Bat Roosting in Bridges: Pros and Cons of Assessment Methods from a New England Regional Study

Abstract

A research project was undertaken to assess bats’ use of bridges for roosting in New England. The project included rapid visual assessments of 191 bridges and more extensive evaluation of 18 of these bridges. Full evaluations included acoustic monitoring, detailed inspections, and emergence studies during early, mid-, and late summer roosting seasons in summer 2015, summer 2016, or both. During this project, 13 bridges were documented and positively identified as bat roosting sites in New England, and an additional two were highly likely (six of them were monitored in the project; 11 were identified by state departments of transportation), with possible roosting at several other sites. The project evaluated monitoring technologies including acoustic methods, infrared imaging, borescope inspection, and visual inspection. To identify bat species, both full-spectrum and zero-crossing acoustic monitoring software programs were used. These programs’ results differed significantly in species identification of identical call sequences; the range agreed with wide discrepancies between automated acoustic bat identification software program results previously reported in the literature. A new bridge inspection survey has been developed for the project to supplement FHWA-FRA survey forms. Recommendations for bridge surveys, training of inspection personnel, interpretation of data, and information to request from consultants have been developed through the project. Final project findings can be used as guidance for transportation agencies developing protocol for construction at potential roosting sites, which will be especially useful if federal listing of bat species and associated 4(d) rulings are modified in the future.