By Merlin Tuttle
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 today quality bat houses, properly located, are achieving up to 80% success. But the biggest problem has been finding reliable vendors whose bat houses truly meet bat needs. Far too many purchasers have become discouraged due to the failure of cheap, poorly constructed houses, or even good houses sold with inadequate instructions.
Characteristics of a Quality Bat House
- Wherever you live, several species, including most temperate-zone bats, will likely prefer roosting crevices 3/4” wide. A few may prefer 1″ to 1-1/2” crevices. Endangered Florida bonneted bats (Eumops floridanus) that live only in central and southern Florida and Pallid bats (Antrozous pallidus) that occur in arid and semi-arid areas of the West, from southern Canada through much of Mexico, prefer these wider crevices. Big brown bats (Eptesicus fuscus) are found throughout most of North America and will readily accept chamber widths of 7/8″ to 1″. Multi-chamber houses that include a wider roosting crevice may additionally appeal to these species. Roosting crevices greater than 1″ wide should be tested only where larger species are targeted.
- Taller houses provide the best thermal options and ideally should be at least 20″ tall. More is often better. Shorter houses should be used only on buildings where risk of overheating is minimal. Taller roosting crevices are most important in single-chamber houses exposed to direct sun.
- Tight fit construction and caulking of all external seams reduces leakage and warping that can lead to early deterioration and abandonment.
- Best resistance to weathering and warping can be provided by outer shells of aluminum or UV-resistant plastic. However, houses made solid wood, at least 3/4″ thick, have proven highly successful without painting. Houses of thinner wood that remain unsealed or painted often become warped and unhibaitable within 2 to 3 years.
- All exterior parts of wooden houses ideally should be at least 1/2” thick. Any plywood used should be exterior grade.
- All interior surfaces and landing areas must be roughened, enabling bats to gain secure footing. Horizontal grooves 1/16″ deep at 1/4″ to 3/4″ intervals are ideal. However, grooves should not be cut more than 1/32″ deep for plywood surfaces to avoid early deterioration. Hand-roughened or rough-cut surfaces also work well.
- All bat houses should have a roughened landing area extending at least 4” below the roosting partitions. Additionally extending the sides, back, and front approximately 4” below the roosting partitions provides a recessed landing area that protects against owl attacks during entry.
What Size to Choose
As bat house quality and knowledge of preferred mounting locations have improved, size has become less important. Both large and small bat houses can be successful as long as the bats’ basic needs are met. Many single-chambered houses are successful when mounted on buildings. Houses with two or more chambers may be more likely to attract nursery colonies. When a bat house is being provided 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 near one another.
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 fall unpredictably, potentially leaving occupants homeless. Knowing there are options nearby appears to increase attractiveness. Before purchasing a house designed to shelter hundreds or thousands of bats, it is best to experiment with several smaller houses in varied sun exposure to evaluate local needs.
Where Bats are Most Easily Attracted
Mixed habitats near freshwater, 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 ten species that occupy bat houses in North America. Areas surrounded by large expanses of buildings or single-crop agriculture are least attractive.
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 to 15 feet above the ground.
All bat houses should be in sunny areas, except in the hottest climates. There is a strong preference for dark brown or black houses in the coldest regions. In hot climates, medium 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 the 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 two to three years or more to succeed. In all cases we are aware of, the largest houses have taken the longest to attract their first occupants. Nevertheless, we’ve guided large-scale constructions that have eventually attracted thousands, and in one instance hundreds of thousands.
If 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.
MTBC-Approved Bat House Vendors
The following retailers meet most of our criteria for bat house success and longevity. There are likely others providing quality houses, and we would be happy to consider adding them to our list once they pass inspection. To seek approval, please follow our Bat House Seal of Approval Instructions for Applicants.