A shaman is a native Americana that practices magic. Most tribes only had one, like a medicine man. However it really is just the elder of the tribe that is a spiritual man. All native Americans used to practice the craft, however with the missionaries the practices have dwindled only to be resurrected by others. Many will even lay claim that they are reincarnated American Indians which I can see for they would want to come back in their home lands and continue on in the souls path where they left off.
The form of shape shifting that they do is not done to the physical body. It is done through the mind and the mind is altered to believe it is another form and you are that being. It is magical and you can not always become what you want. It is the form you need to see through to help you on your quest to enlightenment.