_______ is the largest flightless bird.
Ostrich
Flightless birds are those that have lost the ability to fly during the course of evolution. Ostrich is believed to be the largest living bird, and thus, obviously, the largest among flightless ones too. Emu, endemic to Australia, is the second largest living bird (by height) in the world, and is also flightless. Penguins and turkeys are flightless birds too.