Bat House Warnings – A Reality Check

5/6/21
Merlin Tuttle

Observations of heat-stressed, sometimes dead bats associated with bat houses, have led to unfortunate speculation that bat houses can become ecological traps that lure bats to their death. It is true that numerous bat houses are badly built and sold with unreasonable claims and little, if any, instruction on bat needs. Vendors of such houses defraud customers and threaten the credibility of bat conservation. Both vendors and customers can benefit from education and certification. Nevertheless, there is no evidence that poorly constructed bat houses threaten bat survival. Bats are smart enough to avoid bad bat houses except when desperate from lack of alternatives.

Loss of vast numbers of traditional roosts is a key cause of bat decline. Most species that occupy bat houses today originally relied on loose bark and cavities in snags that were often lost during storms. This likely explains why bats prefer to live where multiple roosting options are available.

 

Radio-tracking studies show that occupants of natural roosts frequently move among several, apparently to escape predators and parasites or find optimal temperatures. During severe weather, large numbers may die even in traditional roosts. With so few remaining, bats often fail to find ideal homes.

 

When providing bat houses, the best way to reduce mortality is to offer a variety of roosting options. Several houses ideally should be colored, positioned, or vented to provide a range of temperature. Needs vary between cool versus hot weather extremes.

 

Northern long-eared myotis (Myotis septentrionalis) emerging from beneath tree bark. Many bats roost beneath lose bark on old snags.
A nursery colony of Fringed myotis (Myotis thysanodes) reared young beneath exfoliating bark on this old snag in Arizona.

Selecting a quality bat house, as well as proper placement, are crucial to success. In our forthcoming book, Danielle Cordani and I will provide plans for houses that maximally meet bat needs over a wide range of temperature. However, a single house is unlikely to prove ideal for all species and locations.

Where sufficient habitat exists, especially within easy reach of overwintering locations, available houses often become crowded, and growing numbers of young may perish for lack of available space. The same occurs at traditional roosts. We simply don’t see it. And we can’t prevent it, though careful monitoring and providing more bat house options can help.

We installed this bat house, donated by BatsBirdsYard.com, for testing in a central Texas pecan orchard. Nut growers increasingly are using bat houses in pest management.

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Collaboration to Improve Housing for Bats

9/20/18

Harrison (standing) and Christopher showing off their new BatBnB houses.

For the past 18 months Merlin has been assisting Harrison Broadhurst and Christopher Rannefors in creating a line of designer bat houses, attractive to both home owners and bats. Their curving architecture isn’t, just pleasing to the eye. At least theoretically, it may increase the area available for roosting, since most bats appear to prefer to line up with faces exposed to the outside air, not to another’s posterior, quite understandable!

The new houses are constructed of kiln-dried, sustainably sourced, three-quarter-inch western red cedar, with each piece dovetailed and caulked, thus minimizing leakage. Roosting chambers are extra tall (20-26 inches) and vented to ensure thermal gradients preferred by bats. All landing and roosting surfaces provide one-eighth-inch-deep cross cuts at half-inch intervals, ensuring maximum footholds for young bats. By avoiding plywood, the possibility of early deterioration and potential off-gassing are finally eliminated. Furthermore, treatment with Thompson’s WaterSeal Semi Transparent Stain & Sealer will additionally protect against warping.  BatBnBs come with complete instructions and are easy to mount.

We’re proud to be a part of Harrison and Chris’ efforts on behalf of bats and a safer environment. They’ve been featured in dozens of leading publications and have even appeared on several television shows. You can learn more about them and obtain your own BatBnB by visiting their online shop. By using the code MERLIN when ordering you will be given a 10% discount, and the company will donate to MTBC an additional 5% of all orders using this code. Feel free to share this discount and BatBnB’s website with friends and let us know about your progress. By purchasing a BatBnB, you can reduce the threat of harmful pesticides in your neighborhood and help MTBC educate the world about these invaluable neighbors.

 

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chiroptorium design

World’s First Artificial Bat Cave

World’s First Artificial Bat Cave Provides Model for Future
By Merlin Tuttle
8/14/2016

 

artificial bat cave design
The chiroptorium covers 3,000 square feet (279 square meters) and offers about 8,000 square feet (743 square meters) of likely roosting surface. A structure of welded-together rebar, coated in heavy plastic, was then sprayed with a foot of gunite to form a permanent shell.

Modern bats face a serious housing shortage. Millions of homeless bats have died when their caves were destroyed or converted to exclusive human use, not to mention when old-growth forests were logged. Often, the single most important action we can take to restore bats today is to provide alternative homes.

We know from long experience that desperate bats often readily occupy human-made structures, from abandoned mines and railroad tunnels to old buildings. Though building backyard bat houses is an excellent way to help, sometimes it is very much in our mutual interest to provide long-lasting structures that can accommodate large numbers, not only for pest control, but also for the pure entertainment large colonies can provide.

construction crew at artificial bat cave
Construction crew working in entrance passage. Vertical braces were removed once the gunite hardened.

When J. David Bamberger was first introduced to an evening emergence of the millions of Brazilian free-tailed bats (Tadarida brasiliensis) at Bracken Cave in the Texas Hill Country, he was awestruck. He fell in love with this wonder of nature and soon began asking if it would be possible to attract a miniature Bracken colony to his ranch. Undaunted by an absence of caves, he asked me about the feasibility of “building” a cave.  Would bats come? (more…)

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