Development of a Bat Guano and Acoustic Sampling Testing Protocol to Identify Species Occupying Virginia Department of Transportation Bridges

Report No: 22-R20

Published in 2022

About the report:

Over half of Virginia’s extant bat species, including six imperiled bat species, have been documented as using bridges as day or night roosts. To prevent or minimize harm to these species, the Virginia Department of Transportation (VDOT) performs surveys to detect bat use in transportation structures when essential infrastructure maintenance must occur. The current indicators used by VDOT to inspect bridges for bat use include staining, guano piles, and the presence of live bats. Notwithstanding,  without trained personnel, it can be difficult to positively identify species use. Without the ability to confidently identify roosting species, regulatory agencies cannot discount transportation structure use by a sensitive species, such as the federally endangered gray bat (Myotisgrisescens). 

Most acoustic detection survey methods allow for the identification of bat species active in an area but cannot confirm that a species is using a transportation structure as a day or night roost with certainty. Because acoustic detection methods are relatively inexpensive and not labor intensive, they can be used to monitor areas for an extended period to determine the likelihood of bat presence and provide region-wide predictive modeling to assess potential risk. However, acoustic monitoring alone cannot confirm roost use, so a subsequent, genetics-based approach could confirm which species have roosted in a structure.

In this project, we tested a combined approach using intensive acoustics in conjunction with DNA barcoding of guano found at the sampled bridges to detect the species present in the areas around 40 bridges in southwestern Virginia’s Bristol District. From March to November 2019, we observed bat activity with acoustic sampling throughout the Clinch, Powell, Holston, Big Sandy, and New River watersheds. Gray bat activity at the bridges was correlated with proximity to the known summer maternity roost in the Bristol area and mean cave density in the surrounding landscape. Combined with pilot acoustic data from 2018 and a partial continuation in 2020, we observed high year-to-year variations in gray bat activity. We found that a long acoustic sampling duration is necessary to discern the monthly presence and relative abundance patterns of imperiled bats species (focusing on the six imperiled species documented as using bridges) across the year from emergence to the initiation of hibernation. The spatiotemporal patterns that we observed can help VDOT assess the risk to gray bats and other bat species from transportation structure management activities.

In total, 283 guano samples were collected from 29 bridges for subsequent DNA analysis. Although 245 of the samples were amplified, only 77 (27% of the collected samples) were of sufficient quality to find a species match. Nine bridges had guano with DNA that matched big brown bats (Eptesicusfuscus), 12 bridges had guano matching gray bats, and three bridges had guano matching the federally threatened northern long-eared bat (Myotis septentrionalis). The bat species at all the sites with guano-derived DNA were also recorded acoustically. For guano DNA analysis, additional work-refining techniques will be needed; however, as proof of concept, the combined approach to bat sampling that we developed may aid VDOT managers in assessing bat use of bridges, which is particularly valuable in areas such as the New River drainage into which the gray bat population is newly expanding.

Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.


  • Hila Taylor, Karen E. Powers, Ph.D., Eric M. Hallerman, Ph.D.

Last updated: November 7, 2023

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