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Giant eagles: not the stuff of legend

Scientists learn genetic ancestry of birds that hunted prey heavier than most humans

Before human settlement 700 years ago, New Zealand had no terrestrial mammals, apart from three species of bats. Instead, about 250 species of birds dominated the terrestrial ecosystem. At the top of the food chain was the extinct Haast’s eagle. With a wingspan of 2.5 to 3 meters (around 3 yards) and a weight of between 10 and 14 kilograms (up to 31 pounds), Haast’s eagle was about 30-40% heavier than the largest living bird of prey, the harpy eagle of Central and South America.  Haast’s eagle was approaching the upper weight limit of powered flight, scientists say.

Haast’s eagle is the only eagle known to have been the top predator in a major terrestrial ecosystem, hunting moa, the giant herbivorous birds of New Zealand, weighing up to 200 kilograms (440 pounds).

Evidence of eagle attacks remain, as holes and rents torn into the bones of moa, which show that the eagle struck from the side, gripped the moa’s pelvic area with one foot, and killed with a single strike by the other foot to the neck or head. The eagle is thought to be the bird called “Hokioi” in the oral history of the Maori, a native people. Its existence is recorded in rock art, and artifacts shaped from eagle bone prove that the bird co-existed with early Polynesians, according to researchers. However, there is no evidence that humans were targets for this huge aerial predator.

In the research paper, New Zealand researchers at Oxford University, U.K., extracted DNA from fossil eagle bones about 2000 years old. Michael Bunce, who performed the research, said, “When I began the research project with graduate student Marta Szulkin, it was to prove the relationship of the extinct Haast’s eagle with the large Australian wedge-tailed eagle. The DNA results were so radical that at first we questioned their authenticity.”

The researchers says they showed 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 under 1 kilogram, about 2 pounds. “Even more striking was how closely genetically related the two eagle species were. We estimate that their common ancestor lived less than a million years ago. It means an eagle arrived in New Zealand and increased in weight by 10-15 times over this period; such rapid size change is unprecedented in terrestrial vertebrates,” Bunce said.

New Zealand-based palaeobiologist Richard Holdaway, who was part of the research team and who has studied the eagle for the past 20 years, speculated as to how and why Haast’s eagle grew to be so big, so quick: “The size of available prey and the absence of predators are, we think, the key factors driving the size increase. The large herbivores were available, and after killing a moa, an eagle would have been able to feed unhindered.”

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