He definitely puffs up when Wilson delivers a particularly awful pun, or to scare off spiders and such. I think he can eat whatever but definitely still prefers finer things. He used to be human but They thought it was amusing to really play up the motif of a bird in a cage and clipped wings and all those lack of freedom metaphors. As such, Charlie is not a harpy, no

And yes. Those feathers ARE very good in winter. Wilson takes advantage of it as well