| Efforts to minimize and mitigate impacts to bats from bridge widening, seismic retrofit, and replacement projects include creating replacement roosts. While several replacement roost designs have been implemented successfully, few monitoring studies have been conducted to evaluate the long-term effectiveness of these designs. For this project, two types of roost structures were installed as mitigation at a bridge complex spanning the Santa Ana River in southern California. Temperature data loggers were installed in a representative sampling of these structures. Regular surveys were performed from March 2015 to August 2017 to determine whether roost structure design or location affected roost temperature and bat occupancy. The results of this study are consistent with other studies showing that thermal stability and high temperatures are important co-factors in maternity-season roost selection. However, data indicate that bats preferred roosts with wider temperature fluctuations during fall and winter, and the thermal stability of a roost structure was correlated more with the location of that roost than with its design. Consequently, site selection on a bridge may be an important factor to consider when mitigating for a maternity roost versus a migratory or overwintering roost. |