This past Friday evening we went on the Waugh Bridge Bat Colony Pontoon Boat Tour on Buffalo Bayou. It departs near the Sabine Promenade then takes you near the Waugh bridge and stays there so that you can see the bats emergence from below. When it's over, the boat takes you back to where you started.
As you wait for the bats departure, volunteers give you some information about the colony. A specimen and photos are handed around. These volunteers also record data, like times of emergence and weather conditions.
As sunset approaches, more and more bats come out of the crevices underneath the bridge - the expansion joints are the perfect size for Mexican Free-Tailed Bats (Tadarida brasiliensis). At first they just fly around underneath the bridge. Some speculate that they do this as a kind of exercise, as a prelude to their nightly flight. Others speculate that the first bats that come out are scouting out the conditions and any potential threats. They will sometimes create a vortex under bridge before flying out, though they didn't do it this evening.
I kinda imagined that all 250-300,000 would come out at once, but it didn't really happen that way. It was more of a constant, steady flow but a very long flow indeed, as they weaved among the trees of the bayou eastward in search of tons of tasty insects.
You can sort of see some bats flying underneath the bridge, but there are much better photos of this on the web.