This one is from animal planet, states scientists had concluded a 3 yr old child at one time was killed by a large bird, It would be interesting to find an article of this incident. The story is known in Indian Country however I have never known any details, just that it was a yound child supposedly taken by a large bird, didn't know scientists had proven it to be true.
Makes you question why something like that would happen, what supernatural reasons?
In the videos above they also depict the thunderbird as a vengeful hunter, and a more fierce being, not so much a protector or holy figure like I believe they are.
I'll be checking for several other videos, see what I can find.
The main thing to remember when dealing with a teratorn is that genetically speaking many species carry their older codes and for whatever reason the old codes are either switched off and/or partially missing. This means occasionally old traits resurface in species. In the case of Teratorns is they have living descendents some of these codes may get reactivated from time to time hypotetically, allowing larger individuals of a current species appear from time. If over time the descendents of such species immigrated north eventually it would not be unusual to find the old traits reappear. Think of them as recessive genes unless the parents of recessive gene have almost similar set old codes the offspring will not activate those codes, and even then one slight variation might not allow those codes to activate.
Hmmm. So in short are you suggesting different gene expression? Not necessarily more or less evolved? Yet some of these larger birds could possibly be linked genetically as the same species? Just some genes are dormant in others and active in others differentiating them? And some codes could be reactivated at times? (didn't make that in short, did I? ) hmm that gives a whole different perspective thinking about all of that. No surprise that makes me contemplate some older legends of the thunder bird -It is said that Medicine Men shape shifted into Thunder Birds, or thunder birds were associated with that mystical power of shape shifting. I'm going to look for one of those articles. Funny how your above explanation can totally relate to that, from my perspective it does.