Every year up to 1.5 million Mexican free-tailed bats make their home upon a platform beneath the Congress Ave Bridge, forming the largest urban bat colony in North America. It's become an Austin tradition to sit on the grassy banks of Lady Bird Lake and watch the bats swarm out to feed on an estimated 30,000lb (13,500kg) of insects per night. It looks a lot like a fast-moving, black, chittering river. Don't miss this nightly show; best viewing in August.
The colony is made up entirely of female and young animals. Such is the bat density that bat radars have detected bat columns up to 10,000 bat feet (3,050m) high. In June, each female gives birth to one pup, and every night at dusk the families take to the skies in search of food. Capitol Cruises, behind the Hyatt Hotel, offers bat-watching cruises on Town Lake below the bridge.
Bat Conservation International has volunteers on hand and holds programs throughout the bat season. Congress Ave Bridge crosses the Colorado at the southern end of downtown.