Baby birds babble. That's the conclusion of some scientists at MIT. This one doesn't come as a surprise to me, having read Donald Kroodsma's book, wherein he makes the same observation.
So when we're out in the field, we have to remember not only changes in plumage, but that we can hear some pretty strange things coming from the brush.
Kroodsma also points out that birds vocalize in local dialects. Black-capped chickadees' sound repertoire includes some whistled notes that vary from place to place. And try listening to common yellowthroats in different areas.
Just as I think I'm becoming a pretty decent birder......I continue to learn how much I don't know. As Kenn Kaufman has said, as humans we try to draw lines and categorize, whereas nature is fuzzy. My brain doesn't do well with fuzzy.