Use of Box-Beam Bridges as Day Roosts by Mexican Free-tailed Bats (Tadarida brasiliensis) in Texas

Authors: Melissa B. Meierhofer, Hsiao-Hsuan Wang, William E. Grant, John H. Young Jr., Lauren H. Johnston, Lilianna K. Wolf , Jonah W. Evans, Brian L. Pierce, Joseph M. Szewczak and Michael L. Morrison

Bridges provide roost structures for bats in temperate regions of the US, including Texas, where Tadarida brasiliensis (Mexican Free-tailed Bats) are common occupants. In March 2018, we documented 1 Mexican Free-tailed Bat with Pseudogymnoascus destructans, the fungal causative agent of white-nose syndrome (WNS), in an artificial structure in Texas, thus making the ability to quantify their movements and occupancy critical for understanding WNS ecology.

To determine which attributes influenced day-roosting activity by Mexican-Free-tailed Bats, we surveyed for roosting bats 70 box-beam bridges in 21 Texas counties and collected structural, weather, and landscape-characteristic data. We analyzed the data with a stepwise multiple logistic regression model to isolate variables significantly correlated with presence of day-roosting Mexican Freetailed Bats. Of 70 bridges sampled, 14 (20%) contained day-roosting Mexican Free-tailed Bats and 17 (24%) bridges had signs indicating bat use. In the best-fitting logistic regression model, bridge width, number of spans, and elevation had a positive influence on bat occupancy, whereas average temperature for the month of July 2016 negatively influenced bat occupancy. Bridge age also had a positive influence on bat occupancy, but the effect lessens in older bridges. These data show that structural and environmental characteristics are significant predictors of bridge use by Mexican Free-tailed Bats.