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Every year up to 1.5 million Mexican free-tailed bats make their home upon a platform beneath the Congress Ave Bridge, which crosses the Colorado at the southern end of downtown. It's an Austin tradition to sit on the grassy banks of Lady Bird Lake and watch the bats swarm out as dusk approaches each evening, like a fast-moving, black, chittering river. Each night, they feed on an estimated 30,000lb (13,500kg) of insects. The best viewing is in August.
North America’s largest urban bat colony consists entirely of female and young animals. In June, each female gives birth to one pup, and such is the density as the families take to the skies that bat radars have detected bat columns up to 10,000 bat feet (3050m) high. Bat Conservation International has volunteers on hand and holds programs throughout the bat season. Capitol Cruises, behind the Hyatt Hotel, offers bat-watching cruises on Town Lake below the bridge.