Kakapo relative wing size of any parrot. Its wing feathers are shorter, more rounded, less asymmetrical, and have fewer distal barbules to lock the feathers together. The sternum is small and has a low, vestigial keel and a shortened spina externa. As in other flightless birds and some flighted parrots, the furcula is not fused but consists of a pair of clavicles lying in contact with each coracoid. As in other flightless birds, the angle between the coracoid and sternum is enlarged. The kakapo has a larger pelvis than other parrots. The proximal bones of the leg and arm are disproportionately
Flightless bird free to increase in abundance until the population was limited by food and territory. A study looking at energy conservation and the evolution of flightlessness hypothesized intraspecific competition selected for a reduced individual energy expenditure, which is achieved by the loss of flight. Some flightless varieties of island birds are closely related to flying varieties, implying flight is a significant biological cost. Flight is the most costly type of locomotion exemplified in the natural world. The energy expenditure required for flight increases proportionally with body size, which is often why flightlessness coincides with body mass. By reducing large pectoral muscles