Schrödinger's Penguin? Divergent population trajectories and the challenge of assessing the threat status of the Erect-crested penguin.

"Schrödinger's Cat" is a quantum mechanics thought experiment in which a cat in a sealed box is considered both alive and dead until the box is opened and its true state observed. In species threat assessments, the reverse paradox can apply. We present the case of the Erect-crested penguin (Eudyptes sclateri), one of four extant penguin species currently listed as "Endangered" on the IUCN Red List - a classification based largely on limited and inconsistent observations from the 1970s. Between 2022 and 2024, we surveyed the entire global population of Erect-crested penguins, a species confined to two subantarctic island groups in the New Zealand region: the Bounty Islands and the Antipodes Islands. Population estimates were derived from drone surveys of the entire global range, conducted primarily during the incubation stage, complemented with ground counts to establish nest-to-bird ratios and estimate breeding population size. On the small Bounty Islands (~50 ha), we found approximately 27,800 breeding pairs - nearly as many as on the Antipodes Islands (~29,800 pairs), which are roughly 40 times larger and historically considered the species' stronghold. Although data since the mid-1990s are sparse, available evidence suggests divergent population trajectories: the Bounty Islands population appears stable over at least the past 25 years, whereas the Antipodes Islands population has declined by more than 40% since the mid-1990s, a trajectory that approaches - and under precautionary assessment likely warrants - "Endangered" classification. These contrasting trajectories likely reflect fundamental differences between the two sites: on the Bounty Islands, birds have access to potentially more productive subtropical waters during the non-breeding season, while on the Antipodes Islands, Subantarctic skua predation on fragmenting colonies generates a self-reinforcing Allee effect and penguins face potentially less favourable oceanic conditions. Thus, like Schrödinger's cat, the Erect-crested penguin population occurs in two states - stable and moving towards extinction - which only became clear once the proverbial box was opened. This situation highlights the challenges of assessing extinction risk when a species comprises spatially segregated populations on distinct demographic trajectories. While the Antipodes population clearly warrants concern, the stability of the Bounty Islands population would likely reduce the species' global conservation status to "Vulnerable" under IUCN criteria. This creates both a communicative and conservation conundrum: the binary population trajectories - one stable, one declining - combine to improve the global threat category while effectively obscuring a severe and genuine population-level crisis, and may even weaken the fisheries protections critical to safeguarding what may be the species' last stable stronghold, the Bounty Islands.