Iceberg-driven constraints on colony–foraging connectivity result in severe decline in chick counts for the Coulman Island emperor penguin colony

Emperor penguin populations are vulnerable not only to regional sea-ice variability but also to localized physical disturbances that affect access to breeding colonies. Here we document an approximate 69% decline in springtime chick counts at Coulman Island in 2025, based on integrated multi-satellite analyses and field surveys. These declines were driven by an iceberg calved from the Nansen Ice Shelf that became grounded in late July along a narrow coastal margin, coinciding with the transition into the chick-rearing period and obstructing the colony’s primary access corridor. Three-dimensional reconstruction of the iceberg reveals pronounced morphological asymmetry that likely constrained movement pathways: gentler ocean-facing slopes may have facilitated ascent, while the colony-facing margin formed a sub-vertical escarpment exceeding 20 m, potentially increasing travel distance and delaying access to the colony. A residual passage of approximately 1 km persisted along the eastern margin but was spatially limited and likely difficult to locate. Field observations and high-resolution imagery indicate evidence consistent with elevated chick mortality during the early rearing stage. These findings suggest a mechanism in which iceberg morphology and local coastal configuration jointly result in a substantial reduction in functional accessibility between the colony and foraging areas. A calved iceberg that obstructed the primary feeding corridor of an emperor penguin colony on Coulman Island during the 2025 breeding season resulted in a 69% decline in chick counts, according to integrated multi-satellite analyses and field surveys.