When Thunderbird flaps its wings, the sound of thunder can be heard. According to the Lost Tapes site, "Some cryptozoologists today compare thunderbirds to pterosaurs, giant birds of prey that roamed the skies 115 million years ago." Thunderbirds are mostly described like giant, murderous eagles.
Thunderbirds are vengeful hunters, "Prone to grabbing children in their talons and carrying them away to kill them." In Lost Tapes, however, the youngest of the three boys who were skateboarding in a forbidden area of the forest was carried off by the creature, but was not killed. He had no recollection of what happened between the moment he was carried off to the moment he woke up in a completely different place.