New bridge takes flight
How does one go about housing 40,000 bridge-dwelling bats? Officials in Sacramento County, Calif., found themselves faced with that question after a 1997 flood damaged the Franklin Boulevard Bridge where northern California’s largest maternal colony of Mexican free-tail bats was living. While federal regulation did not require the critters’ habitat to be replaced when the bridge was rebuilt, local and state officials decided the bats were worth incorporating in the new design.
From March to October of each year, the bats would roost in the bridge’s trestle to mate, give birth and nurture their pups. The bat colony also was an important environmental and agricultural resource for the farming community. Every day, the 4-inch-long bats devoured approximately 1,500 pounds of flying insects that would otherwise feed on local crops. In addition to the bats, at least half a dozen endangered or threatened species — including fish, snakes and hawks — occupied the area.
Before the flood damaged 1,700 feet of trestle and 500 feet of approaches, the bridge was the primary north-south route across the Mokelumme River for the local farm-to-market traffic and a well-traveled alternate to the nearby interstate. Completely replacing the bridge proved to be a complicated process. “This project was noteworthy because of all the environmental issues, and the way we proposed to go about incorporating the mitigations,” says Tom Zlotkowski, director for the Sacramento County Department of Transportation. In-water work, for example, was limited to mid-June to mid-October because of migrating salmon. Also, tree removal to clear the right of way was scheduled so that it would not interfere with the nesting season of endangered raptors.
Before bridge construction began two years ago, the bats had to be removed from the trestle, a process that took almost two years. Zlotkowski and a project engineer from Los Angeles-based DMJM Harris worked closely with Sacramento County’s Department of Environmental Review and Assessment (DERA) to protect the bats by providing housing during trestle demolition. The county’s staff waited until most of the bats had left for their seasonal migration in October, and then used small-mesh bird netting to cover the known roosting sites within the bridge. Keeping the bats away also required caulking every existing niche.
When bats returned to roost, they were put up in eight pre-fabricated houses purchased from Daingerfield, Texas-based Maberry Centre Bat Homes. With an unoccupied weight of 540 pounds each, the 4-foot-square, 5-foot-tall bat condos are supported 15 feet above the ground to prevent flooding and to allow departing bats to fall and catch air in their wings before taking flight.
The $14 million replacement project is now nearing completion. Deep, narrow, vertical crevices created with Redwood planks have been integrated into the soffit of the new bridge deck to form nearly 20,000 square feet of habitat. Accommodating the bats added an estimated $160,000 to the project total.
The bridge, which should open to traffic in late September, already has attracted at least 15,000 bats. The goal is for the colony to re-establish itself at pre-project numbers within 10 years. “The sight of bats leaving to feed every night is amazing,” says Toni Barry, DERA project manager. “With the success of this project we all win — farmers, the environment and even the bats.”
Loretta Hall is an Albuquerque, N.M.-based freelance writer.