Do Native-American Thunderbird legends refer to pterosaurs or giant birds or both? When considering any particular legend, remember the importance of feathers or absence thereof. Also consider the possibility that the same word, “Thunderbird,” may refer to different things in different legends.
I do not imply that every legend of a giant creature contains truth about a real creature. But when modern human residents of North America witness giant flying creatures, bigger than any scientifically classified bird, I strongly suspect that earlier human residents witnessed the same thing, and what would be more natural than the creation of a legend based at least partially on old sightings of an unusually large flying creature? I suspect that at least one Thunderbird legend may be just that: based on a real animal.