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“Some cryptozoologists first theorized that the thunderbird myth is based on sightings of a real animal. Regarding the question whether such a large bird could have flown at all, the prehistoric vulture-like Aiolornis incredibilis (previously known as Teratornis) which was described in 1909 had a wingspan of around 5 m (16 ft) and was capable of flight, but probably would have favored heavy winds to facilitate takeoff. This bird, however, was almost certainly never seen alive by human beings, but a slightly smaller relative, Teratornis merriami must have been frequently encountered by early Amerindians.

It is generally believed that Merriam’s Teratorn was a dynamic soarer, riding upcurrents of heated air. Their finger bones have adapted to bear the load of huge primaries, allowing them to maneuver expertly in strong updrafts, as typically found associated with storms (Campbell & Tonni, 1983).

Paleontologists reject the continuing existence of a large and conspicuous bird like Aiolornis or Teratornis in modern times, and anthropologists point out that American Indian thunderbirds were not especially similar to this creature (and usually not very similar to birds at all). The changes of “thunderbird” records over the years - from pterodactyl-like beings in the late 19th and early 20th century to teratorn-like birds becoming prominent from the middle 20th century onwards, as reconstructions of teratorns were reaching a wider audience - suggest that most records are considerably shaped by underlying expectations. In a similar vein, cryptozoologist John Keel states that his mapping of Thunderbird sightings corresponded chronologically and geographically with storms moving across the United States, but this is only to be expected in either case.”