Despite the large amount of adaptation required for flight, there are nevertheless a large number of birds that have abandoned flight. The older birdfossils dating some thirty million years after Archaeopteryx including gull-like forms (Ichthyornis) which were skilled flyers with a keeled chest bone and no bony tail. In essence they were modern birds. At the same time, however, lived huge swimming birdsHesperornis, which were nearly as big as a man and had already ceased to be able to fly. Fossils of those other non-flying birds, the penguins, also appeared around this time. Fossils of another large flightless birdDiatryma stalked the plains of Wyoming, while a similar birdPhororhacos. This bird was about 2m tall, carnivorous, and equipped with a huge bill. It is possible that this group was successful in the absence of other large carnivores representing either reptile or mammal classes. Large carnivores in the former class were already extinct in the former class and were yet to evolve in the latter class. Diatryma may have been the early ancestor to Gruiformes group of birds (Rails and Cranes) which even today have representatives (e.g. flightless rails of Gough Island) that showed a marked tenancy to lose flight when they colonize islands that have few or predators. The cormorants of the Galapagos Islands have such small wings that they cannot fly any longer. On the Madagscarene Islands, the dodo (Raphus cucullatus), was a very large pigeon that adopted a terrestrial habit and was exterminated by the human introduction of dogs to the island in the seventeenth century. The Elephant bird Aepyornis was about 3m tall and possessed the largest known eggs for any birdspecies (148 times the size of a hens egg by volume). Moas (Diornis) were another giant flightless bird over 3m tall and occurred on New Zealand.
Currently four orders of birdsspecies fall into the general category of wingless and flightless terrestrialbirds. These include ostriches (Struthioniformes), rheas (Reiformes), cassowaries and emus (Casuariiformes) and kiwis (Dinornithiformes)
Moa attacked by a Haast's Eagle