Improving Bat Houses in America:

Nearly 40 Years of Progress and Still Learning

9/14/2020

By Merlin Tuttle

Bat houses are outstanding tools for education. When I introduced them to Americans in 1982, my primary objective was to help people overcome fear and accept bats as valuable neighbors. That goal has been vastly exceeded. Today, hundreds of thousands of American bats live in a wide variety of bat houses.

Individuals who have carefully tested local bat preferences, and adapted accordingly, are reporting close to 90 percent occupancy. Nevertheless, there is still much to be learned. And that is why we’re initiating new collaborations.

Late last month, local member, Debbie Zent, founder of Austin Batworks, reported an impressive event. Her three-chamber nursery house had been caulked, sealed, and painted inside and out, and was mounted high on a streamside ranch building—a nearly perfect combination. But to find it overflowing with occupants just days later was surprising.

This Texas Hill Country bat house became overcrowded within days by Brazilian free-tailed bats (Tadarida brasiliensis).
The house was ideally located approximately 18 feet up on a building beside a permanent creek where it receives only morning to mid-day sun.

Was this extraordinary success due to house or location quality, or were these bats simply desperate?

We are now working with Debbie to test the latest innovations in bat-rich areas, especially where previous attempts have failed. Additionally, we recently collaborated with Tom and Laura Finn, founders of Fly By Night, Inc.to test three of their Deluxe Bat Condos that have peak-season capacity to shelter up to 1,000 bats each. Hundreds have been installed in Florida with 90 percent occupancy. Ours were mounted on poles near rivers. Additionally, Tom helped us mount three small houses on nearby buildings.

Tom Finn digging a 3.5-foot-deep hole for installation of one of his Deluxe Bat Condos overlooking Onion Creek, a few miles southwest of Austin.
Tom demonstrating how one person can use a tapered trench to install a bat condo on top of a 24-foot pole.
Dani Cordani, MTBC’s Bat House Project Coordinator, assisting Tom by adding water to concrete mix already in the hole. The bottom of the fully erected house is 17-18 feet above ground.
Tom explaining condo features, including metal roof, landing pads, sealed and painted surfaces throughout, and unique venting. More details to be available in MTBC’s forthcoming bat house resource.
Pedernales River habitat adjacent to our second condo location.
Tom and helpers inspecting the newly-installed condo at Westcave Preserve, also near the Pedernales River. Assuming occupancy, reserve educators hope to explain bats to visitors during evening emergences.

Tom demonstrating how to use a rope to lift a bat house after having climbed a ladder. This BatBnB two-chamber house has been altered with three-quarter-inch thick furring strips to provide extra roosting space between the house and building. This adds an additional roosting chamber at little cost (above, left). During installation, Tom braces the house on his ladder. This house will be shielded from rain but exposed to morning sun (above, right).

On a broader scale, Jay Gaderre, of WHITEHORSE, has agreed to help test a small bat house design that has proven more than 90 percent successful in Taiwan. Jay will be shipping samples for testing in five U.S. states.

Taiwanese schoolteacher, Heng-Chia (Thrash) Chang, invented an extraordinarily successful bat house that has approximately 90% occupancy.
Thrash mounted more than 100 of his unique bat houses at the grade school where he teaches. The school is now famous for its bats and attracts 20,000 visitors annually to its Formosan Golden Bat Museum. The students are almost continuously surrounded by bats wherever they go and not a single one has ever been harmed or contracted a disease.
Three mother lesser Asiatic yellow bats (Scotophilus kuhlii), each with one pup. Thrash’s small houses are very popular and have attracted 12 species when mounted on buildings. They don’t attract large bat colonies but are very popular because they are lightweight and easy to mount on a wall.
This is the only one of Thrash’s houses mounted on a tree at his school and it’s the only one that has never been occupied.

Why so much new testing? Haven’t we already learned all we need to know about bat house basics? Yes, between 1988 and 2004 we did learn a great deal through the North American Bat House Research Project, led by Mark and Selena Kiser and myself, relying on invaluable reports from thousands of volunteer participants.

Virtually all kinds of bat houses were at least occasionally successful. But the best-used houses were:

·        tightly constructed, caulked, and painted

·        mounted 10 to 25 feet above ground on buildings or poles

·        positioned to receive sun appropriate to local climates

·        roosting chamber widths of ¾ to 1 inch

·        sited within a quarter-mile of a stream, river, or lake

·        located in areas where bats were already attempting to roost on or in buildings

 Failures were strongly associated with:

·       warping due to poor materials or treatment

·        mounting on trees

·       vulnerability to predators (mostly climbing snakes or owls from nearby perches)

Initial results were mixed. Some users uniformly failed while others with similar houses were dramatically successful. As we’ve gradually discovered why, occupancy success has improved.

Temperature was identified as a key determinant. Bat houses could be too hot or too cool, or could simply fluctuate too much. For example, in coastal areas of the Northwest, bats readily occupied houses mounted on poles but rejected them east of the Cascade Mountains. Day-to-night temperature fluctuated too much in arid areas. This can be solved only by using extra-large houses or by mounting them on buildings which serve as heat-sinks that dampen day-to-night temperature fluctuations.

Except in areas of extreme day-to-night changes, pairs of houses mounted back-to-back on poles in full sun, one facing southeast, the other northwest, are increasingly favored by America’s most experienced bat house users. Testing can pay big dividends.

In Florida, Ernie Stevens hung a bat house from a tree limb and attracted a nursery colony of 124 apparently desperate evening bats. When I convinced him to try mounting a pair of houses back-to-back on a pole in full sun, his original colony moved to the house facing northwest and expanded to over 300. These were joined by free-tailed bats in the warmer southeast-facing house. His resulting mixed colony included 800 bats!

 

In Louisiana, Bill Halloway began by mounting two houses on pine trees. No bats were attracted, so he moved them to a pole, mounted back-to-back. Bats promptly began moving in. Encouraged, he added another pair of larger houses on a pole, one black and one white, and ended up with a mixed nursery colony of 800 free-tailed and big brown bats, apparently due to a greater range of temperature options.                

Much can be learned by simply watching bat house occupants on warm versus cool days.

In Pennsylvania, Lisa Williams and Cal Butchkoski noticed that their little brown bats moved up on cool days, down on warm, literally hanging their heads out on extra hot days. By adding ventilation slots, they were able to protect their bats from overheating, even in sun-exposed, black houses on the hottest days. Vents are now standard in all but the coolest climates.

As a result of such reported discoveries, the Kisers and I were able to publish the Bat House Builder’s Handbook in 1993. Over the subsequent 27 years discoveries have continued. Recently, Dani Cordani and I have been interviewing America’s most experienced bat house users in a wide variety of habitats and climates, each with decades of experience with hundreds of bat houses.

Based on their experiences, it’s clear there is still more to learn. We’re working on an updated resource that will:

  • summarize points of broad agreement
  • explain areas still in need of improved understanding
  • introduce longer-lasting materials and designs, and
  • provide detailed construction plans
Lisa Williams and her mentor Cal Butchkoski were among the first bat house pioneers in America. Here they are monitoring temperature in each bat house roosting chamber to determine effects of bat house shape and the positioning of ventilation slots.

 

If you’ve made a potentially important discovery, it’s not too late to share it with us at bathouses@merlintuttle.org. If you’d like to support these endeavors, please donate here and/or join MTBC as a sustaining member.

To better protect both bats and bat house consumers, MTBC is also offering our seal of approval to vendors whose houses meet our strict standards for construction and advice. Our resource, Selecting a Quality Bat House, provides guidance and a list of certified vendors. To apply for certification, contact bathouses@merlintuttle.org.