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

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.