Harpagornis, the New Zealand giant eagle, has been the largest bird of prey to have graced the skies. The latest studies indicate that it hunted like an eagle but ate like a vulture
Harpagornis, Haast's extinct eagle (Hieraaetus moorei), was the largest bird of prey to ever soar through the skies. Their average body mass has been calculated at 12.3 kg for males and 17.8 kg for females. That is, it weighed 3.5 times more than a golden eagle (Aquila chrysaetos).
The distance between the ends of its wings (wing span) could reach three meters and it had huge claws, like those of a tiger, capable of piercing the bones of its prey. Such dimensions dwarf the largest living eagle, the harpy (Harpia harpyja) of the neotropical forests, which does not exceed 7.5 kilos.
It was initially thought that such a formidable bird of prey must have evolved on the South Island of New Zealand from the Australian bold eagle (Aquila audax). But analysis of the sequences of two genes (cytochrome C and ND2) in DNA recovered from subfossil remains pointed to the Australian hawk (Hieraaetus morphnoides), whose mass ranges from 0.6 to 1.3 kg, as its closest living relative. .
This implies that the Harpagornis multiplied its body size by 15 times when adapting to the new environments of the island. What could have caused such growth?
New Zealand, full of birds
With the exception of three species of bats, mammals did not colonize New Zealand, which led to the diversification of its avifauna. In it, an endemic group stands out, the moas (Dinornithiformes), with ten non-flying species that weighed between 20 and 250 kg. The moas occupied the ecological niches exploited by herbivorous mammals on the continent.
Therefore, when the ancestor of the giant eagle arrived in New Zealand, a little over a million years ago, it did not find terrestrial carnivores as competitors. This allowed it to rapidly increase in size, becoming the apex predator of ecosystems. So rapid was the growth that the brain lagged behind: a comparative study using computerized axial tomography (CAT) of the skull showed that its brain, with a volume of 19.9-20.5 ml, was half that expected in a raptor of his bearing.
How did the Harpagornis fly?
Reconstruction of its wings shows that it was comparatively short and wide, and that it had a long tail relative to other eagles. This could suggest that it hunted by maneuvering in leafy environments, like the harpy, where vision is very important.
However, the CT scan revealed that the volume of the eye sockets, the diameter of the optic nerves and the size of the lobe of vision in the brain were also 50% smaller than expected. Therefore, it thrived in environments clear of trees. This is confirmed by its neural canal, which had reduced dimensions at the cervical and thoracic level, as is typical of eagles that fly by gliding in open spaces.
The legends transmitted orally by the Maori about the eagle, which they called pouakai, corroborate this interpretation. Apparently, they only saw it when it flew, dwelling on the tops of the mountains and hunting on the plains, as a Maori told the geologist Sir James Hector in 1872.
The paintings they left inside the caves testify that they respected her and feared her for her power, considering her capable of kidnapping a human being.
Reconstruction of the extinct Haast's eagle with a bare head of feathers as in vultures; a comparison of its claw (top) with that of the Australian bold eagle (bottom) is shown. B: Talons of the giant eagle on a moa pelvis with perforations. C: drawing of the pouakai by a Maori in the cave of the Eagle, represented with a dark body and a head without feathers. Author montage with photos of Katrina Kenny, the Museum of New Zealand and Craigmore /
Was it an eagle or vulture type of raptor?
The skull of the pouakai has an intriguing feature: the opening of its nostrils is partially covered by bone tissue. This is typical of vultures that stick their heads into large carcasses to engulf the internal viscera, as it prevents foreign material from entering the respiratory tract. Eagles, on the other hand, do not need such an adaptation because they consume prey smaller than themselves, whose meat they tear.
This detail led some authors to suggest that it was a scavenger bird that fed on moas killed by other causes. However, vultures have a more developed sense of sight than eagles, as they detect carcasses from great heights.
This was not the case with the pouakai, as we have seen. Nor in terms of smell: the CT scan showed that its olfactory nerves and bulb had smaller proportions than in vultures, as happens in eagles. Likewise, the nerves of the lower extremities were highly developed, unlike in vultures, and very large insertion areas for the muscles were found in the pelvis. This indicates a great ability to hold the prey with its claws, as occurs in eagles.
The answer to the dilemma is that the pouakai hunted like an eagle and ate like a vulture, according to a study using finite element analysis and geometric morphometry of the braincase, beak and talons. Comparing it with modern birds of prey, they found that, in general terms, the beak of the pouakai was of the eagle type, while its neurocranium resembles that of the condor. Thus, it had a very high bite power, as in eagles, and its ability to dismember and tear off portions of carcasses was equally high, as in vultures.
Everything points to the fact that the pouakai could bring down large prey in relation to its own size, such as the moas, then feeding on their muscles and viscera, as the gobbling vultures do. In fact, in the rock shelter known as Eagle Cave at Craigmore there is a painting of the bird. She was represented with a dark body and a colorless head. This suggests that it was bare of feathers, a feature that it would share with the vultures that introduce their heads into the corpses.
The fate of the pouakai
The Maori arrived in New Zealand around 1280 AD, dealing a death blow to their ecosystems. By the year 1400 they had exterminated all the moas, whose population renewal was very slow, as the females reached their sexual maturity late. With the disappearance of the prey it fed on, the pouakai's fate was extinction. Although perhaps some individuals were able to survive in the subalpine environments of the South Island until the 19th century.
This article has been published in 'The Conversation'.