Ancestors of New Zealand's extinct flightless bird, the moa, probably flew here from South America and then lost the ability to fly home.

New research has found that the moa's closest cousins were not the Australian emu and cassowary, as previously thought, but a small, quail-like South American bird called the tinamou.

Massey University's Professor David Penny, co-author of a paper on the issue in the journal Systematic Biology, said the moa's ancestors probably flew here from South America via Antarctica 30 to 40 million years ago when the southern continents were much closer together than today.

He said they probably lost the ability to fly, and developed into much larger birds up to 3.7m high, when they found no natural predators in the small islands that dotted the sea where New Zealand is today.

"Many different kinds of birds have lost flight, so there is nothing unusual about that," he said.

"In the Pacific Islands there are well over 100 species of rail alone, the family that includes the pukeko and the takahe, that became flightless in the islands because they haven't got mammalian predators, and because flight is costly."

The new research is a collaboration led by former Massey student Dr Matt Phillips, now at the Australian National University, with Dr Penny and his postgraduate students Elizabeth Crimp and Gillian Gibb.

Although moa have been extinct for about 500 years, earlier researchers have worked out their genetic structure from bones preserved in swamps.

The Massey/ANU team has now pieced together their family tree.

Dr Penny said the tinamou, which spend most of their time on the ground but can still fly, were always assumed to be related to the flightless "ratites" such as the moa, kiwi and emu.

Until now it had been thought that the moa's ancestors must have come to New Zealand about 80 million years ago before the southern continents started to drift apart.

"What happened is that we got into a way of thinking that some people call 'Moa's Ark' - the idea that perhaps New Zealand's plants and animals were a bit primitive and slow and needed special looking after," he said.

The finding that the moa's closest cousins are the tinamou, rather than any other Australasian birds, indicates that they must have come much later - when the only way they could have got here was by air.

"The fact that they flew here means that our birds and animals are more vibrant, more dynamic, than we thought," he said.

Scientists found about 15 years ago that the kiwi's closest cousin is the Australian emu, suggesting the kiwi's ancestor also flew here, from Australia, probably more recently than the moa.

Dr Penny said the genetic evidence suggested that the ratite group began evolving about 80 million years ago, before a huge meteorite dropped in Central America 65 million years ago and wiped out the dinosaurs.

He said the oldest birds of any kind appeared to have evolved in what is now the Australasian part of the ancient continent of Gondwanaland.