I'm in a quandary. I don't know whether to include this essay in my Cryptozoology or my Anomalies blog. Aficionados of both will be aware that occasionally they overlap. An obvious example is the appearance of bigfoot-like creatures in the vicinity of flying saucers. Similarly, back in the UK, black panthers are usually assumed to be flesh and blood, and placed in the cryptozoology pigeon hole, while black dogs seem to possess spectral qualities, and thus are classified along with ghosts and goblins.
Thus, although I have problems with the idea of thunderbirds with wingspans of six or seven metres, I would not rule them out, and so would classify them as a cryptozoological mystery. However, the accounts you are about to read contain additional elements of weirdness which lead me to record them in the current blog. You be the judge.