A new manual has been written for the California Department of Transportation (Caltrans) to improve bat mitigation for bridge and culvert projects. This document describes bat habitats associated with bridges, bat species' legal status and the implications for bridge and culvert projects, and recommended protocols for bat surveys for bridges, culverts, and the adjacent habitats.
Over 8,000 California bridges are more than 50 years old, and many are expected to require rehabilitation or replacement within 10 years; therefore, there is a need to better understand which mitigation strategies are the most effective for minimizing impacts on bats and their habitats. Ongoing research is enabling us to make more informed impact assessments of bridges and culverts and develop mitigation recommendations that increase the likelihood of success to support intact bat populations in California.
Between late spring and late summer in 2017 and 2018, we surveyed 61 bridges and culverts in 11 of the 12 Caltrans Districts to evaluate the implementation of bat mitigation. Of the 61 sites selected, 39 bridges and culverts (many of which were relatively recently constructed) were evaluated for their effectiveness at mitigating impacts on bats and their roosting habitat. The remaining 22 sites lacked mitigation habitat but were occupied by bats. The bridges and culverts where mitigation was implemented included situations where the original habitat was (1) left intact, (2) lost and new on-site habitat was provided, or (3) lost and new off-site habitat was provided. We also measured the efficacy of using add-on structures, (e.g., Oregon wedge-style bat boxes), as well as cast-in-place day-roosting and night-roosting habitat. The evaluations involved acoustic surveys, exit counts, and day roost counts.
Cast-in-place crevice habitat appeared to be the most successful mitigation approach, followed by Oregon wedge-style bat boxes that were attached to bridges and culverts. Some habitat designs (e.g., open-top bat boxes) were effective in inland regions with warmer nighttime ambient temperatures, but not in coastal regions where the corresponding ambient temperatures were cooler. Off-site bat boxes were almost never occupied by bats, regardless of their design. Issues identified with on-site bat boxes appeared to be the result of: (1) unsuitable design for the target species, (2) inappropriate crevice widths, (3) inadvertent creation of habitat for predatory raptors, (4) the use of unstable materials (e.g., particle board), (5) a design that didn't match the climate, or (6) incidental light pollution. Internal nighttime temperatures and crevice widths appear to be the most critical characteristics for bat occupancy. The results of our evaluation demonstrated the complexity of selecting effective bat mitigation strategies for bridge and culvert projects, and a thorough understanding of the affected species is required for the highest likelihood of success.