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Bats in Bridges


There is a critical need to repair and replace structurally deficient bridges throughout the U.S. in order to protect public safety and maintain the nation’s transportation network; however, the work can bring a host of challenges in any location. Multiple factors must be considered, from seismic and corrosion concerns to impacts on the environment and the public. But what happens when bats inhabit the bridge as well?

Bats are flying mammals probably best known to many as the source of spooky gags during Halloween, but they in fact provide a host of environmental services including pest control, pollination and distribution of perhaps the best fertilizer on the planet. There are actually dozens of bat species inhabiting all 50 states, several of which are protected under the Endangered Species Act due to declining populations and the spread of a lethal fungus responsible for White-Nosed Syndrome. This condition affects wintering bats and has greatly reduced their numbers in some regions.

With voracious appetites, a single bat is able to eat 1,000 mosquitoes in one night. Given that a bridge in California harbors 250,000 bats and another in Texas harbors one million, these mammals are largely responsible for controlling major agricultural pests, including the corn borer moth. For this reason, the knowledgeable public, as well as the nation’s agricultural industry, increasingly want bats protected and accommodated whenever possible. With mosquito-borne illnesses on the rise, bats may become even more appreciated as natural vector control.

Bats are naturally attracted to crevices and deep recesses to protect them from the elements and potential predators. Therefore, bridges with long deep expansion joints, or vent holes leading to hollow interiors are especially favored.

Sometimes it’s necessary to create an alternative roosting habitat for bats

Bats’ inhabitation of bridges can be divided into two major behaviors, day roosting and night roosting. Day roosting bats are sleeping, resting or caring for their pups, which, unable to fly, are left behind while the adults go out to forage. Therefore, day roost locations tend to be very secure, hidden and thermally stable for both the adult and juvenile bats. Night roosting is a more temporary behavior where bats seek out short-term shelters, generally after an early evening foraging, in order to rest and digest before another foraging mission. Night roosts can be seen in open situations under an overhang or other simple cover.

To evaluate whether bats inhabit a standing bridge, first consider the bridge design and materials. Bats that desire day roosting spots typically seek areas of thermal stability – the less variation in temperature over a 24-hour period the better. Therefore, bridges made of concrete or wood, which provide this thermal stability, are favored over bridges made of steel or other metals, which have a tendency to heat up and cool down rapidly.

Many states’ Departments of Natural Resources and Fish and Wildlife agencies require a professional bat expert to evaluate the bridge. Due to endangerment and the many different roosting requirements that bats have, getting help from experts to determine mitigation and exclusion steps is always advised.

The timing of construction is another major consideration. Most bats breed during the summer months, so construction at this time, when adults are feeding their flightless young, could prove catastrophic for the colony. In many parts of the country, bats leave bridges after breeding season to migrate to wintering locations, making late fall through early spring the best time for bridge construction when bats are present. However, the appropriate timing can vary depending on the species involved and the part of the country you are in, so always check with local experts and agencies.

With the proper accommodation and scheduling, bat colonies can be safely harbored nearby while a bridge replacement project is completed. Bat exclusions, which are used to safely remove bats from construction sites, are designed to allow bats to exit the structure without harm and prevent them from re-entering. Funnels made of aluminum flashing, cut-off construction cones, or other materials allow bats to exit. Once the bats are excluded, the crevices can be filled with expandable foam to prevent re-inhabitation.

Depending on the size of the bat colony to be excluded, it may be necessary to create an alternative roosting habitat during the project, for example a “bat house.” Bat houses vary widely in size and design, but effective models have been approved by Bat Conservation International (www.batcon.org). Larger bat houses are multi-chambered nursery style houses made of pine or cedar plywood and painted using a non‐toxic weatherproof paint. A newer design that has proven very effective is the ‘Oregon Bat Wedge,’ which consist of simple panels of wood or concrete placed under and along bridges. Bat houses should be designed with the size of the colony in mind and in a protected location that receives sun from a variety of angles. In addition, regular maintenance checks should be made to ensure the houses are functioning properly.

Adding bat assessment and protection to an already complicated bridge construction project is sometimes met with frustration; however, maintaining healthy bat numbers is a worthwhile investment in the nation’s pest control and pollination needs.

Drew Stokes, B.S., is a wildlife biologist with the San Diego Natural History Museum with more than 20 years of field biology experience. He is considered an expert on bats in Southern California. He holds permits to capture and handle bats, and has extensive field experience using various techniques such as mist-netting, roost surveys, use of night vision equipment, and acoustic bat surveys. Kevin Clark, M.S., director of BioServices at the San Diego Natural History Museum, has more than 20 years of biological experience. He is a former U.S. Fish and Wildlife Service biologist who has worked on the recovery of numerous endangered species. He holds a variety of permits to nest search, monitor, and band rare and endangered birds, mammals, and reptiles.