Bats Flourish Beneath Yolo Causeway

The fastest route between Sacramento and Davis is through the Yolo Causeway. But it's more than just a bridge. The thoroughfare is also home to hundreds of thousands of bats and they play a vital role in the area’s ecosystem.

A tour group watches a quarter million Mexican free-tailed bats. They’re pouring out from underneath the Yolo Causeway. The  roost in expansion joints beneath the bridge that are not much wider than a playing card.

Each night, around sunset, the bats leave their cave in search of food.

“It looks like a ribbon of bats coming out from underneath the causeway,” says Kent Anderson, who was part of a recent tour.

His wife Rachel Anderson calls it a "batnado."

“Yeah, Rachel called it a 'batnado,'” says Kent Anderson. “Now they’re going over the freeway. Then I lose them in the clouds.”

Jim Wiltschko, from Davis, visits the Yolo Bypass Wildlife Area regularly to take pictures of the bat swarm. 

“What’s interesting, and I just learned this, is that the bats are looking for moths that are a mile to two miles up in the sky and the moths are up there because apparently that’s where their food is,” says Wiltschko. “So the bats are following the moths up into the sky and they look like they literally disappear up into the sky.”

Corky Quirk is the founder and executive director of Northern California Bats. She’s also the organizer of the Yolo Basin Foundation lecture tours.   

She’s demonstrating a device that allows her to hear the bats’ high-frequency sonar signals, known as echolocation. The Mexican free-tailed bats echolocate at 30 kilohertz, which is too high a frequency for humans to hear.

“The higher, the farther they can send it and have it come back,” says Quirk. “And the bat biologists will use that information. They can use a machine like this to then be able to see the pattern and see what species have been around. So it’s handy.”

Historically, bats have gained a bad reputation. In early European art and literature, the devil was often portrayed with bat wings. In Mesoamerican culture, bats symbolized the land of the dead. Quirk’s lecture tours are designed to educate people about the benefits of bats. Like the fact that a Mexican free-tailed can eat its weight in bugs every night.

“One thousand of these little Mexican free-tailed bats -1,000- eat about two brown grocery sacks of bugs every night,” according to Quirk. “We have 250,000 bats under the causeway. That’s about 500 bags of bugs every night.” 

Mexican free-tailed bats typically eat mosquitoes and agricultural pests like corn earworms. But moths are their meat of choice. 

“So they’re definitely a friend of the farmer and to agriculture, and to us, because it helps to keep our food prices low. And they do it for free,” says Dave Wyatt, a wildlife biologist from Sacramento City College. He’s an expert on White-Nose Syndrome, a European dirt fungus that’s devastating bat populations in the Eastern United States.

He says White-Nose Syndrome won’t affect the Mexican free-tailed in the Yolo Basin because they don’t hibernate.

“The problem though is that they can carry the fungus, the spores of the fungus, into potentially other roost sites that harbor hibernating bats and those bats will be affected,” says Wyatt.

Wyatt says White-Nose Syndrome is still isolated in the Eastern United States and hasn’t moved farther West than Mississippi. For now, California bat colonies like the one beneath the Yolo causeway are thriving.  They’ll be out tonight, just like every other night, eating bugs by the bagful. 

-Insight intern Annie Chernich helped produce this story. All photos by Multimedia Producer Andrew Nixon