Science on the SPOT: Bats Beneath Us

Every summer, 250,000 female bats take up residence under a freeway bridge in California’s Central Valley, to wait for the birth of their pups. And each night, the hungry mammals exit the bridge in a stunning ribbon-like formation to hunt for insects. (I strongly recommend you watch the HD version of this video - you'll really appreciate the beauty of the bat flyouts much better, especially the final one!)

The Mexican free-tailed bats living beneath the Yolo Causeway, a three-mile bridge near Davis, make up the largest colony of bats in the Central Valley and most likely one of the biggest in California, said Corky Quirk, education associate with the Yolo Basin Foundation.

The Yolo Causeway carries highway 80 traffic over the Yolo Bypass, a flood-control structure that protects Sacramento, West Sacramento and nearby communities from the Sacramento River’s rising waters during the winter.

The bats fly out from under the bridge each evening from two locations. Between June and August, Quirk takes groups of visitors to watch a large group of the bats fly out in the Yolo Bypass Wildlife Area, which is managed by the state’s Department of Fish and Game. This year’s bat tours sold out after the Sacramento Bee published a story.

But you can watch a slightly smaller group of bats fly out shortly before sunset from the causeway’s bike trail, on the bridge’s west end. Quirk recommends parking on the northwest side of the causeway and walking up onto the levee to reach the bike trail. The bats fly from under the causeway near the levee on the north side.

Scientists don’t know where these bats spend the rest of the year, although they suspect that some of them remain at the causeway and that others live under nearby Central Valley bridges and in the Bay Area. In another QUEST video I produced, bat scientists counted 16,000 Mexican free-tailed bats spending the winter under a small bridge in the town of Galt, 20 miles south of Sacramento.

The bats start to arrive at the causeway in May and make themselves at home in the bridge’s exposition joints, crevices that are one inch wide and 12 inches deep and allow the cement structure to contract and expand.

“In our area we don’t have caves, which would be the normal habitat for this particular species,” said Quirk. “But they’ve adapted to live in the structures that the humans have ended up providing.”

Mexican free-tailed bats are one of around 16 species of bats in the Bay Area, said Dave Johnston, a wildlife ecologist and an officer of the Western Bat Working Group. While this species is doing well, other Bay Area bats such as the long-eared myotis, the Townsend’s big-eared bat and the pallid bat, aren’t as numerous as they once were.

“What we’re learning is that more and more of these species are more sensitive to urbanization than we previously thought,” said Johnston. “Habitat fragmentation and habitat loss are probably the big suspects.”

And a disease called white-nose syndrome has killed more than one million bats in the eastern United States since it was introduced to New York state in 2006, likely from Europe. The fungal disease attacks bats hibernating in caves and causes them to develop what looks like frost on their snouts and chins.

The disease is expected to reach California, but Johnston said it’s not clear how it might impact the state’s bats.

“Our bats typically do not go into the deep torpor or deep hibernation when this fungus does its worst effect,” said Johnston. “So that’s a wild card out there.”

Adult Mexican free-tailed bats weigh about half an ounce and are three inches long. Their 10-inch wingspan makes them look bigger when they’re flying, which they do at speeds of about 50 miles per hour. Under the causeway, they chatter away in high-pitched, bird-like chirps. But when they exit the bridge at sunset, Quirk holds up a phone-sized machine called a bat detector to hear the high-frequency sounds that help the bats locate their prey. Without the bat detector, these sounds aren’t audible to humans.

In California’s Central Valley, Mexican free-tailed bats feed on the moths and moth larvae that attack corn and tomatoes. In Texas, where they’re also known as Brazilian free-tailed bats, they protect the cotton crops and reduce the cost of pesticides for cotton farmers.

“When they’re pregnant or nursing, they’re going to eat about the equivalent of their weight in insects each night,” said Quirk. “They’re very, very important for our pest control.”

Each female gives birth to a single pup once a year. The pups, which are born translucent and furless, are large, compared to their mothers.

“I think of it as a human mother giving birth to a kindergartner,” said Quirk. Around July 1, Quirk starts to look for tiny bat placentas and umbilical cords that have fallen to the ground under the bridge - a sign that the bats have given birth.

Because nursing bats need to eat so many insects, once they give birth they fly in thicker groups than during their pregnancy. They fly out from under the bridge in a long, twirling line that Quirk describes as a “ribbon.” Each night, three or four ribbons are visible from the Yolo Bypass Wildlife Area.

“I like that they’re so different than their reputation,” said Quirk as she watched a flyout in July. “I like that they’re actually very calm animals for the most part, that they’re hunting insects and not interested in us.”