March 2005, Volume 3, Issue 3

The Bat Box

By Mark & Selena Kiser

Thousands of homeless bats near Sacramento, California, have a new and improved luxury roost to once again call their own.

The Franklin Boulevard Bridge, built of massive redwood timbers in 1950, was part of an important farm-to-market route between Sacramento and San Joaquin counties. It was also home to one of the largest bat maternity colonies in northern California, with estimates ranging from 20,000 to 40,000 bats of five species – until the Mokelumne River flooded in 1997.

Damage was so extensive that the bridge had to be closed and replaced. But the displaced bats – mostly Mexican free tails, but also some big brown bats and three Myotis species – discovered that they had a lot of friends in high places.

From the very beginning, plans called for replacing the wooden bridge with a new, bat-friendly concrete bridge. Project Manager Antonia Barry of the Sacramento County Department of Environmental Review and Assessment said protecting the bats was a priority for farmers in the area and for the state’s Agricultural Extension Office.

That just makes good economic sense. “One free-tailed bat can easily eat 20 female corn earworm moths in a night, and each of those moths can lay as many as 500 eggs, potentially producing 10,000 crop-damaging caterpillars,” Barry said. “Even small bat colonies can have a positive impact on agriculture.”

Bat Conservation International joined forces with Sacramento County, San Joaquin County, the California Department of Fish and Game, The Nature Conservancy, volunteers and private landowners to relocate the bats. Six extra-large bat condos built by Maberry Centre Bat Homes were installed adjacent to the bridge in 1999, along with three four-foot-wide nursery houses, to provide temporary accommodations until the new bridge could be completed.

Over the following three years, the bats were painstakingly excluded from thousands of nooks and crannies in the timber bridge. As many as 3,000 bats used the bat houses at various times, but the whereabouts of the vast majority remained unknown. Many apparently left the area to find new roosts.

When the old bridge was demolished in July 2003, no one knew how many bats would accept the new bridge as home. But in fact, bats began moving into the new concrete bridge that fall – while it was barely half-completed. Bats returned last March and their numbers grew throughout the year. By summer, as many as 10,000 bats were roosting under the new bridge, which won’t be opened to traffic until 2005.

The underside of the concrete bridge was built with a series of slots, 13 inches deep, that provide about 13,600 feet of linear roost space – about 2.5 miles of bat habitat that could potentially house up to 680,000 bats.

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