When giant eagles ruled the roost

Living in isolation can lead to the evolution of super-sized creatures, as illustrated by new research into New Zealand's now extinct giant eagle.

Life away from its pint-sized forebears helped the Haast's Eagle to become the top predator on the block.

An analysis of ancient DNA taken from an eagle that died about 2,000 years ago helped scientists learn about the development of the Haast's Eagle. The research by anthropologist Michael Bunce from McMaster University, Ontario, Canada, and colleagues is published this week in the journal Public Library of Science Biology.

Before human settlement, about 7,000 years ago, New Zealand lacked terrestrial mammals. Apart from bats, the only inhabitants were about 250 species of bird.

The Haast's Eagle sat at the pinnacle of this food chain, the only eagle known to have been a top predator in a major terrestrial ecosystem.

The eagle dominated all it surveyed when it coasted on updrafts across New Zealand. It weighed between 9 kg and 13.5 kg, making it 30 to 40 per cent heavier than the largest living bird of prey in the skies today, the Harpy Eagle of Central and South America.

The eagle hunted moa, the herbivorous, flightless birds of New Zealand, which can weigh 180 kg, making them a challenge for any predator and especially one less than a tenth the size of the prey.

The working assumption was that the large New Zealand bird was related to a similar raptor living in Australia. The initial challenge was to extract useable DNA from old eagle bones dating back two millennia in order to prove this relationship. The initial results were so surprising, however, that the researchers doubted what they had found.

"When we began the project it was to prove the relationship of the extinct Haast's Eagle with the large Australian Wedge-tailed Eagle," says Bunce.

"But the DNA results were so radical that, at first, we questioned their authenticity." The data showed that the New Zealand giant was no cousin of the Australian eagle. Rather it sprang from the same family as one of the world's smallest eagles, the Little Eagle from Australia and New Guinea. This carnivore weighs in at less than one kg.

"Even more striking was how closely related genetically the two species were," adds Bunce. "We estimate that their common ancestor lived less than a million years ago. It means that an eagle arrived in New Zealand and increased in weight by 10 to 15 times over this period, which is very fast in evolutionary terms. Such rapid size change is unprecedented in birds and animals."

This type of gigantism is frequently seen when a species finds its way to a new, isolated environment free from predation. The Pacific islands, Antipodes and Madagascar all have species that have changed in this way from the Dodo to the Komodo dragon.