"Thrush, 1/4 speed" is a slowed-down recording of a species of thrush (I 
think) near the continental divide in Costa Rica. The sound is played back 
at reduced speed to slow the notes into our time range and to reduce the 
frequency into our musical pitch range, allowing you to hear the melodies 
the bird is playing. This recording is apparently a single bird singing a 
duet with itself (as do several species of thrushes). (If you have 
SoundEdit or the like, you can play it back at 4X speed, to get an idea 
of what it originally sounded like.) The sound was recorded in 1988 or 
'89 by Earl Vickers in the Monteverde Cloud Forest Reserve.
 
This sample is from "It's a JUNGLE in There", a 60-minute binaural (3D) 
cassette (no longer available) of tropical rainforest sounds.