Big Flightless Birds Come From High-Flying Ancestors
Halibutboy Flatfish stashed this in Science
Flightlessness evolved separately several times.
They still don't know why:
One remaining question is why these birds around the world would independently lose the ability to fly and get big.
Cooper notes that this happened right after the extinction of the dinosaurs 65 million years ago. Between that event and the eventual rise of mammals, there would be a time with no predators, when birds could become big and flightless without being eaten.