New research highlights bird species' dramatic evolution into New Zealand whoppers

Saturday, 23 February 2019

Genetic research has highlighted the dramatic evolution of how certain bird species grew much larger after their ancestors arrived in New Zealand.

The most remarkable change was in the mysterious New Zealand adzebill​ – with its enormous reinforced skull and beak – but the mighty Haast's eagle and rather impressive Eyles' harrier also bulked up significantly.

Of the three, the flightless adzebill was in this country by far the longest. The North Island species typically weighed around 16kg, while the South Island version typically got to 19kg. It's been suggested the South Island species' top weight might have been 25kg – about 10 times smaller than the largest moa.

Research led by the University of Adelaide, with New Zealand and US involvement, and published in the journal Diversity during the week, indicates some of the adzebill's closest living relatives are tiny flufftails, which live in Africa and Madagascar, and can weigh as little as 25 grams.

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'We know that adzebills have been in New Zealand for a relatively long time since we previously discovered a 19 million-year-old adzebill fossil on the South Island,' co-author Associate Professor Trevor Worthy, of Flinders University, said.

'A key question is whether they've been present since New Zealand broke away from the other fragments of the supercontinent Gondwana (100-80 million years ago) or whether their ancestors flew to New Zealand from elsewhere later on.'

The researchers estimated the ancestors of the adzebills split from the Sarothruridae group – which includes the flufftails and Madagascan wood rails – 39.6 million years ago.

That suggested the ancestors of the adzebills got themselves to New Zealand, rather than having been around when the country split from Gondwana.

That may have involved them travelling from another Gondwanan landmass via Antarctica. Evidence suggested some coastal regions and offshore islands of Antarctica had a temperate climate that supported southern beech forests until the end of the Eocene – about 40 million years ago.

Ancestors of kiwi and moa may also have arrived via Antarctica.

If Madagascan wood rails, which weigh up to about 280g, were used as a proxy for the size of the ancestor of the adzebills, that suggested the mass of adzebills had increased in mass more than 50-fold in 20-24 million years, the study said.

The calculation was based on the time between the adzebill's split from the Sarothruridae group, and the 16-19 million-year-old age of the adzebill fossil found in the South Island.

The time for the evolution may have been even less, if flying ancestors of the adzebills only arrived after the inundation of large areas of New Zealand, if not the whole country, which peaked 22-25 million years ago, the study said.

While quite large flightless rails had been found on other oceanic islands, none rivalled the adzebill for size. That could indicate the adzebill's ancestors may have exploited some unique circumstances, the paper said.

Adzebills had a 'massive' beak and skull that, along with the neck vertebrae, were heavily reinforced and likely supported powerful muscles.

'Adzebills also had robust legs and feet, which may have been used with the massive bill to excavate or immobilise food or prey,' the paper said. They were likely predators or scavengers, although their exact feeding strategy was unknown.

'Archaeological deposits confirm that adzebills were hunted by early Maori, who also rapidly cleared the dry, lowland podocarp forests in eastern NZ, which were the birds' main habitat during the Holocene.'

Both activities led to the extinction of the adzebill around the same time as the moa.

Haast's eagle and Eyles' harrier also went extinct after the arrival of Maori. A separate study published during the week, and involving scientists from the University of Otago and Canterbury Museum, found both birds probably split from their smaller Australian cousins at the start of the most recent Ice Age, about 2.5 million years ago.

Haast's – the world's largest eagle – had a wingspan of up to three meters and weighed up to 15kg. The research, published in Molecular Phylogenetics and Evolution, found it was a close relative of the Australian little eagle – one of the smallest eagles in the world weighing only 1kg.

For Eyles' harrier – the largest harrier in the world, weighing 3.5kg and with a two-meter wingspan – the closest relative was found to be the small to medium-sized Australian spotted harrier. Eyles' was more than three times larger than the swamp harriers common in New Zealand today.

Canterbury Museum senior curator of natural history Dr Paul Scofield said that at the time the ancestor of Haast's eagle arrived in New Zealand, advancing glaciers were clearing swathes of dense forest. That likely provided suitable open habitat for the ancestor eagle to establish itself.

Before the eagle's arrival, there had been no apex predator, preying on moa, Haast's eagle grew to fill the apex predator niche, which on other landmasses was occupied by mammals such as lions and wolves.

'These giant birds of prey went through some really dramatic changes in a relatively short time,' Scofield said.

'Haast's eagle and Eyles' harrier are examples of what scientists call island gigantism – a biological phenomenon where the size of an animal species isolated on an island increases dramatically in comparison to its relatives elsewhere.'