The absence of mammalian predators and competitors in New Zealand, allowed dominant taxa to evolve from other animal groups that were functionally equal to mammals. With the exception of a few oceanic islands, they have no functional or taxonomic equivalent anywhere in the world. The moas were the most significant mammal replacements on New Zealand, taking the role of the largest dominant herbivores, the same role as large animals such as deer and elephants in other lands.

The 11 extinct species of moa are another of New Zealand's biological peculiarities, which evolutionary scientists relate to the long isolation and size of the country.   It is Earth's largest oceanic archipelago, and the most distant from any continental land mass, which together with a mixed topography, provided the conditions for natural selection processes that produced varied evolutionary outcomes. As Dr Jared Diamond of UCLA points out  "... the only approach to moas elsewhere in the world were the elephant birds of Madagascar plus the surviving ratites of the continents, but none of these other groups of very large flightless herbivorous birds radiated to anything like the degree that the moas did ..." Ratites were a notable group of early birds that have no ridge (keel) on their sternum (breast bone) to which wing muscles are attached in birds that fly.  It is thought that they evolved from flying ancestors whose breast muscles and wings degenerated when they increased in size and improved their ability to run to escape. Evidence points to the probability that moas walked into New Zealand before it was separated from the other Gondwana countries.  Kiwi are also ratites, but flew to New Zealand much later, about 40 million years ago.  Kiwi now have vestigal wings, whereas moas had no wings whatsoever.  Other ratite relatives of the moa that indicate a Gondwana connection include the emu of Australia, cassowaries of New Guinea and Australia, ostrich of Africa, rheas of southern South America, and the fossil ratite Sylvornis of New Caledonia. Moas were sexually dimorphic, with males different in size and shape than females of the same species. This led to confusion in the identification of species with up to 29 listed in the classic classification during the last 60 years.  More recent research now identifies 11 species. Moa eggs were normally cream colored, however, some light green and teal blue shell has been found.  The eggs of the biggest species were 24 centimetres (10 inches) long, and the largest egg that has been found has a capacity of 4,302 cubic centimetres (1.8 cubic feet), 100 times the capacity of an average sized chicken egg.

MOA EXTERMINATION

The descendants of the original ratites proliferated and developed distinctively during more than 80 million years of isolation on New Zealand, until the arrival of humans.

Moa had only one natural predator - the huge Haast's eagle (Harpagornis moorei), the largest eagle ever known with a wing span of ten feet and talons as big as tiger's claws.  But life in a bird's paradise could not last, and after millions of years moas quickly ended up in human cooking pits, and much of their habitat was destroyed by fire.

R. N. Holdaway & C. Jacomb, Rapid Extinction of the Moas (Aves: Dinornithiformes): Model, Test, and Implications, Science, March 24 2000

The Holdaway and Jacomb study explored the impact on moa population of low exploitation levels by an initial population of 100 people, coupled with the habitat loss caused by them.  Conservative analysis used low to medium human population growth rates, minimal rates of habitat removal in two areas of the two main islands, and the lowest cropping rates.  The total population of all species of moas at the time of human settlement was 158,000 birds, and only consumption of adult moas over one year old was considered.   It is known that consumption of moa eggs was considerable, but was ignored in the study.

Moas, like most long-lived birds, were most vulnerable to any increase in adult mortality.  When subjected to a low level of human predation, moas required an impossible increase in births to maintain their numbers.  Even without habitat loss that is known to have occured during the extinction period, the most conservative analysis suggests that moas were extinct within 160 years of human arrival.

Revised radiocarbon dating of charcoal in campsites, place the earliest arrival of moa hunters, Polynesians who were the ancestors of Maori, in the 13th century instead of the 11th century as previously thought.  Archeological research of dismembered moa bones in campsites show that they were not killed after the 14th century.  The short period during which moa were eaten out of existence is a mere blink in the geological time of moa life.

In a commentary on the study, Dr Jared Diamond claims that in New Zealand, Madagascar and many Pacific Islands, few would deny that the first arriving humans caused mass extinction, and the only question is how fast it occurred and whether hunting was the only cause.  He said the new study "shows that moa extinctions were very fast and were mainly by hunting".

Dr Ross MacPhee who is a mammalogist at the American Museum of Natural History in New York and has studied ancient extinctions, says "there is no way of interpreting this record other than that it had to happen virtually overnight ... this is easily the best instance of overkill, of blitzkreig, on the record".

Moa extinction was more rapid than the rate of extermination of large prehistoric mammals such as mammoths, camels, ground sloths, mastodons and giant beavers that some scientists think was caused by hunters in a North American blitzkreig 13,000 years ago.  The predators that depended on these animals, which included saber-toothed cats, cheetahs, maned lions, wolves and short-faced bears are thought to have become extinct in 400 years.