Ostriches, emus and cassowaries all retain wings in reduced form, and even the extinct great auk kept functional stubs. Moa skeletons have no forelimb at all. The shoulder girdle is vestigial and the humerus is absent or reduced to a tiny remnant in some specimens, making the group unique among known birds.
New Zealand had no terrestrial mammals apart from bats before human arrival, so there was no ground predator to flee. The only serious threat was Haast's eagle, striking from above, against which wings would not have helped. Moa filled browsing roles that deer and cattle occupy elsewhere, and the nine recognised species partitioned habitat between forest, shrubland and subalpine zones.
Genetic sexing of moa bones produced a surprise: specimens once assigned to three separate Dinornis species proved to be males and females of fewer species, with females roughly 150 per cent taller and up to nearly three times the mass of males. Extinction followed Polynesian settlement around 1300 and was complete within about a century or two, driven by hunting and egg predation on birds with very slow reproduction.