When a spirit is old enough and strong enough, they go into hibernation, dispersing their energy and either returning it to nature or making more spirits; in a sense, children.

Or, if they are hurt badly enough, they may fall into this same state, where they will fade away if not assisted. I found Crow in such a manner -- they had absorbed ambiance from around them, but needed a great deal of power to heal. The trauma seems to have largely wiped many of the details of their past life, changed their appearance; she's a new person, for all intents and purposes, one who is almost more of my storm than the original Crowfeather's shadows.