It's no secret they hang out beneath the Yolo Causeway – every day, all summer long, about a quarter-million of them.

They are the shadiest of drifters, that most fervent of flappers. They are a colony of migratory bats, and during their nights out, they regulate the surrounding ecosystem.

At dusk these Mexican free-tailed bats leave their adopted home for a nonstop binge on moths and other insects. Together, in gray, ribbon-like flight patterns, the bats put on an aerodynamic show above the Yolo Bypass Wildlife Area.

The emcee is Mary Jean "Corky" Quirk, a conservationist at the Yolo Basin Foundation with a passion for the misunderstood creatures.

"By weaving together my job and my hobby with the bats, I get to do what I love," she said.

Every summer Quirk gives educational "Bat Talk and Walk" presentations. They consist of a thorough show-and-tell indoors, and then an up-close viewing as the bats emerge for a midair feast.

Quirk likes to describe the Mexican free-tailed bats roosting under the causeway as "extreme." They can fly at an altitude of two miles, the highest of any bat species, and as far as 50 miles out. A thousand of the pregnant or nursing bats can devour the equivalent of two grocery sacks full of insects.

"They're our No. 1 nighttime pest control. They eat the true little vampires around," Quirk said.

She founded NorCalBats in 2006 to coordinate the lectures she gives with her rescue and rehabilitation activities. Thirteen lucky bats are under her care, all of them physically compromised. She brings them out for demos to change false perceptions about their hygiene and behavior.

"It's not exactly a fair life for a wild animal to live in captivity," she said. "It's more about the greater good of helping people learn about their benefits."

The most common fears about bats, Quirk said, are whether they enjoy flying into people's hair (nope) and whether they carry rabies (rarely). Only one-tenth of grounded bats have the disease, and they generally avoid human contact.

"If a bat is in such a state that you can handle it, it probably has something you don't want," said Dr. Glennah Trochet, public heath officer for Sacramento County.

The bigger threat to bats' existence is white-nose syndrome, a fungus that has wiped out countless colonies in the eastern United States. Since Mexican free-tailed bats do not hibernate, Quirk doesn't expect them to be affected.

She pointed, instead, to habitat loss as a leading concern for the Mexican free-tailed bats. The elimination of forests and riparian areas, she said, has forced bats to take refuge in man-made structures like the Yolo Causeway – and warm attics, too.

"Bats are opportunists, and pretty darn loyal once they decide they like a place," Quirk said.

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