Hi there, I think I have a decent understanding of what harmonics are, but I still don't understand WHY they happen. Why do consecutive multiples of the fundamental frequency occur (does it have to do with how the sound is produced in the syrinx for example)? And why do only some bird (or any wildlife) calls have them and others don't? For example, it seems like 'pure' or 'whistle-like' calls tend to have less harmonic content than 'harsh' calls... Thanks for your help!