2 June 2012

Death of the giant eagle

700 years ago the largest bird of prey and one of the largest aerial predators that has ever lived hunted in the skies over New Zealand's South Island - Harpagornis moorei, more commonly known as Haast's Eagle. The bird, weighing up to 15kg (which is very heavy for a bird as they have hollow bones), had a wingspan of 3m and was the top predator in its ecosystem - hunting the moa (Dinornis novaezealandiae), a 12 foot high flightless bird that weighed in at almost 230kg!

Haast's Eagle was a specialist predator and almost exclusively hunted moa, soaring at high altitude before dropping out of the sky at speeds that are estimated to be as fast as 50mph, striking the flightless bird with enormous force! Wounds on the bones of moa suggest that once the eagle was within striking distance of the hapless bird, it grabbed its prey's pelvis with one talon and the crushed the back of its neck with the other. It is believed that the eagle then landed on top of the moa and, if it was still alive, quickly finished it off using its very large and razor sharp beak. Unlike many modern-day predators that have to compete with scavengers for their kill, the isolated island habitat of New Zealand did not have such animals, which enabled H. moorei to have consumed all of its kill by itself, returning to the carcass for up to a week after it was killed! The fact that the eagle could utilise the vast majority of its kill is believed to be one of the reasons that the species evolved to be so large.

An artists impression of Haast's Eagle hunting moa. Despite the eagle's very large size, which is pushing the boundaries of body mass for powered flight, it had a very short wing span. This is believed to be an adaptation for hunting over the scrubland and forests of New Zealand because it allowed them to hunt in dense vegetation.

The other reason that the eagle grew to such an impressive size is believed to be due to a phenomenon called 'island gigantism', which is likely the main driving force behind the evolution of its growth. Island gigantism is a relatively common biological trait where species that live on isolated islands with no contact to the mainlands grow to be unusually large. This may seem strange, but makes sense biologically as these isolated islands often develop their own unique ecosystems due to the fact that the more abundant species that live on mainlands cannot get across to them. As a consequence of this, there are relatively few species inhabiting the island so that such animals are under little competition for resources. Thus, animals on isolated islands are able to fuel large body growth and often evolve to be unusually large. The isolated nature of such islands also means that many of the species that live there have evolved independently from those on the continent so that many of the organisms found on cut-off islands are unique, being found nowhere else. A good example of such novel species are the strange species of marsupials that are found in Australia, which broke away from Africa 184 million years ago. These marsupials have therefore, evolved independently from continental species for a very long period of time.

Growing to lengths of 3 metres the Komodo dragon, Varanus komodensis, is the world's largest species of lizard and the world's biggest poisonous animal. The lizard is found spread across certain islands in Indonesia and is an excellent example of island gigantism, with its large size being attributed to a lack of competition over prey as there are no other species that fill its niche on the islands.

Bizarrely, phylogenetic analysis of the DNA of H. moorei has found that the eagle is not related to other large species of predatory eagle as you might expect, but is instead most closely related to the Little Eagle, Hieraaetus morphnoides. This rather small bird of prey is about the same size as a Peregrine Falcon, weighing a mere 815g! Although it is slightly odd that the ancestor of the Little Eagle remained at such a diminutive size while one of its cousins became one of the most massive aerial predators ever, it in fact supports the idea that island gigantism fuelled the evolution of the colossal size of H. moorei: a small number of the ancestors of the two eagles were trapped in New Zealand after it separated from Antarctica between 130 and 85 million years ago, whilst others remained over the larger continent. Those over the continent had greater competition for resources so could not fuel the growth needed to reach such huge sizes and consequently, evolved into the Little Eagle. Those trapped in New Zealand however, had an abundance of food and evolved into the giant Haast's Eagle.

This shows the foot of H. moorei (left) compared with that of a Little Eagle (right). It has been calculated that the massive eagle's talons could have pierced and crushed bone up to 6mm thick under 50mm of skin and flesh!

Although the large size of the eagle is very impressive it also, rather unfortunately, led to its downfall. Unsurprisingly its extinction was due to the arrival of man to New Zealand, as one of the main characteristics of human invasion into a new environment is the extinction of its endemic megafauna - such large animals provide excellent sources of food and are typically very vulnerable to the alterations that humans make to their habitat. In this case however, humans did not hunt and kill Haast's Eagle directly; instead killing it by wiping out the moa, leaving the eagle with nothing to eat. Obviously, a 12 foot flightless bird was easy pickings for the early Maori settlers (who came from Hawaii) and they exploited the bird, hunting it mercilessly. This over-hunting would have wiped the moa out eventually, but the problem was made even worse because moa eggs were also considered as a delicacy and were raided from the birds nests. This was a huge factor in the moa's rapid extinction because they only laid a few eggs every year; meaning that there were nowhere near enough young moa to replace the adults that were being killed by humans for meat!

Once the moa became extinct it was only a matter of time before Haast's Eagle followed it into the abyss,  mainly because it had evolved to a specialist predator that hunted moa almost exclusively. However even if H. moorei knew how to have hunted the other animals resident to New Zealand, none of them were large enough to have fed it for long so the birds would have eventually starved to death anyway. Although such an end to a species is not uncommon and has happened many times in the past, it is still a sad and rather undignified end for such a majestic species and Haast's Eagles have not soared above the far-flung islands of New Zealand for over 600 years...

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