Scientists Counted Penguins on 2 Remote Islands—Then Discovered a Mysterious 40% Decline

Two groups of endangered penguins live on remote islands in the Southern Ocean, separated by only a stretch of open water. For years, researchers had reason to think the larger island group was the species' main stronghold. But when scientists returned with drones and systematically counted the breeding colonies, the picture changed. One small archipelago held far more breeding penguins than its size suggested. And a comparison with older records revealed something even more important. The birds living on these neighboring island groups were not sharing the same fate. The bird at the center of the discovery is the erect crested penguin, a species whose scientific name is Eudyptes sclateri. It is one of four living penguin species listed as endangered on the International Union for Conservation of Nature's Red List, making an accurate understanding of its population especially important. Unlike penguin species spread across multiple regions, erect crested penguins breed in just two isolated groups of sub-Antarctic islands, the Bounty Islands and the Antipodes Islands. Both lie in the Southern Ocean, approximately 800 kilometers southeast of the South Island of New Zealand. About 200 kilometers of open ocean separates one archipelago from the other. that distance may seem small on a map of the Southern Ocean but the islands differ greatly in their physical size and in the environments their penguins encounter those differences would become central to understanding what was happening to the species for a long time the Antipodes Islands were regarded as the penguins principal stronghold they cover roughly forty times the area of the Bounty Islands and an estimate from 1995 put their population at 52,081 breeding pairs By comparison, the Bounty Islands occupy just 50 hectares. The relative size of the two island groups helped shape expectations about where the greatest numbers of breeding birds would be found. But those expectations rested partly on historical estimates. For a species breeding so far from major population centers, counting all its colonies is not a simple undertaking. An accurate species-wide picture required researchers to examine both island groups rather than treat the larger one as automatically more important. Scientists from the University of Otago in New Zealand worked with the Antarctic Research Trust and the Global Penguin Society to carry out that assessment. Between 2022 and 2024, they used drones to photograph and count every breeding colony across both archipelagos. Their findings were published in the scientific journal PLOS ONE. The approach produced a new basis for comparing the two populations. Instead of relying on the assumption that the larger island group must support a much larger breeding population, the researchers could examine colony counts across the species' entire breeding range. The first major surprise came from the Bounty Islands. despite their tiny area they held about twenty seven thousand eight hundred breeding pairs the far larger Antipodes Islands held about twenty nine thousand eight hundred pairs in other words the archipelago forty times smaller in area supported almost as many breeding pairs as the one long considered the species stronghold Thomas Mattern, an ecologist at the University of Otago and lead author of the study, described this result as overturning a long-standing assumption. The importance of the finding was not simply that the smaller islands had been underestimated. It meant that the balance between the species' two breeding populations was very different from what their relative size might suggest, yet the numbers from a single survey could only tell part of the story. The researchers also compared their recent findings with historical records reaching back to the middle of the 1990s. That comparison exposed a much deeper contrast. On the Bounty Islands, the breeding population appeared to have remained stable, with little overall change over at least 25 years. The Antipodes population, however, had lost more than 40 percent of its individuals over roughly three decades. The two island groups were home to the same endangered species, but their population trends were moving in sharply different directions. That distinction changed the conservation question. It was no longer enough to ask how many erect crested penguins remained across their entire breeding range. Researchers also needed to ask why one population appeared stable while the other had declined so severely. The study identified several pressures that could help explain the difficulties facing the Antipodes birds. One involves danger on land, especially from skuas. Skuas are native sub-Antarctic birds that prey on penguin eggs and chicks. Their presence is not new, but the consequences of their predation can change as penguin colonies become smaller. In a large colony, breeding penguins can benefit from collective defense. As a colony shrinks, that shared protection weakens. Mattern explained that declining colony sizes on the Antipodes may have made eggs and chicks more vulnerable to skuas. Additional, losses can then leave colonies even smaller and more fragmented, continuing the problem. The effect can become particularly severe in the smallest breeding groups. according to the researchers colonies containing fewer than ten breeding pairs can experience almost complete chick mortality from skua predation such losses matter