Flightless birds are birds that through evolution lost the ability to fly. There are many species to look at including the well known ratites such as the ostrich, emu, and rhea. The largest (both heaviest and tallest) flightless bird, which is also the largest living bird, is the ostrich (2.7 m, 156 kg). Many domesticated birds, such as the domestic chicken and domestic duck, have lost the ability to fly for extended periods, although their ancestral species, the red junglefowl and mallard, respectively, are capable of extended flight. Flightlessness has evolved in many different birds independently.