By Merlin Tuttle

2/15/19

Ever since I first introduced the idea of attracting bats to American yards in 1982, one of the most frequently asked questions has been, “Where can I purchase a good bat house?” The next, of course, is “How do I know bats will come?”

Over decades of time we’ve learned a great deal about bat preferences, and in fact today, quality bat houses, properly located, are achieving close to 90 percent success. But the biggest problem has been finding reliable vendors who’s bat houses truly meet bat needs. Far too many purchasers have become discouraged due to failure of cheap, poorly constructed houses, or even good houses sold with inadequate instructions.

Characteristics of a Quality Bat House

Most temperate-zone bats prefer roosting crevices 0.75” wide. Wherever you live, several species likely will prefer this spacing. A few may prefer 1.0 to 1.5” crevices. These mostly include just three species. The endangered Florida bonneted bat (Eumops floridanus) lives only in central and southern Florida. Big brown bats (Eptesicus fuscus) are frequent bat house users throughout most of North America and are often found in 1 to 1.5” wide crevices. Pallid bats (Antrozous pallidus) occur in arid and semi-arid areas of the West, from southern Canada through much of Mexico. Multi-chamber houses that include one or two of the wider roosting crevices may additionally appeal to larger species.

Taller houses provide the best thermal options. This is especially important in single-chamber houses (best at least 20” tall, though shorter ones will work in some locations).

Tight fit construction, with all seams caulked, reduces leakage and warping that leads to abandonment.

Best resistance to weathering and warping is provided by houses with a UV-resistant plastic outer shell. However, houses made exclusively of wood can be highly successful and long lasting if treated with a water-based sealant or three coats of paint. Houses that remain unsealed or unpainted often become warped and uninhabitable within 2-3 years. A water-based sealant is best for landing areas. Paint can render surfaces too smooth for secure footing. But a light coat of dark spray paint on landing and roosting areas can aid longevity and may enhance attractiveness (by darkening the interior).

All exterior parts of wooden houses ideally should be at least 0.5” thick. Any plywood used should be exterior grade.

All interior surfaces and landing areas must be roughened enabling bats, especially young, to gain secure footing. One-sixteenth-inch deep horizontal grooves at 0.5” intervals are ideal. Hand-roughened (scratched) surfaces or high quality, tightly-fitted, plastic screen (such as Gutter Guard) also work well.

All bat houses should have a roughened landing area extending at least 4” below the entrance, with one exception. Multi-chamber houses permit an ideal option. By extending the roughened surface of the back approximately 4” below the roosting partitions and permitting the sides, back and front to also extend approximately 4” below the roosting partitions, a recessed landing area can be provided. This ensures ideal protection against owl attack during entry.

Choosing a Size

As bat house quality and knowledge of preferred mounting locations, have improved, size has become less important. Many single-chambered houses are now successful when mounted on buildings. Houses with two or more chambers are more likely to attract nursery colonies. When a bat house is being provided as a new home for a colony about to be excluded from a building, it should be large enough to shelter the number of bats being excluded. Alternatively multiple houses can be used.

An early bat house study showed that people who provided two or more houses were twice as successful, possibly because most crevice-roosting bats are accustomed to living in snags or beneath loose bark, both of which tend to unpredictably fall, potentially leaving occupants homeless. Knowing there are options nearby appears to increase attractiveness.

Before purchasing a house designed to shelter thousands of bats, it is best to experiment with smaller two to four-chamber houses. If they succeed, larger houses are nearly always successful.

Where Bats are Most Easily Attracted

Mixed habitats near fresh water, such as lakes or rivers, are ideal. Areas where bats have attempted to form colonies in buildings are also especially successful. Not all bats roost in crevices. However, crevice roosting species are typically the most abundant nearly everywhere, including at least 10 species that occupy bat houses in North America. There is a chance of attracting bats almost anywhere. Areas surrounded by large expanses of buildings or single-crop agriculture are least likely.

Where to Mount Bat Houses

Outer walls of buildings are the best mounting sites for small houses and are the only locations likely to succeed in arid areas or cold climates. Buildings act as heat sinks, stabilizing temperature. In addition, bat houses in cold climates should be dark brown or black and positioned to maximize solar heating.

In more moderate and tropical climates, bat houses do very well on poles, ideally at least 12-15 feet above ground.

All bat houses should be in sunny areas, even in hot climates. There is a strong preference for dark brown or black houses in the coldest regions. In hot climates dark brown can still prove attractive if provided with vents and a tin roof to protect from mid-day sun. Unless protected by a tin roof lighter-brown (or equivalent), well-vented houses are preferred.

A North American study revealed that bat houses mounted on trees are least successful and take, on average, twice as long to attract bats even when successful. Bat houses on tree trunks are typically more shaded and may also be more vulnerable to predators.

Bats fear owls and climbing predators, probably at least one reason why it is best to keep bat houses at least 20 feet from the nearest trees. Nearby perches may help owls to sit and wait for bats to enter. Young ones are a bit slower and risk being pounced on. In warm climates, pole-mounted houses may require snake guards like those used to protect purple martins.

When to Expect Occupants

Where needs for homes are extreme, even poorly designed bat houses sometimes attract first occupants within just a few days. Two to six months is closer to average, and some very successful houses have taken years to gain first occupancy. Early success is likely in areas where bats have already experienced bat houses. Some extraordinarily successful houses, especially extra-large ones, have taken 2-3 years or more to succeed. In all cases I am aware of, the largest houses have taken the longest to attract their first occupants, though those I’ve been involved with have eventually attracted thousands, in one instance hundreds of thousands.

If you know you have a good bat house mounted in a good location, be patient. There is no evidence that adding bat guano will attract bats faster. Bats will likely arrive eventually. If an unoccupied house isn’t receiving at least seven hours of direct, daily sun, it may need to be moved prior to achieving success.