The secret of New Zealand's geological past has been found in the unlikely hiding place of a tiny native bird's DNA.

A new DNA analysis of living and extinct species of New Zealand's acanthisittid wrens has provided compelling evidence that, contrary to some suggestions, our country was not completely submerged under the ocean around 21 to 25 million years ago.

Acanthisittid wrens - which include rifleman - are a group of tiny, largely flightless, birds found nowhere else in the world.

They're called wrens because of their similarity in appearance and behaviour to "true wrens", but they don't belong to the same family.

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"Of the seven species living before humans arrived in New Zealand, only two now remain, the rock wren and the rifleman," said the study's lead author, Dr Kieren Mitchell, of the University of Adelaide.

"Consequently, little is known about their evolution."

Published in the journal Molecular Phylogenetics and Evolution, the researchers analysed DNA from three of the extinct species along with the two living species.

"Most surprisingly, we found that some of the wren species were only distantly related to each other, potentially sharing a common ancestor over 25 million years ago," Mitchell said.

"Previously, researchers have suggested that New Zealand was completely submerged 21 to 25 million years ago, which implies that all of New Zealand's unique plants and animals must have immigrated and diversified more recently than that time.

"This theory is consistent, for instance, with what is known about the moa, where the different species all shared a common ancestor much more recently than 21 million years ago.

"But the ancient divergences we found among the wrens suggest that they have been resident in New Zealand for more than 25 million years, and possibly as long as 50 million years (when New Zealand became disconnected from the rest of Gondwana).

"As the wrens were largely very poor fliers, or even flightless, some land must have remained throughout that period.

"This has important consequences for our understanding of the evolution of New Zealand's unique ecosystems."

The research was in collaboration with the Museum of New Zealand Te Papa Tongarewa, Canterbury Museum and the Department of Conservation (DOC).

Meanwhile, a species of wren in the Homer Saddle and Gertrude Valley are making a comeback thanks to a trapping project run by the Southland Section of the New Zealand Alpine Club (SSNZAC).

Just four years ago, the rock wren population in the area were in catastrophic decline, and a team of Department of Conservation researchers found that all of the nests were being attacked by predators.

To protect these vulnerable birds from local extinction, SSNZAC set up a predator trapping programme.

The number of rock wren in the area is now steadily increasing, growing from 10 breeding pairs to 33 pairs today.

"We are really excited by the success of the rock wren population in our monitored areas," SSNZAC Committee member Stanley Mulvany said.

"This programme seems to have made a significant difference to the survival of this, New Zealand's only true alpine bird."

DOC will continue to monitor the rock wren population in the Homer-Gertrude area through the next breeding season.