HOW BIG CAN AN EAGLE GET?

by Whit Gibbons

October 4, 2009

Certain animals serve as a reminder that despite what some people like to think about humans being special, some predators view us as simply another meal. Sharks, crocodiles, and giant snakes are clearly in the top 10 of animals that under the right circumstances would view us as just another prey species. A Komodo dragon, the largest lizard in the world, in Indonesia and a pet Burmese python in a Florida residence each killed someone this year, presumably considering the people as potential prey.

A scientific article in the September issue of the Journal of Vertebrate Paleontology by R. Paul Scofield of Canterbury Museum in New Zealand and Ken W. S. Ashwell of the University of New South Wales in Australia adds a new dimension to the potential list of human predators. Information collected during their study suggests that a giant eagle, now extinct but alive in New Zealand as recently as 500 years ago, may have preyed occasionally upon children and small adults. The scientific emphasis of the published study was on other aspects of the ecology and evolution of the eagles, but an aerial predator capable of swooping down and carrying someone off for a meal is a chilling thought.

Biological information about Haast's eagle, as the predator is called, is based on skeletal material the scientists examined. As with modern birds of prey, including hawks, owls, and eagles, females typically got larger than males. A male Haast's eagle is estimated to have reached a body weight of 27 pounds. The females are believed to have weighed in at just over 39 pounds. Because the estimated weights of Haast's eagle are based on only a few specimens, it seems safe to say that the largest ones were probably over 40 pounds.

Forty pounds is huge for a flying bird. To put their size in perspective, the largest bald eagles are around three feet in body length, have an outstretched wingspan of slightly under eight feet (which is right big in itself), but average under 15 pounds in total weight. That's pretty paltry compared to 40 pounds. Haast's eagles are indisputably the largest eagles known to science.

The thought comes to mind that the Maoris, the original settlers of New Zealand, probably got cricks in their necks from keeping a close watch on the skies for incoming eagles. But the primary target of Haast's eagles were flightless birds native to the islands--the moas.

Ostriches are the largest birds on earth today. Moas were even larger; they were the biggest birds ever known to have lived on earth. Some of the species were over 10 feet tall and weighed more than 400 pounds. Haast's eagles were the only natural predator of the moas, which used their enormous legs to move around quickly, like modern-day ostriches and emus. One advantage of flight is the ability to escape ground predators. About 10 species of moas had evolved on the islands of New Zealand because, without flying, they were able to fend off any natural predators--except the eagles. Things would probably have persisted for centuries in equilibrium, with big eagles eating big flightless birds, if the Maoris had not arrived in New Zealand in the late 1200s. The Maoris could easily capture and kill the moas, which had never encountered such a relentless land predator and had never evolved the ability to fly. So by the time Columbus landed in America, all species of giant moas had been extinct for a century.

Moas and Haast's eagles are gone now, the former because of relentless hunting and the latter because its main prey base was driven to extinction. An aerial predator that can swoop down and carry off a small human is a staple of certain myths, fairy tales, and speculative fiction. But such a creature wasn't mythical; it was real. Knowing that a bird twice the size of a bald eagle once existed is an intriguing thought. But the idea that one might swoop down and carry away your walking partner goes one step beyond what feels comfortable.

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