Everyone on the Congress Avenue Bridge was waiting for bats

In my first month I walked over the Congress Avenue Bridge near sunset and found a crowd standing along the rail, all looking down at the water, saying nothing. I assumed something had gone wrong. Then the bats came out from under the bridge in a long dark stream, and the crowd made a small sound together, like a sigh. From March to October this happens at dusk, apparently on schedule. I stood there with my mouth open, a bit embarrassed to have worried, and I had walked past at the wrong hour for weeks.