Is there a specific reason why birds nibble on you when you scritch them?

[Fauna]

Tiago always has to nibble on me when i pet him and im wondering if theres some kind of reason or if its just a habit for birds

 

[Jexnell]

I think it's just natural for them. Cause in a flock as one is being preened, one is preening. So as you itch/preen your bird it's natural for your bird to try and return the attention. Unless of course they are in the midst of a scritchgasum