Many bat species use bridges, culverts, and other transportation structures for daytime and/or night-time roosting habitat and for seasonal hibernacula and maternity sites. These structures provide valuable habitat for bats whose populations have been declining in many regions as a result of loss of natural habitat and infection by the fungal disease known as white-nose syndrome. In 2015, the U.S. Fish and Wildlife Service listed the northern long-eared bat (Myotis septentrionalis) as threatened under the federal Endangered Species Act; additional bat species may warrant listing in the coming years. Many states also have legal protections for bat species.
When state departments of transportation (DOTs) need to repair or replace transportation structures, they must take measures to avoid and minimize impacts to bats if protected species are present. Such measures are particularly important during the maternity season to avoid further losses to bat populations. To meet all of these objectives, state DOTs can restrict construction or maintenance activities to periods when bats are not present. However, temporal avoidance is challenging in regions where winter conditions and cold temperatures limit the construction season.
Alternatives to temporal avoidance include methods that temporarily deter or exclude bats from a structure for the shortest length of time needed for construction or maintenance activities, with the intent to allow bats to return once the project is complete. Methods to accomplish this include physically blocking bats from the structure or cavities with exclusion or filler material; modifying roosting habitat by changing microclimatic conditions (e.g., installing fans or removing expansion joint glands); and deterring bats with lights or noise. One promising method is the use of non-lethal ultrasonic acoustic devices.
However, effectively excluding or deterring bats can be challenging. Some deterrence methods require costly equipment or materials and considerable labor for installation and maintenance. Some project locations may not have a readily available power source to operate deterrence equipment. At some sites, expensive or sensitive deterrence equipment can be vulnerable to vandalism and/or theft. Further, if a method is not effective and bats continue to use the bridge, the project may be delayed. Additionally, if the intent is to allow bats to return to the structure after the project is completed, the deterrence should be temporary in its effect. Selecting a method that will be effective in deterring or excluding bats and be feasible for the project requires consideration of bat behavior and biological needs, project characteristics, and site conditions.
Methods that are appropriate for the project and target species, properly timed, and effective at temporarily deterring and/or excluding bats from a transportation structure provide several benefits. In addition to reducing impacts on imperiled bat species and successfully meeting regulatory requirements, improved environmental stewardship can strengthen interagency collaboration and partnerships. Effective methods can make construction schedules and costs more predictable, thereby increasing overall cost-effectiveness and reliability in delivering transportation projects and programs for the traveling public.
Previous research has investigated a number of bat deterrence and exclusion methods, although in practice, outcomes have been somewhat mixed and documented examples are limited. Research is needed to evaluate current and promising methods for temporarily deterring and/or excluding bats from a range of structure types and to provide guidelines for state DOTs on selecting and implementing appropriate and effective methods.
The objective of the research is to develop a handbook for state DOT environmental staff and design and maintenance engineers on how to select and implement methods to temporarily deter and/or exclude bats from transportation structures ahead of and during construction and maintenance activities. The handbook will describe methods that are (1) sensitive to the biological needs of bats and (2) effective for a range of geographical locations, project types, and site conditions. The research will include field evaluations of a variety of methods, with a focus on non-lethal ultrasonic acoustic devices used alone and in combination with other methods.