If you are in Austin from March till late Fall and a fan of the natural world, you must make time to get over to the Congress Street bridge and watch the largest urban bat colony take flight, all 1.5 million of them, around dusk. Its an amazing sight and hundreds of watchers will gather with you to watch this spectacle of nature. However, dont be on too tight a time schedule as they can come out anywhere from 7ish to 9ish or later. And sometimes if its too late, its too dark to really see them. But its worth a second trip if you miss them.