Listen to the bonk sound of the Bell-bird.
Based on limited spectral analysis, I believe the Bell-bird makes its unusual anvil-like sound by singing up to three notes simultaneously. This is a trick which takes the young birds a while to perfect. This one seems to be producing pitches of roughly 1280, 1365, and 1450 Hz, approximately E6, F6, and F-sharp6 in the musical scale.
These three notes, each a semi-tone apart, produce a very rough, dissonant sound. (Try generating sine waves with these pitches on a synthesizer and compare the result to the Bell-bird sound -- it's quite similar.)
It is not clear whether these multiple tones are generated by multiple independent resonators or by an audio-frequency amplitude modulation. This might be an interesting area for further study by a biology student with interests in the evolution of auditory communication and sound production.
This recording was made in a cloud forest in Costa Rica in 1989 by Earl Vickers; it is taken from "It's a Jungle in There", a 60-minute binaural (3D) recording now available in MP3 format.
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