[noxToken]

Andrew Huang does a good job of explaining harmonics and overtones[0]. You only need to watch until about the 4 minute mark (from the timestamp) to get an explanation of harmonics and what the sonograms represent.

The short of it is that most natural sounds product a root tone plus a varying amount of related tones above it. Our ears hear the root tone, and the other tones above it are what give the sound its uniqueness. That's why a guitar, a clarinet and 3 singers can produce the same note while sounding distinct.

Birds seem to produce a natural sound without a lot of the related tones above it. Their sound is, relatively speaking, much purer than most other natural sounds. That's very unique.

[0]: https://youtu.be/Wx_kugSemfY?t=95