Off-Structure Bat Mitigation

Focus Area

Wildlife & Ecosystems


Natural Resources






1-2 years

Research Idea Scope

The California Department of Transportation (Caltrans) owns thousands of bridges throughout California, many of which provide habitat for bats.  Bats seek out bridges for thermal regulation during reproduction, food digestion, migratory roosts, and as hibernaculum.  A single species may dominate a day roost, however both day and night roosts usually support more than one species. Bridge maintenance and replacement often leads to temporary, or even permanent impacts to bats.  Short-term impacts can result from seasonal exclusion, however the greatest potential for long-term impact occurs when a bridge is replaced.  Traditionally, providing new roosts on replacement structures mitigates bat colony impacts.  However, there is a desire for off-structure mitigation to avoid worker exposure to bats and bat waste products in confined spaces that is cost effective, require minimal, if any maintenance, and is suitable in a wide range of conditions including coastal and inland climates. A qualified bat biologist will be contracted to work closely with Caltrans in the development of a suitable cost effective off-structure design.  Concurrently, the biologist will monitor the bat population on the State Highway 46 Cholame Creek Bridge in San Luis Obispo County, California for one year to determine the species and number of each species using the bridge, along with the seasonality of each species and its numbers.  This bridge is planned for future demolition during a planned realignment of Hwy 46.  Construction of a mitigation structure will begin when sufficient information is available about current usage and target species.  Once complete, the mitigation structure will be monitored for at least one-year to determine the success of the structure.  A successful structure will accommodate the same species and, at a minimum, the same approximate numbers at comparable times throughout the year. The proposed work can be undertaken in two phases; one to study a bridge with a bat population and design a mitigation structure, the other to construct the mitigation structure and monitor success post construction.

Urgency and Payoff

Attempts at off-structure mitigation in California have been largely unsuccessful.  Those attempts have included pole mounted wooden bat houses and concrete bunkers.  There are several benefits to successful off-structure mitigation.  By far the greatest is eliminating the need for periodic disturbance of roosting bats.  In addition, successful off-structure mitigation can increase daytime roosting habitat and help stabilize at-risk species.  Other benefits include eliminating worker exposure to bats and their waste products and developing an off-structure design that can be altered to accommodate any bat species and adapted to a specific site and the species present.

Suggested By

Thomas M. Edell, California Department of Transportation