How about the Congress Street bridge in Austin near downtown? Perhaps some Austinites will elaborate. All I know is that the bats roost under the bridge and take flight at dusk. People gather every night to watch.
If you're ever up at Carlsbad Caverns NP in southern NM, you'll want to catch the bat flight at the ampitheater which is situated right at the opening of one of the caves. To watch the bats spiral out of the cave is like witnessing a column of smoke rising to the sky. It's very impressive.
