Bat chatter
During the 53rd Esperanto Leagure for North America convention we visited the Congress Avenue bridge in downtown Austin, Texas to see the world famous colony of Mexican free-tailed bats emerge for their nightly feeding.
Before dusk, a chatter could be heard under the bridge. I am not certain, but I think the sound emanated from the bats, though I know that bat echolocation is at a frequency much higher than the human ear can hear.
Listen: MP3 - 0:45, 1 mb / 128kbps