Volume 12, Issue 4, Winter 1994

Research Begins on Bat Friendly Bridge Designs

THE LARGEST urban bat colony in the world, over a million Mexican free-tailed bats (Tadarida brasiliensis), roosts beneath a bridge in the heart of Austin, Texas-now a matter of civic pride [BATS, Summer 1990]. The presence of the bats is a direct result of bridge reconstruction in the early 1980s, which inadvertently created ideal crevice widths for bats. As traditional bat roosts in caves or tree hollows are disturbed or destroyed, it becomes increasingly clear that bridges have great potential to provide alternate homes in regions with warm climates.

The success of Austin's Congress Avenue Bridge as a major bat roost has attracted much interest from other communities as well as from state and federal agencies. Both BCI and the Texas Department of Transportation have been receiving growing numbers of requests from across the country for information and advice on how to duplicate the success. After Merlin Tuttle addressed the annual Bridge Designer's Conference of the Texas Department of Transportation, department engineers began thinking seriously about how to attract bats to other bridges.

BCI and the Texas Department of Transportation began a multi-year cooperative project this spring to survey other bridges throughout Texas and determine which designs best attract bats, and why. Funding is being provided by the Texas Department of Transportation, The Winslow Foundation, and The National Fish and Wildlife Foundation. The new project is a follow-up to a pilot study conducted in 1991, which concentrated primarily on Central Texas. In that study, certain bridge designs were shown to have high occupancy rates.

Over this past spring and summer, more than 600 bridges were examined throughout Texas. Although only a small number (about 50) of the bridges provided the right roosting conditions for bats, those that did accommodated an enormous number of bats-possibly as many as six million individuals-mostly free-tails. This represents a significant population and demonstrates the importance of bridges as roost habitat in Texas.

Using the additional data and detailed measurements resulting from the new study, BCI is working with bridge design engineers to discover the most effective and practical methods to meet bat needs without adding to bridge costs as new ones are built or others are repaired. The Texas Department of Transportation is already making slight adjustments when building bridge types appropriate for bat roosts, and this year alone may have created enough new habitat to accommodate a million additional bats. Without this cooperative project, older bridges would gradually have been lost as bat roosts. Experiments are also underway to retrofit some existing bridges with structures that will accommodate bats. In the first test on a bridge near Austin, bats moved in within five days.

Building significant wildlife habitat into Texas bridges could become a national and international model. The U.S. Fish and Wildlife Service is already using the findings to advise state highway departments on bridge construction projects. Mark Bloschock, a bridge design engineer for the Texas Department of Transportation, attended an October meeting in Mexico City initiated by BCI and the U.S. State Department. The meeting was called to take action on the loss of migratory free-tailed bat populations [see "A Binational Partnership to Protect Mexican Freetailed Bats," page 6]. Bloschock presented the preliminary results and success of the project as an example of how to create bat habitat. Freetailed bat populations in Mexico, where bats from the United States migrate each winter, have suffered tremendous declines from habitat loss.

BCI invites members and others to contact us with information about bridges occupied by bats anywhere in the country. Although current evidence tends to indicate that bridges will prove to be good bat habitat only in southern regions or in warmer areas of the West, there may be exceptions. Eventually, the project will expand to include other parts of the country. Contact the Bats and Bridges Project Coordinator at BCI, P.O. Box 162603, Austin, Texas 78716 or 512-327-9721.

The Texas Department of Transportation is cosponsoring research on bat friendly bridge designs. A bridge previously unused by bats was fitted with special crevices, which attracted bats within a week.

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