Flightless bird that require a significant amount of overall metabolic energy, ratites decrease their basal metabolic rate and conserve energy. A study looking at the basal rates of birds found a significant correlation between low basal rate and pectoral muscle mass in kiwis. On the contrary, flightless penguins exude an intermediate basal rate. This is likely because penguins have well-developed pectoral muscles for hunting and diving in the water. For ground feeding birds, a cursorial lifestyle is more economical and allows for easier access to dietary requirements. Flying birds have different wing and feather structures that make flying easier, while flightless birds'
Flightless bird the lineage. This indicates that the distinctive flightless nature of ratites is the result of convergent evolution. Two key differences between flying and flightless birds are the smaller wing bones of flightless birds and the absent (or greatly reduced) keel on their breastbone. (The keel anchors muscles needed for wing movement.) Adapting to a cursorial lifestyle causes two inverse morphological changes to occur in the skeleto-muscular system: the pectoral apparatus used to power flight is paedorphically reduced while peramorphosis leads to enlargement of the pelvic girdle for running. Repeated selection for cursorial traits across ratites suggests these adaptions comprise a