By the time the mesolithic was in gear ostriches were mostly the only big flightless birds around for people to encounter. The southern tip of South America had a special treat though. The last of the terror birds (phorusrhacid) lived there until a little under 12,000 years ago. 10 feet tall and instant death to any human who might have made it to the end of the world that is Patagonia at the time.