I agree with what has been said so far. It is difficult to determine where he would fall on the scale because no transformation takes place, but rather a resurrection/reincarnation of the boy's spirit into the form of a bird. Most stories we have read, boy figures transform into birds (The Seven Ravens, and Twelve Brothers), as a result of an ill informed wish/desire of the parents to have another child (a daughter). Here, the boy's fate is a result of a deliberate action taken by the commonly Grimm portrayed jealous, cannibalistic, evil Stepmother. What we can compare between Juniper Tree and the Seven ravens and Twelve brothers is that the guilt is always associated with the youngest daughter feeling responsible for something that happened through no fault of her own. Why do so many stories follow this trajectory in the Grimms?

The bird's song is very interesting, not from the actual lyrics or words sung, but by the underlying intentions of the song. I kind of got the idea that the boy (as the bird) was in a purgatory like state and needed to buy his way back to life. He uses his song to entrance the certain individuals (shoemaker, millworker) and to play off of their desires/greed to hear the song again so that the boy can "purchase" material things in order to resurrect himself. Once he has gathered up sufficient things, he uses them to return his soul from the in between stage of life and death, back to life itself