Counting Free-tailed Bats in Bridges

By Merlin Tuttle

Merlin inspects Brazilian free-tailed bats (Tadarida brasiliensis) moments after removing them from their bridge crevice roost for counting.

For many years we’ve wondered just how many Brazilian free-tailed bats (Tadarida brasiliensis) could cram into a single 18-inch-deep bridge crevice. Accurate counts of large colonies are difficult no matter how they’re made. However, when estimating bridge colonies, it would help if we knew the number, using an average horizontal foot of crevice.

The solution seemed easy. Two years ago, Glen Novinger, an MTBC member and I, inserted two, three-quarter-inch-thick wooden frames, each encompassing a square foot of interior space, into bridge crevices of the same width while the bats were out feeding. The idea was to later slowly remove them, forcing those roosting inside to exit into a cloth-lined bag from which we would count them.

However, the bats were full of surprises. The first night we waited patiently till half an hour after we’d seen the last ones leave—or at least that was what we thought! But when we approached to install our devices, roughly half remained inside. I couldn’t help but wonder how many emergence counts had missed those that, for whatever their reasons, didn’t emerge at sundown. (more…)

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Bat-Friendly Bridges Help Farmers

Merlin Tuttle


Dave Wyatt (holding light) and Gregg Erickson inspecting an old wooden causeway for bats in 1996. Thanks to their efforts, the California Department of Transportation worked diligently during replacement to ensure bats were accommodated. Today, area farmers are grateful.

Building bat roosts into highway bridges in farmlands can benefit farmers at little or no cost to taxpayers. Mark Bloschock, a supervising bridge design engineer at the Texas Department of Transportation, discovered the potential for bridges to help bats when he worked on Austin’s now famous Congress Avenue Bridge. As hundreds of thousands of bats unexpectedly moved in, he contacted me for advice. He soon discovered that, by simply making small adjustments in the spacing between box beams, large numbers of bats could be attracted where needed, and where they weren’t wanted, they could be discouraged by simply changing the spacing.

In 1998, when highway US 90 required two new bridges over Seco Creek, near D’Hanis, Texas, he wrote specifications that placed the box beams three quarters to 1.5 inches apart, hoping to attract Brazilian free-tailed bats (Tadarida brasiliensis) to this important agricultural area. The bats quickly moved in and soon exceeded half a million, today as many as two million.

For decades, Rachael Long, an agriculture consultant, has been a leader in educating area farmers regarding the values of bats and how best to meet their needs. Here, she inspects a colony in an abandoned warehouse.
I visited the Seco Creek bridges a few weeks ago, finding many thousands more free-tailed bats than could be accommodated in existing crevices. There is clearly a housing shortage and additional crop protection opportunity.



We first learned of Mark’s success when I received a phone call from Rodney Sams, an agricultural consultant specializing in pest management. He explained that he and Joel Curtis, a pecan orchard owner he worked for, were baffled. Their pheromone traps, used to evaluate needs for pesticides, normally held 80 to 100 moths each, but that spring they were down to just a handful. They had seen a large bat emergence from the nearby Seco Creek Bridges and wondered if they were the reason for the dramatic reduction in chemical needs.

I checked with Mark, and he said yes, that was where he had designed bat habitat into a pair of bridges.

Christopher Anderson, a San Antonio Express News writer, interviewed each of us for his story published on April 11, 2000. He reported “Sams strongly believes the voracious insect eaters [bats] are keeping Curtis’ pecans from being assaulted.”

Mark believes that box beam bridges, with suitable spacing, are preferred, not just alternative housing for bats. Luckily for the world of bats, he became a passionate advocate for helping bridge bats help people, and thanks in large measure to his enthusiasm, there are now important bat colonies safely living in bridges from Florida to California.

The latest reported success began in 1996 when David Wyatt and Gregg Erickson, of the California Department of Transportation, and Rachael Long, a farm advisor with the University of California Cooperative Extension Service, invited me to join them to investigate the needs of bats living in old bridges, including those that lived in an old wooden causeway that needed replacement. Thanks to their enthusiasm, Yolo County’s bridge bats now have safe homes in bat-friendly bridges, one of which has become a tourist attraction.

Mark Bloschock inspecting bats in a device used to count bats in bridges.
On August 16, 2018 the Atlas Obscura, in its online magazine, published an article titled, Batnadoes Can Protect California’s Crops, Bats are farmers’ new best friends by Anne Ewbank. In the article, local farmer, Mike DeWit swears that the 250,000 free-tailed bats from the Yolo Causeway play an “outsized role” in keeping his wild rice crops pest free. Organic farmers of the region are particularly intrigued, increasingly seeking advice on how to attract bats. For the sake of both bats and humans, we’re hoping the news will spread as each year we learn more about bat values and how to attract them.

Here’s a guide to Texas’ best bat watching bridges and Merlin’s photos showing the species that live there.

Take action on behalf of bats!

We encourage you to send praise and thanks to the author and editor of the Atlas Obscura article by filling out a feedback form. Of course sharing on social media is always a great way to help bats get the positive publicity they deserve. Bats need all the friends they can get!

Seco Creek bridge crevices overflowing with thousands of free-tailed bats. At approximately 20-year intervals vegetation will need to be trimmed to avoid blocking bat access.

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