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

The Earth system is undergoing rapid change driven by anthropogenic forcing, with mounting evidence that these changes are intensifying environmental pressures on polar ecosystems^1,2,3. In Antarctica, recent decades have been marked by documented atmospheric warming⁴, declines in sea-ice extent and thickness^5,6, and an increased frequency of extreme weather events⁷. Collectively, these environmental shifts are associated with changes in ecosystem structure and have been linked to variation in survival and reproductive performance across Antarctic species^5,8,9,10,11. Among Antarctic marine predators, the emperor penguin (Aptenodytes forsteri) has been described as a sentinel species, reflecting broader changes within the Southern Ocean ecosystem¹². Its life history is closely linked to the availability of stable sea ice, which supports key stages of its annual cycle, including breeding, incubation, chick rearing, and molting^{5,8,13,14,15,16}. Consequently, alterations in sea-ice conditions may increase the risk of reduced breeding performance and could influence long-term population trajectories^{14,17,18,19,20,21}. Given this ecological dependence, emperor penguins have become a focal species for investigating the biological consequences of sea-ice variability and ongoing environmental change in Antarctica.

Emperor penguins depend on stable landfast sea ice for breeding, and reproductive success may be compromised when sea ice forms late in the season or breaks up before chicks fledge^8,11. Emperor penguins follow a highly seasonal breeding cycle, with egg laying in May–June, incubation through winter, hatching in July–August, and chick rearing through spring until fledging in December–January (Fig. 1a). Because the sea-ice zone supports key foraging activities for this species, reductions in sea ice may influence long-term population viability by constraining access to prey resources²². In addition to changes in sea-ice conditions, large iceberg calving events along parts of the Antarctic coastline have been associated with increased cryospheric variability²³. Such physical disturbances can generate short-term, site-specific ecological impacts by obstructing access to breeding habitats or altering local environmental conditions that support reproductive success, including for apex predators such as emperor penguins.

In the context of these cumulative pressures, emperor penguins were classified as Near Threatened on the IUCN Red List^8,11,24, and have recently been evaluated as Endangered status²⁵. In response, parties to the Antarctic Treaty System, including those operating under the Protocol on Environmental Protection and the Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR), have discussed or implemented management measures intended to reduce additional stressors that may compound the effects of climate change on emperor penguin populations^8,26.

Research on emperor penguins has traditionally relied on sea-ice metrics, including extent, formation timing, and stability, to explain breeding success and population variability^5,14,17,21. These approaches have provided robust and quantitatively supported insights, suggesting that sea-ice conditions can explain a substantial portion of demographic variability. For example, previous studies have shown that models based on sea-ice and fast-ice conditions can account for approximately 40–50% of the variance in breeding success R² ≈ 0.4–0.5;^27,28. Such frameworks are widely used not because sea ice is assumed to fully determine population dynamics, but because these variables are consistently observable and can be reasonably represented in Earth system models. Nevertheless, the remaining unexplained variability suggests that additional environmental processes, beyond large-scale sea-ice metrics alone, contribute to observed demographic patterns.

In recent decades, additional episodic habitat disturbances, not readily explained by sea-ice variability alone, have been documented. For example, the grounding of large icebergs near Cape Crozier and Beaufort Island disrupted access to foraging areas and coincided with substantial reductions in breeding success²⁹. While iceberg calving and grounding are natural components of Antarctic dynamics, such events can, under specific spatial and temporal configurations, lead to short-term ecological impacts. The frequency of such cryospheric disturbances has been projected to increase in some regions under continued climate change^30,31. A well-documented example occurred in 2001 near Cape Crozier in the Ross Sea, where the grounding of giant icebergs (B15A and C16) disrupted access to the colony and resulted in two consecutive years of complete breeding failure in the local emperor penguin population²⁹. In subsequent years, chick productivity declined to 0–40% of pre-2000 levels, suggesting that iceberg presence was associated with persistent physical habitat modification beyond sea-ice stability alone and affected multiple stages of the breeding cycle^14,29. Notably, the impacts of large icebergs on penguin habitats have also been documented outside the immediate breeding period. For example, the collision of a large iceberg with the Mertz Glacier Tongue led to substantial habitat reconfiguration when breeding birds were absent, with subsequent ecological consequences for nearby penguin colonies, including the potential relocation of colonies³².

