The kakapo is a large, rotund parrot; the adult can measure from 58 to 64 cm (23 to 25 in) in length, and weight can vary from 0.95 to 4 kg (2 to 9 lb) at maturity.[17][18] Males are larger than females. Twenty-eight males were found to average 2 kg (4.4 lb) in one study, and 39 males were found to average 2.06 kg (4.5 lb) in another. In the same studies, 28 females were found to average 1.5 kg (3.3 lb) and 18 females averaged 1.28 kg (2.8 lb).[19][20] However, one source states that females will not breed until they attain a threshold weight of 1.5 kg (3.3 lb).[21] They are the heaviest living species of parrot; while the largest males attain much heavier weights than any other extant parrot, kakapos average about 400 g (14 oz) more than a hyacinth macaw.[19] The kakapo cannot fly, having short wings for its size and lacking the pronounced keel bone (sternum) that anchors the flight muscles of other birds.