Abstract

The gray bat (Myotis grisescens) is a federally endangered species that uses caves almost exclusively for roosting. In 2016, gray bats were discovered using bridges for roosting in the French Broad River basin. Objectives of this study were to document characteristics of bridges used by gray bats for day roosting and compare those characteristics to randomly selected bridges not used by gray bats. During the summer of 2018, we surveyed bridges for bats over rivers and streams within the French Broad River basin. At each bridge length, width, height above water, aspect, size of gaps between joints under the bridge, and underdeck and beam material were recorded. The presence of bats based on fresh sign (e.g., presence of guano) or visual observation was also recorded. If bats were present, experts from Indiana State University determined species identification visually, and audio detectors were used to confirm identifications. Exit counts during the early evenings also were conducted to estimate the number of gray bats roosting at each site. A total of 93 bridges were surveyed. Of those, eight bridges (9%) were used by gray bats for roosting; six spanned a river and two spanned a stream. The number of gray bats roosting ranged from 1 to more than 1000 per bridge. Occupied bridges were larger and higher above the water than unoccupied bridges, and a greater proportion was constructed of concrete and steel, but never wood. These results suggest that gray bats prefer to roost under large bridges made of concrete and steel, and avoid those made out of wood. This information increases our understanding of gray bat roosting ecology and may help to facilitate conservation efforts by NCDOT when planning new bridge projects.

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