Tildes Talks - Rails, Giants, and Ghosts: Evolution on our Isolated Island

Aotearoa has been a land apart for millions of years. About 80 million years ago, the continent of  Zealandia, also known as Te Riu-a-Māui, split off from the supercontinent Gondwana and mostly sank, leaving only its highest peaks above water; these now form the islands of Aotearoa and New Caledonia. 

Isolated islands, as a rule, produce some extremely strange species. When a fragment of land breaks off, the species on the new island evolve in isolation. This is also the case for species that subsequently manage to reach it. The original few species and those that later arrive evolve to fill all available niches in the new environment. 

There are two trends in island evolution. The first trend is island dwarfism, where large species marooned on islands become small due to a lack of food, such as ancient pygmy elephants on the islands of the Mediterranean. The second trend is island gigantism, where small species grow larger to fill the niche left open by a lack of large animals.  

In Aotearoa, there are striking examples of island gigantism — take the moa, which evolved to fill a similar ecological role to large browsing herbivores like cattle or horses, or the Haast eagle, the largest eagle known to have ever lived, which ruled as the apex predator.

One particular group of birds that illustrates the trends in island evolution perfectly is the rail. Typically, rails are terrestrial or wetland wading birds that feed on vegetation and insects. Their evolutionary journey in Aotearoa is particularly rich and fascinating.

In 1844, Paleontologist Richard Owen was presented with a 16-million-year-old leg bone from a fossil bed in Otago. He presumed that it was a moa, but it was later discovered that it was, in fact, an 18-kilo predatory bird that hunted smaller birds and large reptiles like the tuatara. Its beak was large and hooked like an axe, earning it the name adzebill. 

The adzebill is closely related to the living kagu in New Caledonia, so they both likely evolved from a species living on the continent of Zealandia before it sank, separating them. They belong to a particularly ancient branch of the rail family.

Other rail species arrived in Aotearoa after it had become isolated. The North Island takahē, now extinct for at least 200 years, was found through genetic analysis to be closely related to the Australian swamp hen. One of these swamp hens was likely blown across the Tasman during a storm and, once here, gradually evolved into a large, flightless, browsing bird.

For many years, the South Island takahē was also thought to be extinct—until a surviving population was rediscovered in the mountains in 1948. It was initially assumed that the North and South Island takahē were closely related. However, genetic analysis later revealed that while the North Island population descended from birds that arrived from Australia, the South Island takahē is more closely related to swamp hens from South Africa. This indicates that there were multiple, separate arrivals in Aotearoa, each leading to remarkably similar evolutionary outcomes, where birds became large and flightless, even in the absence of predators.

Takahē share a noticeable resemblance to pūkeko, which are found in Aotearoa, Australia, and across Pacific islands. Both birds have striking blue-green plumage and bright red legs, but pūkeko are smaller and still capable of flight. Fossil evidence suggests pūkeko have only been in Aotearoa for about 400 years, making them relatively recent arrivals—possibly even brought over on Māori waka. The physical differences between the two birds indicate that pūkeko haven’t been here long enough to evolve into large, flightless birds like the takahē. It’s fascinating to imagine how they might evolve over millions of years, although with the presence of mammalian predators today, they may need to retain their ability to fly in order to survive.

A similar pattern can also be observed in the large, flightless weka and the smaller flighted banded rail. 

The most recent arrival in the family is the Australasian coot, one of our newest native birds. The first breeding pair was seen in Lake Hayes, Otago, in 1958. There are 9 records of Australian coots between 1875 and their first breeding in 1958, showing just how common it is for these species to reach Aotearoa. 

From towering moa to tiny coots, the story of rails in Aotearoa is a vivid illustration of evolution in action, shaped by isolation, adaptation, and chance.