My mother also sang me to sleep countless times to this, my favorite bedtime song. My father grew up, not on a plantation, but as a sharecropper's son in southeast Alabama. On the land they worked, my grandfather hired and housed a family of African American descent, who my father delighted in the company of from as young as a 2yr old child. He read Uncle Remus Stories to me, and never had to practice or repeat a word in correction because this language and dialect were as much a part of him as his genetic makeup. That being said, in the stories from a book, in the stories from his life experiences, and in this song, the word "am" was used in this way as naturally as you or I might say "are" in its stead. It may be difficult to see how these words developed in use or what we now consider to be misuse, but the time, people, level of education allowed, etc., seem to have carved this niche into this cultural Southern language and style of speech and inflection. I simply adore it.