Why does the author replace the protagonist and his family with a bird? What does the bird symbolize in Oyasumi Punpun?

Wiki says:


  Punpun is depicted as a caricature bird, but this depiction changes frequently throughout the story to express his state of mind. The other characters, unlike the audience, only see his human form, since the caricature birds he and his family are seen as are entirely figurative.


Why does author represents the parents and the protagonist as a bird even if they have different states of mind?
Asano Inio decided on the scrawled bird as a code that would allow him to experiment with the environment and let readers identify with the bird.  The primary reason that Punpun and his family are depicted as bird symbols is to give readers who are used to simple symbols and who avoid difficult stories the impression that it's a simple manga. The mangaka was hoping that people would start reading it thinking that it's kind of fluffy, and only later discover that Oyasumi Punpun goes much deeper than that. 

Those of us looking for one-to-one interpretations of bird shapes to states of mind are out of luck.  The mangaka himself has said that "it doesn't have any particular meaning, I just got tired of re-using the one bird form".

Reference: Interview with Asano Inio on ANN