By Nic Rawlence 31/05/2021

The eastern moa is stuck fast in the swamp, its thick legs having punched through the peat into the liquid blue clay beneath. Death is inevitable, whether from starvation or from above.

Unable to move, the moa can only eat what it can reach around it, if anything. The forests that covered this area during warmer times are but a dim memory in the recesses of time. Instead, the swamp is surrounded by tussock grass and celery pine. Occasionally the moa tries to escape in vain from the swamp’s tight grasp, bumping against the bones of its brethren preserved in this death trap.

Suddenly, something slams into the back of the moa, pushing it further into the swamp. Large talons rip through flesh and bone. The moa’s arch-nemesis, the King of the Eagles, has just arrived for dinner.

14,000 years later, in the middle of a bone-dry North Canterbury paddock, I’m excavating the Glencrieff moa swamp as part of my PhD research. Removing a large eastern moa pelvis from its Ice Age tomb, I turn it over in my muddy hands to reveal four deep gouge marks left by that moa killing machine, Haast’s eagle.

Imagining this potential, and let’s face it, one-sided ‘battle’ between two avian giants in Aotearoa New Zealand is a glimpse into a prehistoric world, one ruled by titans. Why did this isolated archipelago in the South Pacific have so many chonky birds? This menagerie included, among others, key players like Eyles’ Harrier (the largest harrier in the world), adzebill, takahē, giant goose, and Poūwa, oh my.

One of the keys to why New Zealand has so many feathered giants is our dynamic geological history. In the break-up of a long-term relationship, Aotearoa had started separating from the supercontinent Gondwana around 80 million years ago, before many of the flying ancestors of our unique birds arrived on our shores.

It’s not all about moa: Chonky birds came in all shapes and sizes like Cnemiornis goose. Artwork by Paul Martinson © Te Papa CC BY-NC-ND 4.0.

Lithornids, a worldwide group of extinct birds similar to modern-day tinamous, arrived in New Zealand 58 million years ago and evolved into moa, and again only eight million years later, this time evolving into our national bird, the kiwi. Another widespread group, the tiny flufftails (yes, they are a thing) arrived here 40 million years ago and evolved into the enigmatic predatory adzebill (incidentally, flufftails also got to Haiti and evolved into the now extinct Haitian cave rail). The tūpuna of our giant goose, the Cape Barren goose, possibly arrived here at least 19 million years ago (though this is still up for debate given the fragmentary nature of fossil remains). The rest of our chonky feathered menagerie seems to have joined the party only recently around the Plio-Pleistocene boundary 2.5 million years ago in what I have affectingly coined The Great Australianisation of New Zealand…but more on that later.

But landing here only gets you so far. In Survivor Aotearoa, filling an available niche or job vacancy is key, otherwise, you will be voted off the island. The lithornids that gave rise to moa filled a niche for a large ground-dwelling herbivore, previously occupied by dinosaurs (this isn’t far from the truth…upon seeing a cassowary in Melbourne Zoo my then three-year-old said “that’s not a bird Daddy, that’s a dinosaur”). In a case of founder takes all or first-in, first-served, when the ancestors of kiwi arrived they had to fill the niche of a ‘small’ nocturnal bird. The same pattern occurred in South America with the lithornid ancestors of the large flightless rhea and the small tinamous.

The Great Australianisation of New Zealand beautify illustrates Survivor Aotearoa. For millions of years, the landscape was dominated by a deep, dark forest. It wasn’t until the Pleistocene Ice Age started around 2.5 million years ago that forest retreated into refugia and the landscape looked a lot like the sunburnt country of Australia – vast and open. With the sudden availability of open habitat niches, a whole suite of applicants crossed the ditch, applied, and got the job as the progenitors of some of our most iconic birds. The little or booted eagle (the smallest eagle in the world), spotted harrier, purple swamphen, and the black swan, rapidly evolved into the chonky Haast’s eagle, Eyles’ harrier, takahē, and the All Black of the bird world, the Poūwa.

Arriving on a strange island, what made these feathered freshmen decide to embrace the chonk? Unlike other parts of the world, New Zealand lacked terrestrial mammals, the only exceptions being bats and a diminutive waddling mouse…that we know of. In a case of use it or lose it, many of our avian tūpuna become flightless or flight-reduced (like the Poūwa), and bigger than their forebears. Flying is expensive, so why bother if you can eat your breakfast in relative peace, without being harassed or eaten by furry predators. In a land where the top predators were birds who hunted by sight, echoing a scene from Jurassic Park, it paid to keep absolutely still and not move (aka the freezing behaviour of kākāpō).

One of the necessities of life, food, also played an important role in obtaining large size. Over evolutionary time, eating unpalatable herbivorous food often results in animals becoming chonky. Think of moa, takahē and kākāpō. In the case of Haast’s eagle and Eyles’ harrier, their ancestors filled the niche of top avian predators. Many of our birds were already relative giants – by 16-19 million years ago moa, adzebill, and goose were large. To hunt in a land of giants, these raptors had to be pimped up, and quickly, giving Popeye a run for his money. The extremely large size of female giant moa compared to males (termed reverse sexual dimorphism) is thought to be due to a competition between females to give their offspring the best possible start in life.

Super chonk vs Superman: Our feathered giants have been the inspiration for some of Superman’s greatest villains.

Being big wasn’t all it was cracked up to be though. Many of our feathered giants were K-selected (i.e. slow breeding) compared to r-selected boom-bust species (think of the current mice plagues ravaging rural Australia). This made our chonky birds very prone to extinction when Polynesians arrived in Aotearoa. Take away a bird’s food source (in the case of our large raptors) or decimate their numbers below replacement level, and extinction was all but guaranteed.

Today, only bones remain of the majority of our giant birds for scientists to piece together their long-forgotten tales. Takahē and kākāpō are refugee species that have survived in relict populations, saved no doubt due to their Eyrie in the Murchison Mountains, and nocturnal behaviour, respectively. I for one think Aotearoa is much poorer without our lost feathered giants. To come across moa in the forest and be honked at by angry Poūwa, to seeing Haast’s eagle soar high in the sky above the mountains would certainly bring an exciting, new element to the great outdoors. Who knows, in another place, another time, maybe insurance companies would offer moa and eagle insurance. You were giants among birds. Lost but not forgotten.