Moas, flightless birds that lived in New Zealand, were driven to extinction shortly after the arrival there of humans -- and it didn't take all that many humans to make the birds vanish, research has demonstrated.

The large flightless birds, related to ostriches and emus, had completely disappeared by the time the human population of New Zealand had reached two and a half thousand people at the most, scientists say.

The Polynesian people whose hunting activities led to the extinction of the moa in less than 100 years had one of the lowest human population densities ever recorded, the researchers say; when moa hunting was at its peak there were fewer than 1,500 Polynesian settlers living in New Zealand, a study published in Nature Communications says.

When the first people discovered and colonized New Zealand in the 13th century, their density never exceeded about 1 person per every 100 square kilometers, the researchers estimate.

The total extinction of the moas was probably complete around 1423, barely 100 years after the earliest well-dated archeological site of New Zealand being settled by people coming from tropical East Polynesia, the study suggests.

"This is the first time we have been able to show that extinction was both rapid and synchronous across New Zealand," says George Perry of the University of Auckland's School of Environment and School of Biological Sciences.

"Moa were hunted to the point of being critically endangered within 150 years of settlement, after which only a few small populations clung on in remote mountain regions, but only for another 50 years before they vanished forever," he says.

Some 10 species of moa were thought to have inhabited New Zeland, ranging in size from smaller than a turkey up to giant moas standing 6 feet tall and weighing up to 550 pounds.

Even with the low human population density, it would have taken just five birds harvested per person per year to drive moas to extinction within 200 years, Perry says.

One cautionary outcome of the study is the strong evidence that small human population size can no longer be used as an argument against human involvement in extinctions elsewhere, the researchers emphasize.

The study findings may mean even small human populations could have been involved in the extinction of ancient megafauna like mammoths and giants sloths in North America and giant marsupials that once roamed Australia, the researchers suggest.