Efforts to minimize and mitigate impacts to bats from bridge widening, seismic retrofit, and replacement projects often include creating replacement roosts. While several replacement habitat designs have been implemented successfully, few long-term monitoring studies have been conducted on bat roosting habitat mitigation. For this project funded by a partnership between the California Department of Transportation and the San Bernardino County Transportation Authority, and implemented in coordination with the California Department of Fish and Wildlife, two types of replacement roost structures were installed as mitigation at a bridge complex spanning the Santa Ana River in southern California. Temperature data loggers were installed within a representative sampling of these structures, and regular surveys were performed from March 2015 to August 2017 to determine whether roost structure design and/or where that roost structure was sited had any effect on 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, during the fall/winter seasons, data indicate that these bats preferred roosts with wider temperature fluctuations rather than simply choosing cooler roosts. In this study, the thermal stability of a roost structure was more correlated with the location of that roost than with its design; consequently, site selection on a given bridge may be an important factor to consider when mitigating for a maternity roost versus a migratory or overwintering roost site.
Mitigation for Transportation Projects