There is evidence that eagles are part of a wave of feather invaders.
Above: Due to the lack of other large predators, the now extinct Haast Giant Eagle hunts moa birds in New Zealand. Today, scientists are studying the ancient history of island birds to better understand how the biological invasion of "nature" occurred.
New Zealand has long been known for its bird habitat. Before the arrival of humans 700 years ago, this archipelago had a unique ecosystem with few mammals. More than 200 species of birds support a food web of their own. Not a cow or antelope, but a flightless bird known as the "moa (MOA)". New Zealand also does not have top predators like tigers, replaced by "Haast's eagles".
The eagle has been a magnet for researchers ever since a group of farm workers drained the swamp in the late 1860s and found its buried bones. Explorer and geologist Julius Haast, who first published an annotation on the species, described it as a "giant bird of prey." Today, biologists estimate that eagles can weigh up to 33 pounds (15 kilograms), about 50 percent heavier than any bird of prey known today. But with a wingspan of only 2 to 3 meters (just beyond the range of the vulture), it's a bird of odd proportions.
The shape of the Haast giant eagle is one of the many challenges scientists face when studying this long-extinct species, leaving only a few bones and scattered fragments. For nearly a century, people have debated whether such a large bird can fly, and even after this dispute was settled, there are still doubts about whether the bird has the ability to kill moa, and in some cases, moa are more than 15 times larger than the eagle itself. Now, new science and technology, combined with a clearer understanding of New Zealand's geological history, have placed the Haast giant eagle in a larger ecological discussion: how species "invaded" the new territory.
Scientists now believe that this top bird was one of a wave of feathered invaders who conquered New Zealand in a relatively short period of time. But that's not the only wave of invasions. Michael Knapp, a biologist who studies eagles at the University of Otago, says the Haast giant eagle , though it's gone for centuries , has revealed that we live in a more connected world than we once thought. If these seemingly isolated islands repeatedly attract so many alien species, he said, then "natural invasion" must be a major force in ecosystems around the world.
Dig for answers
New Zealand has always occupied an important place in scientists' understanding of species extinctions. When Western scientists first encountered moa birds, the idea that the species might go extinct only came up for decades. Their skeletons soon became a popular commodity. Paul Scofield, a paleontologist and senior curator at the Canterbury Museum in Christchurch, said: "It is what makes our museum possible. Haast founded the museum himself and collected the original collection by exchanging moa fossils for various other archaeological and zoological treasures.
Above: A kiwi that can't fly.
New Zealand has preserved some unusual species, including the famous flightless kiwi. Combining these existing oddities, the moa fossils helped establish the idea that New Zealand was a lost world where ancient creatures, sheltered from the rest of the world, managed to survive mass extinction events. Later geologists confirmed that the rocky islands were once part of what they called the "Gondwana" supercontinent, but separated about 80 million years ago. In 1990, a television series described New Zealand's islands as "Moa's Ark", promoting the catchy name of this time-honored model that describes how New Zealand's bird-filled ecosystem came into being.
By the late 1990s, however, scientists realized that during the Oligocene period, some 25 million years ago, geological and climatic changes could have submerged the whole of New Zealand. Such a flood would wipe out most, if not all, of the island's species. The theory came to be known as "Oligocene drowning," but was opposed by some scientists, sparking a heated debate about land cover.
Fortunately, the advent of new technologies answers this question. Scientists began extracting DNA from fossils and sequencing them. This means that researchers can compare ancient DNA with modern genomes and create genealogies of evolutionary relationships between extant and extinct species. Such "phylogeny" can roughly determine when two species were separated from their common ancestor, and the data helps to resolve the debate surrounding New Zealand's geological history.
In 2005, a team of scientists published a paper comparing DNA sequences extracted from two Haast eagle fossils with the genomes of 16 modern eagles. The scientists confirmed that, as expected, the closest surviving relatives of this large extinct bird include the Australian species. Genomic data suggest that genealogies have split over the past few million years. Subsequent analyses suggest that this difference occurred about 2.2 million years ago.
Thus, the Oligocene drowning hypothesis is one point: eagles appear to arrive after the proposed drowning time. But later analysis of several other species in New Zealand revealed that they had diverged for about tens of millions of years. Some species still exist during the Oligocene.
By 2014, the geological evidence had convinced most scientists: Yes, much of New Zealand had been submerged, but there was still a small patch of land (about 20%) exposed to the water. Although some species on the island can be traced back to Gondwana, many other species, including the Haast giant eagle, are newcomers.
Above: Australia's largest surviving bird of prey, the wedge-tailed eagle.
