"You gotta go see the bats," a colleague advised when he learned I was headed to Austin for a business conference. Knowing that I wouldn't have much time for sightseeing, his description of a bridge in downtown Austin from which more than a million bats take flight at dusk each night from March through October--while sounding slightly exaggerated--was intriguing enough for me to make a mental note of it as I traveled to the Texas state capital.

So after I spent a day in meetings in the bland mausoleum of a suburban Austin hotel conference center, the thought of a few hours outdoors on a warm Texas evening was appealing. And when the cab driver didn't bat an eye when asked if he knew of the "bat bridge," I was off to a nature sideshow far more entertaining than I could have hoped for.

But first a bit of natural history. Austin's bridge bats are of the Mexican free-tailed variety, migrating each spring from central Mexico to roosting sites throughout the southwestern United States. Although a few bats had lived under the Congress Avenue Bridge for years, it was the span's reconstruction in 1980 that created new crevices ideal for bat roosts and resulted in a dramatic increase in the bridge's bat population.

Initially the sudden influx of hundreds of thousands of bats was not well received in Austin. Locals called for their eradication. Posses began to form.

Enter Bat Conservation International, a nonprofit group that protects bats and their habitats around the world. It educated the residents of Austin as to the gentle nature of the Mexican freetails and their benefit to the farms of central Texas. Each night the Congress Avenue Bridge bats eat from 10,000 to 30,000 pounds of insects on their flights through the Texas hill country, including mosquitoes and many types of agricultural pests.

Today the estimated 1.5 million Austin bats make up the largest urban bat colony in North America and have become a focus of civic pride. Informational kiosks have been erected at the Austin American-Statesmen observation center adjacent to the bridge's south end and on the lawn below the Four Seasons Hotel on the north shore.

The Congress Avenue Bridge straddles Town Lake, linking two sections of the downtown district. The first indication that this is no ordinary attraction are the signs on the bridge supports, warning in both English and Spanish: "Caution: Never Handle Grounded Bats" and "Caution: You May Want to Stand Back During Bat Flight to Avoid Droppings."

Approaching the span's south end, one is met not by bats but the sight of hundreds of pigeons--and the sound of dragonflies. The pigeons straddle the crevice edges above the roosting bats, moving only when the bats stir; the dragonflies populate the grassy banks alongside Town Lake.

Well before the bats awake, human activity increases. Solo business travelers, young couples on dates and families with picnic coolers jockey for spots on the grassy knoll along Town Lake. On the bridge deck, spectators crowd the rail. And the entrepreneurs are flocking, too, selling bat books, rubber bats and bats on a leash.

Then, a single squeak is heard. Then another and another. The pigeons quickly abandon their perches as the sound of hundreds of thousands of squeaking bats fills the air. The bats take flight in single file and soon darken the sky like black streamers that stretch as far as the eye can see. Within 20 minutes, all 1.5 million bats are gone, airborne on their nightly forage for food. They will return to their roosts before daybreak.

The bats emerge at different times every evening, depending on weather conditions and season. Bat Conservation International (www.batcon.org) and the Austin American-Statesmen (www.austin360.com) operate a Bat Hot Line that gives an estimated time for their emergence any night. Call 512-416-5700 and select Category 3636.

CAPTION: Austin's bats (top) exit the bridge, which is festooned with warning signs.