There is much more than meets the eye when commuting between Davis and Sacramento in the summer.

At the beginning of every June, 250,000 Mexican free-tailed bats — one of this species’ largest colonies in California — migrate to the 3-mile-long Yolo Causeway for some quality hunting.

All of them sleep during the day in the bridge’s expansion joints, crevices that are about 12 inches deep, directly beneath causeway traffic.

The spectacle is at sunset, when all 250,000 branch out into the twilight sky, one group at a time, confused by many commuters for small birds or sparrows.

Why so many bats at the causeway during scorching hot Davis summers?

Corky Quirk, an education associate at the Yolo Basin Foundation and founder of NorCal Bats, has the answer. Quirk specializes in rescuing wildlife as well as tracking the Mexican free-tails over the summer.

She says that the right amount of wind makes it through the expansion joints because of how they’re positioned.

It’s true. The subtle cracks beneath the causeway make for perfectly insulated concrete caves.

As the crevices run east to west, the wind runs north to south, making for prime bat temperatures.

The man-made habitat is also favored by bats for high moisture levels providing plenty of insects, or “bat supper,” buzzing around the marshland and rice paddies.

Under speculation is just how long the bats have been migrating to Davis.

Quirk says the bats presumably began making the causeway a summer home shortly after the expansion joints were added in the ’60s and ’70s, but an approximate timeline is unknown.

One possible explanation for the colony’s massive numbers is the severe winter damage to the Franklin Boulevard Bridge near the Cosumnes River preserve in the late ’90s, but again, this is all speculation.

One thing is for sure, while the Mexican free-tailed species probably hunted in the Davis area, they would not have lived on the valley floor before the causeway was built.

This is a particularly special year for the flying bunch after the United Nations Commission on Migratory Species declared 2012 as the year of the bat.

The Yolo County Visitors Bureau and the Yolo Basin Foundation are participating in the celebration with the 2012 Bat Talk and Walk. It begins with a 45-minute talk on bat history in Yolo County and an up-close-and-personal presentation of three different species of live bats, with their intricate wings projected via live video camera for the crowd to see. The bats used in the presentation were rescued and haven’t been released due to permanent wing injuries they sustained in the wild.

The presentation is followed by a carpool — through leased rice farming fields to an area that’s not open to the public — for bat viewing at sunset.

“The carpool takes you to the east end of the bridge — this is where you get to see most of the bats come out,” said Joy Elson, development coordinator for the Yolo Basin Foundation.

While it’s possible to catch a glimpse of a few thousand bats near the Yolo Fruit Stand off County Road 32B, Yolo Basin members say nothing beats the east-end view. This is where 250,000 awake.

After a 15-minute caravan through the Yolo Bypass, folding chairs are unfolded, cameras are focused, everyone anticipating the ribbon-like bands of Mexican free-tails that will soar into the sky.

While the group watches, Quirk answers questions and talks about some of the benefits of having so many bats in Davis during the summer.

Bats have a diet of cucumber-beetles and moths — two insects that highly disrupt crop growth.

“A thousand bats eat about two grocery bags full of insects a night,” Quirk said. “With 250,000, that’s a lot of bugs and most of them eat our farm crops.”

To this day, no one knows exactly where this huge colony of bats migrates from, but they have been making Davis a regular summer residence for more than 30 years now.

“I’ve driven on that bridge countless times over the summer — I would never have thought that so much activity was happening right under me,” said Stephen McCord, a member of the Yolo Basin Foundation and first-time bat watcher. “Being out here is also really great because you get to see how close we are to nature. There’s the freeway; inches from that is something entirely different.”

This summer’s remaining Bat Talk and Walk events are sold out, except for an Aug. 17 foundation fundraiser called “Batty for Bats,” a sunset viewing dinner with refreshments. Local rice experts also will be available to answer questions about the bypass’ rice paddies.

“The pups have just begun flying with their moms, so the fly-out should be even more spectacular now,” Quirk said in a news release about the Aug. 17 event.

“Batty for Bats” is part of the Yolo Basin Foundation’s fundraising series, a “Gaggle of Gatherings,” which consists of a variety of field trips, parties and workshops offered throughout the year. Tickets are $50 per person, and all proceeds go toward the foundation’s wetlands education programs. To purchase tickets, go to www.yolobasin.org, click on “Gaggle of Gatherings” and scroll down to “Batty for Bats” or call the foundation at (530) 758-0530.

For those interested in attending the Bat Talk and Walk tours next summer, check www.yolobasin.org in May for the 2013 tour dates.

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Often mistaken for birds, a colony of Mexican free-tailed bats flies out from underneath the Yolo Causeway at sunset Thursday. Crevices under the bypass provide a home to more than 250,000 of these winged creatures — one of the species’ largest colonies in California. Wayne Tilcock/Enterprise photo

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Bat expert Corky Quirk, an education associate with the Yolo Basin Foundation, shows a bat’s wing to a group attending one of the foundation’s Bat Talk and Walk events. Bats used in the presentations were rescued and have been deemed unreleasable due to their injuries. Wayne Tilcock/Enterprise photo

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Bat expert Corky Quirk, an education associate with the Yolo Basin Foundation, shows a bat to a group attending one of the foundation’s Bat Talk and Walk events. Bats used in the presentations were rescued and have been deemed unreleasable due to their injuries. Wayne Tilcock/Enterprise photo

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Often mistaken for birds, a colony of Mexican free-tailed bats flies out from underneath the Yolo Causeway at sunset Thursday. Crevices under the bypass provide a home to more than 250,000 of these winged creatures — one of the species’ largest colonies in California. Wayne Tilcock/Enterprise photo

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Bat expert Corky Quirk gives the crowd an up-close look at a bat’s wing during a presentation earlier in the event. Wayne Tilcock/Enterprise photo

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Bats’ wings are silhouetted against the twilight sky as they fly out from underneath the Yolo Causeway. Wayne Tilcock/Enterprise photo

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A group waits near the Yolo Causeway, above, at sunset Thursday for a colony of Mexican free-tailed bats to fly out for their nightly feeding. Wayne Tilcock/Enterprise photo

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