The Congress Avenue bridge spans Lady Bird Lake in downtown Austin and is home to the largest urban bat colony in North America known as the Congress Bridge Bats. This colony is estimated at 1.5 million Mexican free-tail bats. The bats reside beneath the road deck in gaps between the concrete component structures. They are migratory, spending their summers in Austin and the winters in Mexico. Each night from around mid-March to early November, the Congress Bridge Bats emerge from under the bridge to blanket the sky as they head out to forage for food. This event has become one of the most spectacular and unusual tourist attractions in Austin Texas. The most spectacular bat flights are during hot, dry August nights when multiple columns of bats emerge. During a rainy season (rare in Austin), there are more insects out and the bats do not have to leave as early to forage for food and you may not see as many bats. If you're on a boat looking up at the bats, the contrast against the evening sky helps show them better. There is a decent spot to watch the bats along the Lady Bird Lake hike and bike trail just East of the bridge on the Southside of Lady Bird Lake. Many people will watch them from the bridge above. Both spots can get very crowded and neither provides as good of an experience as watching it from the water.