because a colony may still contain breeding adults while producing very few surviving young the team also raised another possible change in the relationship between skuas and penguins on the Antipodes the eastern rockhopper penguin another species breeding in the archipelago underwent a dramatic decline during the second half of the twentieth century fewer than one thousand breeding pairs of eastern rockhoppers now remain there Clemens Putz of the Antarctic Research Trust, a co-author of the study, suggested that as rockhoppers disappeared, skuas may have begun targeting erect crested penguin eggs and chicks more often. This was presented as a possibility, not a proven single cause. But it illustrates how the decline of one bird population could potentially increase pressure on another. Predation is only part of the challenge. The Antipodes Islands have also faced increasingly severe storms, sometimes followed by landslides. These events can permanently change the land itself, breaking up breeding areas and leaving penguin groups more exposed. A colony that has already shrunk may be especially vulnerable when the physical spaces it uses are altered. The differences between the two island groups extend beyond the breeding grounds. Once the breeding season ends, the penguins do not all travel through the Southern Ocean in the same way. Their movements place them in different marine environments, with different implications for access to food. Birds from the Bounty Islands tend to remain relatively close to their breeding grounds. On average, they travel about 300 kilometers away after breeding. Their islands sit next to the subtropical front, an oceanic zone rich in feeding opportunities. This nearby resource helps explain why the bountiful birds do not need to make the much longer journeys recorded for the other population. Antipodes penguins follow a contrasting route. After breeding, they travel more than 1,000 kilometers south, reaching cold waters near Antarctica. That takes them far from their colonies and into ocean areas increasingly disrupted by marine heat waves across the South Pacific. These marine heat waves can affect food supplies, potentially contributing to lower survival rates among the Antipodes penguins. The research does not establish a single, simple explanation for the population decline. Instead, it points to several pressures acting across different parts of the birds' lives, from nests threatened by predators to distant feeding areas affected by changing ocean conditions. Put together, the differences help explain why geography matters so much. Two populations of the same species can breed within a few hundred kilometers of one another, yet face substantially different hazards on land and at sea. A species-level population estimate alone cannot show those differences. That led the researchers to a larger question about how conservation risk is assessed. The erect crested penguin is currently classified as endangered on the IUCN Red List. But a global classification assigns one category to the species as a whole, even when its separate populations are experiencing very different trends. The team gave this problem an unusual name, Schrödinger's penguin. It refers to the famous thought experiment involving a cat in a box, described as existing in two possible states until the box is opened and its condition is checked. The penguin comparison works in reverse, Before the comprehensive counts, treating the species as a single unit encouraged the assumption that there was one overall answer about its condition. Once the scientists looked closely at each island group, they found two different realities. The Bounty Islands population appeared stable. The Antipodes Islands population was declining. The distinction has consequences for what might happen when the species is reassessed. Matter noted that the current endangered classification, most recently assessed in 2020, is based on inflated population estimates dating back to the 1970s. Using the newer figures in a general reassessment would, according to the researchers, most likely move the species to the less severe category of vulnerable. That might sound like good news, but a single improved label could hide the seriousness of the decline on the Antipodes Islands. It could also make the relatively stable Bounty Islands population seem less important to protect, even though those small islands support nearly half of the breeding pairs counted in the new survey. The apparent contradiction is the heart of the researchers' warning. a more favorable classification for the entire species would not necessarily mean that every part of its breeding range had become safer nor would it mean that the threats driving the Antipodes decline had disappeared The drone surveys provided something a single global label could not, a clear view of how differently the two island groups are doing. One tiny archipelago has maintained its breeding population over decades. The much larger one has experienced substantial losses while facing a combination of threats that may reinforce one another. For the erect crested penguin, the new counts do more than revise the total. they show why conservation decisions need to consider populations separately especially when those populations depend on different habitats and face different pressures the same species can appear more secure in one place and increasingly vulnerable in another recognizing that split is essential to understanding what the numbers really mean