Icebergs may obstruct access routes between breeding sites, foraging areas, and overwintering habitats. At Beaufort Island, for example, the arrival of large icebergs, including B15A and C16 in January 2001, led to the formation of an approximately 150 km barrier between the Ross Sea polynya and the island, blocking the primary access route to the colony. This barrier persisted for over two years, until late 2003, constraining access to both feeding and breeding areas and forcing penguins to undertake longer detours^29,33. These conditions were associated with marked declines in breeding pair numbers, elevated chick mortality, and, in some cases, redistribution or relocation of individuals between colonies^16,29. Collectively, these cases highlight that, while sea-ice conditions remain a primary driver of emperor penguin breeding success, colony-scale outcomes may also be influenced by additional physical disturbances acting in combination.

Coulman Island (73°28′ S, 169°45′ E), located along the North Victoria Land coast within the Ross Sea Marine Protected Area (CCAMLR Conservation Measure 91-05), has been reported as one of the largest emperor penguin colonies globally Fig. 1b;³⁴. Its large colony size has been attributed to favorable regional oceanographic conditions, including high marine productivity and the persistence of stable landfast sea ice between Cape Jones and Coulman Island^16,29,34,35. Chick abundance at this colony has been monitored since the 1980s, providing one of the longest available census records for any emperor penguin colony. Standardized aerial surveys have been conducted annually by the Korea Polar Research Institute (KOPRI) since 2017, using helicopter access from Jang Bogo Station (~220 km away). Owing to its size, multi-decadal monitoring record, and position within a designated marine protected area, the Coulman Island colony represents a valuable site for detecting and interpreting the ecological consequences of environmental disturbance in the Ross Sea.

We report on field surveys conducted in November and December 2025 in the vicinity of Coulman Island, which documented substantially lower chick counts than in recent breeding seasons. Reduced breeding success was associated with a large grounded iceberg north of the colony that constrained access to the breeding site. This reduction in accessibility coincided with low chick counts recorded during that season, indicating a temporal association between iceberg-induced obstruction and reduced chick productivity. This study is explicitly framed as an observational case study focused on a single breeding season and does not seek to establish definitive causality between iceberg grounding and reduced chick productivity. Our aim is to examine plausible mechanisms through which iceberg-driven disruptions to accessibility may affect emperor penguin breeding processes and to present observational evidence of the role that such access constraints may have played in the reduced chick productivity observed at Coulman Island. We therefore examine the role of iceberg grounding in constraining access to the colony and its potential influence on breeding outcomes, given that fast ice appeared largely intact during the survey period, as observed visually.

Grounded Iceberg Configuration and Observed Breeding Conditions in November 2025

During the annual survey to census emperor penguin chicks at Coulman Island on 12 November 2025 (see Methods), a grounded iceberg was identified approximately 3–5 km north of the colony (Fig. 1c). The iceberg measured approximately 13.9 km in length and 4.0 km in width, with an estimated area of approximately 42.7 km². Its southern margin formed a steep ice cliff, reaching several tens of meters in height. The iceberg was positioned along a primary commuting route between the breeding site and offshore foraging areas extending northward and northeastward from the colony. Adult emperor penguins were observed congregating along the southern margin of the iceberg, where the steep ice cliff extended continuously (Supplementary Fig. 1). The eastern margin of the iceberg coincided with the fast-ice edge, forming a continuous boundary between the iceberg and the surrounding landfast ice during the survey period. Repeated satellite observations and in situ surveys indicated that the iceberg remained grounded at the same location throughout winter and into early spring. It began to move on 2 January 2026, drifting eastward offshore of Coulman Island and subsequently moving northward from 18 January (Supplementary Fig. 2).

