The legend of Poūwa: ancient myths of New Zealand's black swan confirmed by fossil DNA

A tall, bulky and probably near-flightless black swan once roamed New Zealand. But it was hunted to extinction not long after people arrived during the late 13th century, and then replaced by its Australian cousin.

Our research, based on ancient DNA and morphology, shows that the New Zealand species — dubbed Poūwa — was unique and genetically separate from the Australian species.

Poūwa was heavier and larger, a bit like an All Blacks rugby player, and it was well on the way to becoming flightless when it became extinct.

A potted history of the black swan

The black swan, Cygnus atratus, is a quintessential Australasian bird. Until their first voyages to Australia, 18th century European explorers assumed that all swans were white.

By the time Europeans arrived in New Zealand, there were no black swans. But naturalisation societies introduced Australian birds from Victoria during the 1860s.

At the same time, black swan bones were being found in fossil and archaeological deposits in New Zealand and the Chatham Islands. Scientists concluded that the Australian black swan formerly inhabited the New Zealand region but that the local population had been hunted to extinction. Until recently, this was still the prevailing view.

However, my group’s ancient DNA research on New Zealand’s marine megafauna (penguins, sea lions and shags) strongly suggested that such potted histories might not be so simple.

Ancient DNA analysis is a powerful tool that allows scientists to trace genetic lineages through time. Combined with radiocarbon dating, we are able to reconstruct how species and ecosystems respond to human impact and climate change.

Humans impacts on ecosystems

Scientists have learned a lot about prehistoric human impacts on large animals, known as megafauna, and their habitats worldwide. However, we know comparatively little about the impact of human exploitation on smaller species.

Anatids (ducks, geese and swans) are one such example. This diverse group of birds (150 species in 40 genera globally) features in medieval carvings and Roman mosaics, and has a long history of human exploitation. Anatid bones are found in archaeological middens (rubbish dumps) around the world.

Human exploitation is thought to have driven to extinction some prehistoric waterfowl, especially large, flight-reduced or flightless island species such as the moa-nalos from Hawaii and the dodo from Mauritius. However, such impacts have yet to be well characterised.

Why New Zealand

At work on moa bones, at a site known as the Gradungula Passage, in the Honeycomb Hill caves in the South Island. Nic Rawlence, CC BY-ND New Zealand is an ideal place to investigate people’s impact on anatids.

The archipelago is the last major landmass to be settled around the late 13th century AD, during a period of relatively stable climate. The extinctions and widespread habitat modification that followed also occurred during a period of climate stability.

Compare this to other regions of the world, such as the Americas or Europe, where climate change and human impact often coincide and scientists are still debating the relative contributions of these environmental stress factors to ecosystem change.

We know that a third of the New Zealand region’s anatid fauna has become extinct. This unique genetic resource allows us to answer fundamental questions.

Extinction and recolonisation

Our research was carried out by scientists at the University of Otago, Canterbury Museum and the Museum of New Zealand Te Papa Tongarewa. Both museums, along with Otago Museum and Auckland Museum, hold a wealth of pre-human fossils and archaeological remains, including black swan, from throughout the New Zealand region. Museums here and in Australia also hold extensive modern tissue collections of black swans.

Using ancient DNA (at the Otago Palaeogenetics Laboratory) and modern DNA techniques, our genetic analyses showed that prehistoric swans from New Zealand and the Chatham Islands formed one lineage, while modern black swans from throughout Australasia were a different lineage.

We have seen this genetic pattern before with our research on penguins and sea lions. Our data showed that a unique lineage of black swan in New Zealand and the Chatham Islands was hunted to extinction and replaced by the Australian lineage.

The genetic divergence between the Australian and New Zealand lineages suggests that they separated around one to two million years ago. Could these different lineages be different species, given the divergence time? We needed morphological evidence.

The black swan theory revisited

Analysis of the skull and measurements of wings and legs, from many of the same specimens used for genetic analysis, showed that New Zealand and the Chatham Islands had its own unique species of black swan, supporting our genetic evidence of two distinct lineages.

We gave this new species the scientific name Cygnus sumnerensis after the Sumner suburb in Christchurch, where the first remains of this new species were discovered.

We’ve called our new species Poūwa, based on a Chatham Island Moriori legend about a large black bird that inhabited the lagoon and whose bones were common in the surrounding sand dunes.

Just like 18th-century European explorers who assumed all swans were white, up until now, scientists assumed all black swans were the same species. As in the 18th century, we were proved wrong. Sadly, the Poūwa did not survive the arrival of humans.

Evolution on island ecosystems

The Poūwa was morphologically unique. At 10kg, it was taller, larger and heavier than its Australian cousin. It had elongated legs and proportionally shorter wings.

Such proportional anomalies are common in birds from island ecosystems that lack terrestrial mammals. They are also seen in other New Zealand birds, for example the extinct laughing owl. Prior to human arrival, New Zealand had no land mammals and was truly a land of birds. The Poūwa was on the evolutionary pathway to flightlessness - a very tall and heavy bird indeed.

Our findings raise interesting questions for conservation and ecological restoration efforts, and potentially de-extinction.

Since the Australian swan is not a replicate of the original Poūwa, should it be considered a pest in New Zealand, or something to be protected? It may be the case that ecological restoration and rewilding efforts cannot use closely genetically related species of extinct animals as ecological surrogates.

Perhaps, one day, de-extinction science will allow us to bring back New Zealand’s unique Poūwa.