Solved: riddle of the moa's mortality

One of the largest birds to ever live became extinct because it grew too slowly to sustain a viable population after the arrival of Polynesian hunters in New Zealand 700 years ago.

One of the largest birds to ever live became extinct because it grew too slowly to sustain a viable population after the arrival of Polynesian hunters in New Zealand 700 years ago.

The giant moa - which grew to nearly seven feet tall and weighed a quarter of a ton - took at least 10 years to grow to its full size, making it the slowest growing bird on record. This made it incapable of replacing its breeding population quickly enough to withstand the arrival of Maori hunters in New Zealand in about 1300, scientists said yesterday.

Moa, first identified from a bone fragment by Richard Owen in 1839, are known to have become extinct within a couple of centuries of the Maoris' arrival. Now scientists believe they have found out why - moa just took too long to grow up.

All birds alive today reach full size within a year or so, and sexual maturity soon after. But the moa was an exception to this avian rule, said Samuel Turvey of the Zoological Society of London's Institute of Zoology.

An analysis of the bones of the flightless giant moa, published in the journal Nature, shows concentric rings that represent the annual growth spurts during summers, Dr Turvey said.

"Previously we thought that all birds grew quickly, reaching maturity in about a year. Moa lived on an island without any mammalian predators and this made them unique," he said. "It enabled them to grow slowly over a long period of time. But it also made them vulnerable to humans."

Growth rings are seen in many slow-growing vertebrate animals but not in birds, other than the moa. There were about 10 different species with the two largest species growing to be as big as buffalo.