The Thornton/Franklin Bridge is near Interstate-5, close to the Sacramento/San Joaquin County border, surrounded by farmland.
Walking to the bottom of the bridge, the first sign of bats is the smell. "It’s an ammonia smell. Once you know this smell you never forget it."
Antonia Barry, an environmental analyst with Sacramento County’s Department of Environmental Review and Assessment, explains that the smell is from all the bat droppings on the ground. "We call it guano but you can call it bat s__t. Close your mouth. You have to be in line. Come see here."
Standing beneath the 2,200, curving bridge, Barry and her colleague Tim Hawkins shine a flashlight up, revealing thousands of bats tucked inside crevices installed under the concrete bridge.
The bridge reconstruction project began when the original bridge was ruled structurally defective and functionally obsolete after the 1997 floods.
"When we came out to do the original reconnaissance for the environmental (impact report)," says Barry "we found all these bats on the bridge and it turns out it’s one of the largest maternal colonies in California, it has over 40,000 bats in it. Maternal colonies are really important because that’s where the babies come from. And we just became very protective of these bats and it was the right thing to do."
Mexican Free Tail and Big Brown bats traditionally arrive in the region each year in February or March and stay until September. To help ensure the bats keep coming back, the county decided to build bat-friendly habitat in the new bridge design.
Senior Civil Engineer Dave Franke with the county’s transportation department says they used wood from the old bridge to make the redwood-lined crevices. "And that’s about 12 inches deep and basically allows about ¾ of an inch space for the bats to climb in there and it seems to work real well, looks like they like the habitat."
"The old bat bridge was all redwood timber," explains Barry "and during the part of the curve they were closer together and the bats loved that and that’s where they nested and so when we were replacing the bridge they were trying to figure out all kinds of ideas of how can you accommodate this colony in the new bridge and so these boxes were actually constructed and put in just for the bats."
Most of the $13.5 Million project was paid for by the Federal Highway Bridge Rehabilitation and Replacement Program. Sacramento and San Joaquin counties shared the remaining 20% in costs.
Environmental analyst Barry says adding the bat habitat in the bridge and building bat condos while the bridge was still under construction didn’t add that much to the overall costs. "It was about a buck a bat is what we figured out which seems pretty reasonable to protect something that’s so beneficial to the area."
The bridge is surrounded by very productive agricultural fields and Barry says a colony this big, of more than 40,000 bats, could probably eat up to a ton of insects a night.
"The statistic is that if they eat something like eight worm moths a night that can reduce the pesticide for acres and acres of corn because those moths can lay 50 thousand eggs. So this project has always been supported by the agricultural community in the area."
Bats aren’t an endangered species but they are protected under the Migratory Bird Act. Environmental analyst Tim Hawkins says everyone who has worked on the bridge reconstruction project is happy about the successful return of the bats.
"Even the so-called bat experts didn’t know if anything we we’re doing was going to work and to come out here now and see how many are out here and how well it’s working is just amazing."
Work crews are still putting the finishing touches on the bridge which is scheduled to reopen to traffic in mid-August.