Calving and drift–grounding chronology of the nansen ice shelf iceberg

The calving event and subsequent drift–grounding sequence of the iceberg toward the northern margin of Coulman Island were reconstructed through multi-sensor image analysis, integrating MODIS True Color imagery and Sentinel-1 synthetic aperture radar (SAR) time series. Back-tracking analysis shows that the iceberg originated from the Nansen Ice Shelf in Terra Nova Bay. A MODIS image from 12 March 2025 indicates that calving had occurred (Supplementary Fig. 3). Imagery acquired on 10 and 11 March 2025 shows a contiguous ice-shelf front with no visible evidence of structural separation (Supplementary Fig. 3a, b). However, a Sentinel-1 SAR image from 7 March (Fig. 2a) reveals a distinct fracture at the ice-shelf front, indicating that separation had already initiated before the MODIS observations. In the MODIS image from 12 March (Supplementary Fig. 3c), a dark-toned area at the shelf front is interpreted as open water, consistent with the iceberg having already separated from the ice shelf. The MODIS image from 13 March (Supplementary Fig. 3d) further shows the detached iceberg drifting away from the shelf. Post-calving movement of the iceberg was tracked through visual interpretation of a Sentinel-1 SAR time series (Fig. 2). Imagery from 7 March shows the pre-calving configuration, with the iceberg already partially fractured but still contiguous with the Nansen Ice Shelf front (Fig. 2a). Following detachment, the iceberg drifted northeastward, reaching waters near Cape Washington by 19 March (Fig. 2b). It then continued northward to within approximately 40 km south of Coulman Island (Fig. 2c), and by 16 June was observed in close proximity to the island’s southern coastline (Fig. 2d). The iceberg subsequently progressed northward along the island’s eastern flank (Fig. 2e). By 16 July, the iceberg was in contact with surrounding sea ice near Coulman Island (Fig. 2f) and began to rotate clockwise. The subsequent resumption of drift after the disappearance of surrounding sea ice in early January 2026 suggests a constraining effect of sea ice on iceberg motion, although a contribution from local bathymetry cannot be excluded. During this period, the iceberg rotated, with its sloping face oriented toward the south before pivoting north of Coulman Island. Continued rotation was accompanied by reduced translational motion (Fig. 2g), and by 28 July the iceberg appears to have become grounded adjacent to the island (Fig. 2h). The iceberg remained stationary through November 2025 (Fig. 2i). Following the loss of surrounding sea ice, it resumed drift on 2 January 2026, moving eastward offshore before turning northward later in January (Supplementary Fig. 2).

Long-term Chick Census and the 2025 Decline at Coulman Island

Long-term Chick Census at the Coulman Island Colony. Very-high-resolution Maxar WorldView imagery indicates a marked difference in the overall presence and spatial expression of surface biological indicators (i.e., guano staining) at the breeding site between November 2024 and November 2025. In November 2024 (Supplementary Fig. 4a), guano staining was widespread across the landfast ice, whereas in imagery acquired on 6 November 2025 (Supplementary Fig. 4b), such indicators were largely absent at the same location. While interannual differences in guano distribution can arise from multiple factors, including surface conditions and penguin aggregation behavior, this comparison is interpreted cautiously.

To place these observations in a longer-term context, Fig. 3 presents a long-term census of emperor penguin chicks at Coulman Island spanning 1983 to 2025. The time series comprises data from 21 breeding seasons, integrating historical records reported by Barber-Meyer et al.³⁶ and Kooyman and Ponganis³⁴ with standardized aerial survey monitoring conducted since 2017 by the Korea Polar Research Institute (KOPRI) (see Methods). Across the full observation period, chick abundance at the Coulman Island colony shows substantial interannual variability.

Among observations before 2000, the highest recorded chick count was 34,735 in 1992, followed by 27,920 in 1990 and 21,708 in 1983³⁶. The mean chick abundance between 1983 and 2000 was 24,667  ±  6622. Literature-based records from the period after 2000 and before the initiation of KOPRI surveys also show substantial variability. During this interval, chick counts were 25,244 in 2012 and 22,511 in 2008³⁴. The mean for the post-2000 pre-KOPRI period was 18,305  ±  5651, lower than the 1983–2000 mean, and included several years with low counts, such as 9305 in 2010 and 12,382 in 2011³⁴.

Since 2017, KOPRI has conducted aerial photograph–based surveys that have provided a standardized time series of chick counts at Coulman Island. Recorded values were 16,571 in 2017, 21,286 in 2018, 24,464 in 2019, 23,223 in 2021, 22,903 in 2022, 18,723 in 2023, and 21,242 in 2024, yielding a mean of 21,202  ±  2,745. In contrast, the 2025 count represents the lowest value recorded since 1983. During the November survey, 6081 chicks were counted, and a subsequent December survey identified additional small breeding sub-groups, increasing the final total to 6658 (see Methods). This corresponds to an approximate 69% reduction relative to both the KOPRI-era mean and the 2024 count. The magnitude of decline in 2025 exceeds that reported for 2010, when chick productivity was estimated at 56% of the 2006 minimum and 41% of the 2008 estimate¹⁶. The total number of adults counted in 2025 was 15,242, yielding a chick-to-adult ratio of approximately 0.44, slightly below the previously reported range (0.46–0.99) for the Coulman Island colony³⁴.

