two, and some for much longer. That does not mean you did something wrong, because bats take time to find new roosts. But it can also mean a fixable problem. This guide gives a calm checklist, in order of how often each problem matters.
First, is it really empty?
Bats are small, quiet and often invisible from the ground. Look for signs:
- Droppings (guano) on the ground or the wall under the house. Bat droppings crumble into a powder that shows insect parts and look different from rodent droppings.
- Staining around the entrance or on the landing area.
- Evening emergence. Stand at a distance around dusk on a warm evening and watch the house for about an hour from just before sunset. Bats come out one by one.
Do not open or shine a light into a bat house, and never reach in. Disturbing a roost can harm bats and may be illegal if protected species are present.
Checklist, in order
1. Time
Bat house occupancy varies widely. Some are used in the first year, others take several, and some are never used. Houses near existing roosts and good habitat fill faster. If your house has been up for less than two seasons, patience is a sound strategy.
2. Sun and temperature
Too little sun is the most frequent cause. BCI guidance calls for at least 6 hours of direct sun where July highs are below 100°F, and 10 or more hours where July highs are 80°F or lower. If shade has grown, if a tree has leafed out or the house faces the wrong way, bats may find it too cold. Check at several times of day.
If your summers are very hot, the problem can be too much heat. Houses that heat above a bat's tolerance can empty out in July. Consider a lighter color, more chambers and, where regional advice recommends it, some afternoon shade.
3. Color
Color should match your July average high: black below 85°F, dark brown or dark gray from 85 to 95°F, medium from 95 to 100°F and white or light above 100°F. A mismatch changes interior temperatures. Repainting the outside in fall is a simple fix.
4. Height, mount and clearance
At least 12 feet up, preferably 15 to 20, on a pole or building. Trees are discouraged, and metal siding is a poor choice. Keep 20 to 25 feet of open space below and around the entrance. Overgrown shrubs, new branches or a line of wires can break up the flight path.
5. Water and habitat
A stream, pond or lake within about a quarter mile helps. If the surroundings are mostly sprayed lawn, with few insects, bats may pass through and not stay. Native plants, a pond and less pesticide improve things over time.
6. Design
Look at the house itself:
- Does it have two or more chambers, ideally three or four?
- Is roost spacing about 3/4 to 1 inch?
- Are interior surfaces rough enough to grip?
- Are there vents and a landing platform?
- Is the finish sealed so that it will not leak or let in drafts?
- Is any wood warping or cracked?
A very small, single-chamber house sold as a decoration may never be used.
7. Competition and uninvited guests
Wasps, mud daubers, birds or squirrels sometimes take over bat houses. Check for nests from the ground with binoculars, and wait until late fall or winter, when insects are inactive and no bats are present, before removing nests. Wear gloves and eye protection. If wasps return each year, mounting in a sunnier or breezier spot may help.
8. Predators and disturbance
Raccoons, snakes and cats may climb poles. Smooth metal poles and a mounting height of 15 feet or more help. Avoid bright lights nearby and noisy activity right beside the house.
When to move a bat house
If you have addressed the checklist and the house is still empty after two or three seasons, moving it can make sense. Move it only in late fall or winter, when it is empty, and then pick a spot that scores better on sun, height and clearance. Use our checker to compare locations. Check first that the house has not become occupied. Bats do sometimes use houses unseen.
Adding a second house
BCI and many others recommend putting up two houses, perhaps 10 to 20 feet apart and facing different directions or with different colors, so bats can choose between temperatures. A second house is a cheap experiment if you suspect heat or cold is a problem.
If bats move in
Congratulations. Leave them alone. Do not open the house during summer, and do not let children or pets near it. Clean up guano from below the house with gloves and a mask, since droppings can carry fungal spores. Consider counting emerging bats each summer and reporting to a local bat group or a citizen-science program if one exists in your state.
If you have bats in your building instead
Do not seal them in. Many states restrict exclusion during the pup season. Contact a licensed wildlife professional or your state wildlife agency. Put up a bat house in advance so that excluded bats have somewhere to go.
Frequently asked questions
How long should I wait for bats to move in?
At least two seasons. Some houses take longer, and some are never used.
Can I look inside a bat house?
Do not. Watch it at dusk from a distance instead.
Should I clean out wasp nests?
Yes, but only in late fall or winter, when it is empty and wasps are inactive. Wear gloves and eye protection.
Should I move a house that is empty?
After two or three seasons, if sun, height or clearance is poor. Move it in fall or winter.
What if bats are living in my attic?
Do not seal them in. Call a licensed wildlife professional, especially in summer when pups are present.
