The term 'BIG FLIGHTLESS BIRD' can refer to three different species: the EMU (3 letters), RHEA (4 letters), and OSTRICH (7 letters). The EMU is a large, flightless bird native to Australia. It is the second-largest living bird by height, after the OSTRICH. The RHEA is a large, flightless bird native to South America. It is the largest bird in the Americas and is closely related to the EMU. The OSTRICH is the largest living bird in the world and is native to Africa. All three of these birds are flightless, but the OSTRICH is the largest of the three.