[1] A ratite is any of a diverse group of large, flightless birds of the infraclass Palaeognathae. The name “ratite” refers to their flat sternum that is distinct from the typical sternum of flighted birds because it lacks the keel. Flightless birds are birds that have evolved the inability to fly. They include penguins, kiwis, ostriches, and emus. Certain domestic birds, such as chicken and duck, are also unable to fly for extended periods of time. The tallest and heaviest flightless bird is the ostrich. Extinct species such as the Phorusrhacidae was also a flightless bird. Like the ostrich and emu, these birds developed specific body shapes – large heads, long necks, and legs.
[2] After the mass extinction of non-avian dinosaurs, the loss of flight evolved within the ratite lineage. The sudden disappearance of certain species offered opportunities for Palaeognathae to distribute and take up new environments. Due to environmental and ecological factors, these animals developed new body types and flightlessness to adapt. During the Miocene period, rainforests dried into partially arid deserts. These animals had to travel long distances to find food that was low-lying vegetation.
[3] Therefore, flightlessness and gigantism are correlated, and this is most evident in regions such as islands with fewer predators and competition. Nevertheless, ratites usually reside in regions with a variety of mammals. Not all flightless birds have gigantism, such as the New Zealand kiwi. This may be due to their evolution from a flying bird or due to competitive exclusion. Once a flightless bird arrived in a region, the herbivore or omnivore niche was used, forcing other birds – such as the kiwi – to remain small. Where no other flightless birds were present, they may have been excluded by other herbivorous mammals.
[4] It was initially believed that ratites originated after the separation of the Gondwana supercontinent. However, in 1974, Joel Cracraft suggested that ratites derived from a flighted ancestor and developed flightlessness over time. New Zealand boasts the greatest number of flightless bird species. This is due to the fact that until human habitation one thousand years ago, no large land predators existed in New Zealand.
[5] Notable differences between flightless and flying birds lie in the size of the wing bones. In flightless birds, the wing bones are much smaller and generally, the keel that anchors muscles for wing movement on the breastbone is absent. The pectoral apparatus in ratites used to give power to flight is reduced and the pelvic girdle is wider and stronger for running. Primitive characteristics for flight, such as the fusion of wing elements, an alula on the wing, and a cerebellar structure are still present in flightless birds.
[6] It’s interesting to note that some varieties of flightless island birds are related to the flying ones and that flightlessness coincides with body size and mass. The amount of energy needed for flight increases according to the size of the bird’s body. With the reduction of the pectoral muscles that demand a great deal of energy, the metabolic energy rate of flightless birds reduced and they were able to conserve energy. For example, in kiwis, research indicates a notable correlation between the pectoral muscle mass and a low basal rate. However, flightless penguins show an intermediate basal rate as they have stronger pectoral muscles for diving and hunting.
[7] Apart from New Zealand moas, the wing structure is still present in flightless birds. This is due to the fact that wings are necessary for balance and act as an apparatus to slow the bird down similar to how a parachute works. An example of this is in the ostrich that can run up to 50 kilometers per hour. Wings are also used to attract a mate, as can be seen in rheas and ostriches. Larger birds are generally more successful in reproduction and a bigger male size indicates protective abilities to the female. These birds use the wings in courtship and to maintain body size in reproduction. Flightless birds are monogamous as they need a reliable mate for high parental involvement. The male flightless birds protect the baby birds between 85 and 92 days while the female birds feed. The males can survive up to one week without eating and survive off fat storage. Emus can fast up to 56 days.
