Thunderbird

Okay, I'm usually leery of cryptids that're unique to certain localities. For instance, the thunderbird - a legendary creature (with a wingspan of 15 to 20 feet) in the North American indigenous peoples' history and culture. Why only them? Can't these birds flock outside Tatanka's tribe? Ever heard of or read about the alleged Lawndale sighting and attack in 1977, by the way?