The world's largest eagle hunted unlike any other bird of prey

The world's largest-ever eagle acted like a vulture-raptor hybrid, taking down prey before eating its insides. 

Haast's eagle was a 15-kilogram bird of prey that lived in New Zealand until around 700 years ago and is believed to have preyed on the moa, an extinct group of birds that could measure up to four metres tall.  

A new paper suggests that the challenges of tackling large prey led to the eagles' unusual body shape, which has baffled scientists for centuries. 

Dr Joanne Cooper, who is a Senior Curator of Birds at the Museum and was not involved in the research, says, 'It's always been a puzzle with Haast's eagle as the head end looks quite vulture-like while the feet end looks very eagle-like.  

'When hunting the moa, which could weigh up hundreds of kilograms, the eagle was tackling a problem which no other raptor has had to face. It couldn't carry a moa so the eagle has vulture-like features which allow it to eat its prey immediately, while the business-like talons allow it to attack its prey in the first place.' 

The findings of the international team of researchers were published in Proceedings of the Royal Society B. 

Lord of the Long White Cloud 

Haast's eagle was unlike any bird of prey living today. With a wingspan of up to three metres, it would have been a spectacular sight on New Zealand for hundreds of thousands of years. 

Initially, its enormous size had been explained through suggestions it was closely related to the wedge-tailed eagle, the largest bird of prey currently living in Australia. Joanne says that it was 'quite a shock' when DNA evidence showed that it was instead closely related to the little eagle – one of Australia's smallest. 

'Small eagles getting blown out to New Zealand would have found a very strong niche available with large prey so abundant,' Joanne says. 'But to tackle them it had to evolve to a larger size in under two million years.  

'It's the fastest size gain we know of in a raptor and practically in any vertebrate. It's fair to call it a flying tiger, filling the big predator niche.' 

This is a process known as island gigantism, where smaller species grow bigger to take advantage of their new environment. It generally favours smaller species, with large species showing a reverse trend of miniaturisation to survive. 

Haast's eagle's large size has led to plenty of speculation over the years. Due to its relatively small wingspan when compared to its large body, some have suggested it might have been flightless, or could only fly for short periods, similar to the largest owl which ever lived, the Cuban giant owl. 

Others debated whether the birds were adapting to becoming more like a vulture to specialise in feeding on animal carcasses, and if it would even have been capable of hunting the moa. 

To try and put some of these debates to rest, researchers created 3D digital models of Haast's eagle to assess the shape and strength of its bones in relation to living vultures and eagles. 

Swoop like an eagle, eat like a vulture 

The researchers found that its beak was stronger than living vultures, and more similar to the wedge-tailed eagle. However, its skull would have been less able to cope with tearing flesh by shaking its prey, like eagles, and instead would have had to pull meat from the bone more like vultures. 

It suggests that while Haast's eagle may have attacked moa like an eagle, it would have fed on them like a vulture. In particular, it is likely to have delivered the killing blow using its beak, unlike eagles which prefer to use their claws. 

'Moa were really big but once they were knocked over, they were quite easy to subdue,' Joanne says. 'It's thought the attacks knocked them over, or that the Moa were attacked when stuck in swamps.  

'When they were immobilised the eagles focused on the back of the neck, which is a relatively weak spot and out of the range of the moa's feet.  

'Haast's eagle couldn't kill moa outright with its feet due to the size, but it could do a lot of damage and the use of the beak finishes it off.' 

While other eagles fly away with their prey, the sheer size of the moa meant that wasn't an option. Instead, it's likely that the eagles would have eaten the moa's internal organs at the site, with Maori cave art suggesting that, like vultures, Haast's eagle had a featherless head to help with this. 

The arrival of humans around 750 years ago signalled the beginning of the end for the eagle. The destruction of their forest habitat by Polynesian settlers, as well as hunting of the moa, is likely to have brought the eagle into conflict with man. 

'The eagle had the possibility to hunt people,' Joanne says. 'It's hard to imagine a bird in that role but if it could successfully hunt a 250kg moa, then 80kg humans were possibly on the menu. There is oral tradition which suggests it was the case.' 

While it is unknown when Haast's eagle finally went extinct, with some suggestions a pair were killed as late as the 1860s, its end left a unique hole in the world's biodiversity. As Joanne says, 'There's nothing really like it in the world anymore.'