Giant eagles not just the stuff of legends

Dr Richard Holdaway with a moa bone, the staple diet of the Haast’s eagle. (On the computer screen is an illustration of a Haast’s eagle hunting moa – image courtesy of John Megahan.) Click here to view a larger version

University of Canterbury senior fellow and palaeobiologist Dr Richard Holdaway is among a group of researchers from New Zealand and the UK who have shed new light on the evolution of the now extinct giant eagle that once ruled the skies over New Zealand.

The enormous Haast’s eagle dominated its environment. Weighing in at between 10 and 14kgs, it was 30-40% heavier than the largest living bird of prey around today, the harpy eagle of Central and South America, and was approaching the upper weight limit for powered flight.

Led by Professor Alan Cooper from Oxford University’s Ancient Biomolecules Centre, the New Zealand researchers extracted DNA from fossil eagle bones dating back about 2000 years.

Christchurch evolutionary molecular biologist Dr Michael Bunce, now based at McMaster University, Canada, who was part of the Oxford team that carried out the DNA analysis, said when they began the project it was to prove the relationship of the extinct Haast’s eagle with the large Australian wedge-tailed eagle.

“But the DNA results were so radical that, at first, we questioned their authenticity,” he said.

The results showed that the New Zealand giant was in fact related to one of the world’s smallest eagles – the little eagle from Australia and New Guinea, which typically weighs less than one kilogram.

“Even more striking was how closely related genetically the two species were. 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 -15 times over this period, which is very fast in evolutionary terms. Such rapid size change is unprecedented in birds and animals,” added Dr Bunce.

UC’s Dr Holdaway, co-author of the highly-acclaimed The Lost World of the Moa, was extensively involved in the interpretation of the results and the writing of the paper.

Speculating on why Haast’s eagle grew so quickly to such vast proportions Dr Holdaway said: “The size of available prey and the absence of other predators are, we think, the key factors driving the size increase. The eagles would have been able to feed unhindered on their kill.”

Haast’s eagle is the only eagle known to have been the top predator in a major terrestrial ecosystem. They hunted moa which could weigh up to 200kg. With a truncated wingspan of around three metres for flying under the forest canopy, the eagles struck their prey from the side, tearing into the pelvic flesh and gripping the bone with claws the size of a tiger’s paw. Once caught, the moa would be killed by a single strike to the head or neck from the eagle’s other claw.

The scientists believe the eagle died out within two centuries of human settlement of New Zealand. Forest fires destroyed its habitat and humans exterminated its food supply. There is some evidence to suggest the eagles were hunted too.

“There are so many unanswered questions about our biological past that ancient DNA can help provide answers to, and it’s great to see New Zealand’s birds being the focus of this international research,” Dr Holdaway said.