‘Schrödinger’s penguin’: Why one of New Zealand’s least-studied penguin species is disappearing from one island but stable on another just 130 miles away

The Erect-crested penguin, one of New Zealand’s least-studied species, is behaving completely differently on two different islands. A new University of Otago study has found that one population has remained stable for at least 25 years, while another has lost more than 40% of its birds over the past three decades.

The research, published in the journal PLOS ONE on October 21, found that the penguins breeding on the Bounty Islands have remained largely unchanged since the mid-1990s. However, their counterparts on the Antipodes Islands have seen a steep decline over the same period.

The study was led by Thomas Mattern of the Tawaki Project, with collaboration from researchers including the University of Otago’s Departments of Zoology and Marine Science, the Antarctic Research Trust and the Global Penguin Society.

Erect-crested penguins' population increased and decreased in two New Zealand Islands 130 miles apart

Erect-crested penguins breed only on the Bounty and Antipodes Islands, two remote islands separated by 130 miles in the Pacific about 500 miles southeast of New Zealand’s South Island, near the International Date Line.

According to the University of Otago, this finding comes after the first survey that was done between 2022 and 2024 with drones to count every breeding colony of the entire species.

“The results overturn a long-standing assumption,” Mattern said in a statement.

“The tiny Bounty Islands, at just 50 hectares, were found to hold almost as many breeding pairs (about 27,800) as the Antipodes Islands (about 29,800), even though the latter is 40 times larger and historically considered the species’ stronghold,” he added in the striking discovery.

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But the two penguin populations have been moving in opposite directions. “Comparing the new counts with historical records going back to the mid-1990s, the Bounty Islands population has remained essentially unchanged for at least 25 years.

“The Antipodes Islands population, by contrast, has declined by more than 40 per cent since 1995.”

Antipodean penguins are declining, but why

As per Mattern, several factors, including predators, weather conditions, and migration routes, could be the reason why Antipodean penguins are declining.

“As Antipodes colonies have shrunk over recent decades, the collective defence they once provided against native subantarctic bird, the skua, a natural predator of eggs and chicks, has weakened, allowing predation losses to increase and colonies to fragment further,” he said.

Co-author Klemens Pütz of the Antarctic Research Trust explained that skuas might have become aggressive toward Erect-crested penguins after the number of Eastern Rockhopper penguins dropped, one of the smallest species of penguins that the predatory bird preyed on mostly.

As per the researchers, Rockhopper populations crashed “catastrophically” during the second half of the 20th century. The trend was seen across the species’ Southern Ocean breeding range, likely due to a warm ocean, which eventually left fewer than 1,000 breeding pairs on the Antipodes Islands.

“As Rockhoppers disappeared, skuas may have turned to Erect-crested penguin eggs and chicks instead,” Dr Pütz said.

Severe storms might have played another major role in the decline of the penguin population. Researchers said the storms, which are consistent with the Southern Ocean’s climate trends, caused landslides that disrupted breeding colonies and left them more vulnerable.

Differences in the penguins’ feeding grounds and migration routes may also help explain why the two populations are faring differently. According to Mattern, Antipodean penguins, while not in the breeding season, travel far into the Antarctic Polar Frontal Zone.

Bounty Islands penguins, by contrast, prefer staying closer to their breeding grounds, in more sheltered waters that may offer better food supplies.

Researchers call it ‘Schrödinger’s penguin’

Researchers have dubbed the contrasting fortunes of the two populations as “Schrödinger's Penguin”, referring to the famous thought experiment of Erwin Schrödinger, the Austrian theoretical physicist, in which a cat sealed inside a box is considered both alive and dead until someone opens the box to check.

As per the lead author, the penguin populations are in the opposite situation. “With Schrödinger's cat, opening the box settles the question one way or the other.

“With Erect-crested penguins, it’s the opposite. From a distance, without proper surveys, everyone assumed there was a single answer for the whole species,” he said.

“Once we actually opened the metaphorical box and looked, we found there wasn’t one state at all, but two: stable on the Bounty Islands, declining on the Antipodes Islands.”

The conservation status of Erect-crested penguins might be misleading

On the International Union for Conservation of Nature (IUCN) Red List, erect-crested penguins are among just four penguin species worldwide classified as “Endangered.” It includes another species from New Zealand, the hoiho, or yellow-eyed penguin, which also carries the same classification.

Mattern said the species’ current threat classification is based on inflated population estimates from the 1970s. The new survey suggests the Endangered classification remains appropriate for the Antipodes population, but not for the Bounty Islands population, where the birds have actually maintained their population for 25 years.

“Averaged across both populations, the global ranking would actually improve to ‘Vulnerable’, masking the severity of the Antipodes decline, and downplaying how exposed the Bounty Islands, likely the species’ last secure stronghold, are to industrial fishing,” the researcher said.

“A single, ostensibly improved threat ranking is like putting the lid back on Schrödinger’s box, trading a convenient illusion for the stronger protection the Bounty Islands population actually needs,” Mattern concluded.