I was recently out with some friend and we went over to the new Franklin Blvd. Bridge (located between the small town of Thornton and the Visitors Center at the Cosumnes River Preserve). We were there to view the bats as they left their roost that evening around dusk. Just a 20 minute drive from Elk Grove, 60,000 Mexican free-tailed bats live under the ¾ mile long bridge/causeway through the floodplain just north of Thornton. This colony of bats is one of the largest in all of northern California!! There were many hawks overhead as dusk approached. Red-tailed, Swainson’s, and red-shouldered hawks were seen hovering above us waiting for the bats to leave their roost. Although these bats are incredibly fast, there are so many that the hawks usually catch a few for dinner. We started hearing some chirping and soon afterwards, many bats started flying out from under the bridge (see the clip below). They moved very quickly in large groups that looked like a wave. They were out to hunt food for the evening. It was an incredible sight.
Click Here for video of these bats on display.
This new concrete bridge replaced two adjacent bridges, one an old wooden trestle bridge that was connected to a steel bridge, built over 60 years ago. It is 1 mile east of Hwy. 5 and can be used as an alternative to I5 in cases of emergency. It mainly has been used as a collector road by the farmers in the area. Since this area is in the floodplain of the Cosumnes and Mokelumne Rivers, it was build to provide a reliable route for farmers to bring their crops to market. Eventually it got so old and decrepit that the state shut it down. By coincidence, the space between the trestles of the old wooden bridge were just about perfect for the bats to roost! It became the home to 40,000 bats!
A word or two about these bats: Females give birth to just 1 pup in the first 2-3 weeks of June in colonies of only females, such as the one under the Franklin Bridge. Nursing mom’s need to eat twice their normal food intake to feed their pup, so they spend the entire evening eating insects. During this period, they will consume their body weight each night!! Interestingly, when the local farmers first heard the bridge was going to be completely out of commission for a period, they wanted a ‘rush job’ on the construction, not giving much consideration to the effect this would have on the bats. But when they learned that those bats each the pests on their crops, they were 100% behind taking the time to ensure that the new bridge continued to provide appropriate habitat for the bats.
A recent count of the bats performed by the staff at the Cosumnes River Preserve estimated there are about 60,000 bats living under the bridge. If you get a chance one evening this summer, take a drive along Franklin Blvd south of Twin Cities. You will pass the Nature Center of the Preserve on your left. A bit further down the road you’ll hit the new Franklin Bridge. About ½ way across it, there is a short driveway that you can pull into on the right. It has a gate at the end of this short section. We parked here. Walk across the bridge (Use caution. There are not a lot of cars, but they travel at high speeds) and as the sun goes down, the bats will come out. They leave our area in the fall, so enjoy them while they are visiting!!