The now-extinct Haast’s eagle of New Zealand was, at up to 18 kilograms, the largest eagle that ever lived – twice the size of any living eagle.
Unlike its modern relatives, Haast's eagle specialised in routinely killing prey far bigger than itself, including the also-extinct gigantic flightless moa, which could weigh more than 200 kg.
Maori drove the moa to extinction in New Zealand about 500 years ago, and Haast's eagle disappeared soon after, presumably because its main food source had disappeared. But without moa, the eagle may have turned to other food sources before it vanished into the fossil record: Maori legend records a giant eagle that was a child-killer.
Ambitious killing habits aside, Haast’s eagle may have differed from living eagles in another curious respect. Some previous researchers have suggested that the giant raptor’s skull was more like that of a scavenging vulture’s than that of an eagle.
Professor Stephen Wroe, who leads the University of New England’s Function, Evolution and Anatomy Research (FEAR) Lab, have sought to determine whether this was so, and if so, why, working with other researchers to build 3D computer models of the extinct eagle's skull and talons.
The models were used in biomechanical simulations that compared the extinct eagle's killing and feeding capabilities with those of modern eagles and vultures.
Prof. Wroe said that simulations of the talons left no doubt "that they were capable of delivering an awesome, bone-cracking grip", a conclusion supported by the presence of moa bones that appear to have been punctured by the eagle’s talons.
However, analysis of the Haast's eagle's skull showed that it was indeed more like that of a vulture than an eagle, well adapted to the leisurely evisceration of a moa, with a vulture-like beak and head that enabled it to rip and pull flesh from deep inside a carcase.
"This is how vultures feed when wresting high nutrient value soft internal organs, like heart, kidneys and liver, from large dead animals."
Image: A 3D model of a Haast's eagle skull reconstructed and built from CT scans. This model shows strain distributions in the skull under a simulated pulling behaviour. Relatively low strains in the giant eagle's skull were more similar to those of large scavenging birds than to those of the eagles and hawks included in the analysis. This suggests that it fed more like a vulture, poking its head deep into large carcasses to extract nutrient rich soft tissues. Hotter colours indicate regions of higher strain.
So why this chimera-like combination of features? Large living eagles, such as the Harpy, Golden, or Wedge-tail, are formidable predators, Prof. Wroe observes, but they only rarely kill prey bigger than themselves. "It’s too risky. Their relatively fragile bones can damage easily and a broken wing is a death sentence."
"Haast’s eagle was different. It routinely took down much larger animals, up to ten times as big as itself. The battle between a 200 kg moa and an 18 kg Haast’s eagle would have been a titanic struggle, and quite a gamble for the predator, which would have risked serious injury in such encounters.”
But it was a gamble with a hefty payoff that explains the vulture-like characteristics of Haast's eagle.
New Zealand had no terrestrial predators, in fact no competition at all. So, once it had killed a moa, the giant eagle had days worth of food, for itself, its mate and its chicks, and all the time it needed to eat it.
“For any living eagle species, it’s generally pointless to take prey larger than it can carry to a perch. Large terrestrial predators, big cats, wolves or hyaenas, will steal it. Why take the risk?”
Based on their findings, the researchers suggest that although Haast’s eagle was undoubtedly king of the skies, any claims to a proportionately regal appearance may have been compromised by its feeding habits.
“Our findings show that its head was in many ways shaped like a vulture’s, and the mechanics say it fed like one. I suspect that like most vultures it was bald, and its receding ‘feather-line’ avoided the accumulation of gore as it pushed its head deep into the body cavities of dead moa.”
The findings are reported in the Proceedings of the Royal Society, Series B.
Image: Approximate size of Haast's eagle compared to an adult human.