But genetic analysis reveals a new mystery — one scientists don't even think to think about. Scientists often compare the extinct bird to Australia's largest surviving bird of prey, the wedge-tailed eagle. It is clear that it is a close relative of the eagle. Instead, these genes are more closely linked to boot falcons and hawks, both of which weigh around 2 pounds. (The eaglet, as the name suggests, is one of the smallest eagle species in existence.) ）
The discovery suggests that the Haast giant eagle has made a huge leap in size over its nearest relatives: it has grown 15-fold in 2 million years. Biologist Michael Knapp noted in a recent paper that this is an "astonishing" rate of change. We observed that dogs change in size very quickly, but this is a process driven by human selection. To his knowledge, Knapp said, there are no other examples of natural selection leading to such significant growth in such a short period of time. It is possible that all three eagles have an as-yet-unknown ancestral species whose body size is somewhere in the middle, and whose descendants may evolve in different directions. But Knapp thinks that's unlikely. The first wayward eagles set off from Australia and were blown across the Tasman Sea on a day or two journey, landing in favorable conditions for growing birds.
Biologist Michael Knapp points out that other birds of prey, possibly owls or falcons, may feed on the island's smallest birds. But there are plenty of moa running around, ranging in size from turkeys to ostriches — they're too big to be killed by most birds of prey. "There's a lot of meat that's not being eaten," Mr. Nap said. For the largest eagles, such scenes are quickly chosen because they are the easiest to eat these prey.
Above: Moa restoration.
Knapp is now turning to the smallest scale of the mystery: by comparing the genomes of different species of eagles, he wants to pinpoint exactly which genes have changed, thus facilitating the rapid growth of the Haast giant eagle. "Finding out how it works at the molecular level is the real next step," he said. ”
Another bird
However, eagle genes have helped us deepen our understanding of New Zealand's ecological history. The second extinct New Zealand bird, known as Eyles' Harrier, is the largest harrier known in history. But the bird is reminiscent of the Haast eagle than just its sheer size: the harrier also seems to have evolved from a smaller bird. Knapp and his colleagues reported in 2019 that relatives of falcons closest to Ayers who are currently alive are nearly five times smaller than they are. The two harriers appear to have separated from a common ancestor about 2.4 million years ago — relatively close to the time of differentiation of the Haast eagle.
When Knapp was ready to speak at a conference on the work, he became interested in the time shared. So he looked for other examples of similar divergent times. "I've found a lot of people like that," he said. When he began to consider what he called this "suspicious gathering," his colleague Paul Schofield pointed to another commonality: Recent migratory birds are all open-habitat species.
The two scientists and other colleagues came up with a hypothesis, which was published in the journal Frontiers in Ecology and Evolution in 2019: As geology and climate change reshape the island's habitat, a wave of bird invaders arrived in New Zealand in a relatively short period of time.
About 10 million years ago, Australia began to become arid. Meanwhile, New Zealand remained heavily forested – at least until at least 2.5 million years ago, when the Ice Age began. Subsequently, large swaths of the island cooled, causing glaciers to blossom at the top of New Zealand's mountain peaks and some forests to die. Suddenly, vast meadows appeared on these islands: a whole new habitat.
In a paper published in the 2021 Annual Review of Ecology, Evolution and Systematics, Knapp cites the Haast Giant Eagle and Ayers' Falcon as case studies. These, he argues, are examples of "natural invasions." Australian species often cross the Tasman Sea, but they often struggle to compete with existing species on the island. But when the cool temperatures killed some of the forest, the newcomers discovered a familiar niche — a niche that a New Zealand species hadn't yet evolved to fill.
This process is completely natural, but it has implications for conservation. Humans are known to carry species across the globe – directly leading to biological invasions. But the way the Haast Giant Eagle and Ayles's Falcon appeared suggests that humans can play a more subtle role: We can also change habitats. We open and close the niche. By doing so, we can indirectly attract species into new territories.
Since the first humans arrived, much of New Zealand's forests have been burned down, creating more open habitats. After humans arrived in New Zealand (including the Haast Giant Eagle and the Ayers Falcon), about half of the island birds were wiped out. Now, history is repeating itself. Over the centuries, New Zealand has become home to Australian egrets, white-faced herons and swallows.
Above: Haast Giant Eagle, Ayers Falcon and another bird of prey compared to humans.
Biologist Michael Knapp said: "The same thing we saw 2.5 million years ago is happening again now. ”
One thing about the Haast Eagle has never been mysterious is what species wiped it out. Maybe these eagles were hunted. Of course, moa are like this, which will be enough to make the predator perish. "If you're evolving to fit into a specific, very rare segment, you're going to have a tough time when that segment disappears," Knapp said. In either way, humanity is responsible.
So while you can take the recent wave of Australian migration as a reminder that ecosystems are adapting (life continues as new species fill the gap), the story is also a cautionary tale. Evolutionary history is full of strange twists and turns, but there are also dead ends.
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