Restricted accessibility and its potential role in reduced chick productivity

We define this process as “restricted accessibility,” in which breeding and foraging habitats remain physically present, but functional connectivity between them is disrupted by transient physical barriers. The 2025 event at Coulman Island suggests that, even under relatively stable landfast sea-ice conditions, the grounding of a single large iceberg may substantially constrain access between breeding and foraging habitats and be associated with reduced chick productivity. Notably, the timing of iceberg immobilization (late July) coincided with the transition from incubation to early chick rearing (refer to Fig. 1a), a period recognized as particularly sensitive to changes in habitat accessibility and environmental conditions e.g.,^5,8,19. During this period, successful parental coordination and food provisioning depend on continued access between the breeding site and adjacent marine habitat^27,37. Food demand increases as chicks grow, and sustained provisioning is required throughout this period to ensure productivity. Disruption of this connectivity, therefore, represents a plausible mechanism linking altered accessibility to reduced chick productivity.

This interpretation is consistent with the observed timing and scale of reproductive loss in 2025. Following egg laying in May–June, males incubate eggs through June–July while females undertake extended foraging trips at sea (Fig. 1a). Given the timing of iceberg grounding in late July, incubation and hatching likely proceeded largely as normal. However, when females returned to relieve incubating males, iceberg-induced access constraints may have delayed or disrupted parental changeover. Likewise, males completing incubation may have experienced difficulty accessing viable routes to the sea, potentially compounding delays in coordinated foraging and return exchanges. Disruption of parental coordination during early chick rearing could reduce food delivery to newly hatched chicks, thereby increasing the risk of starvation^20,38,39. During the initial field survey in November 2025, multiple chick carcasses were observed within the colony area (Supplementary Fig. 5), consistent with elevated early-stage mortality.

Iceberg morphology and access constraints

Beyond the mere presence of a grounded iceberg, its particular morphology appears to have influenced the degree to which access to the Coulman Island breeding site was constrained. Photogrammetry-based elevation analysis (see Methods) reveals marked asymmetry in iceberg geometry along its grounding axis (Fig. 4). In the western sector (transects T1–T2), the ocean-facing side forms a relatively gentle slope, whereas the colony-facing southern margin rises abruptly as a sub-vertical escarpment exceeding 20 m in height, effectively limiting direct access to the breeding site. In the eastern sector (transects T3–T4), steep ice faces are present on both the colony-facing southern side and the ocean-facing northern side, forming a more topographically confined configuration.

Notably, a narrow passage of approximately 1 km intermittently persisted along the eastern margin between the iceberg and Coulman Island (see Fig. 1c). This corridor was not fully obstructed and may have allowed limited movement between the colony and offshore waters. However, accessing this passage likely required substantial lateral movement along the iceberg margin, increasing travel distance and time. Given the asymmetric slope configuration, individuals approaching along the gently sloping western face may have been directed toward the impassable southern escarpment, thereby increasing travel time before locating the ~1 km-wide eastern opening, as inferred from satellite imagery.

Even where access remained possible via the eastern corridor, the spatial configuration of the fast-ice edge suggests that commuting distances between the colony and open water were longer than in previous years, increasing from approximately 9.3 km in 2024 to 14.9 km in 2025 based on satellite-derived geometry (Fig. 5a). However, this represents a minimum estimate assuming direct access, and actual travel distances were likely substantially greater due to route-finding behavior. Although access was not entirely blocked, the combined configuration of the grounded iceberg and surrounding sea ice likely increased navigational complexity, potentially forcing individuals to follow indirect or suboptimal paths. Penguins rely on visual cues and learned spatial memory for navigation^40,41, although the extent to which this applies under highly altered icescape conditions remains uncertain, and such cues may have been partially obscured or disrupted in this setting.

Field observations documenting adult penguins remaining in prolonged stasis along the southern ice walls (Fig. 5b–d; Supplementary Fig. 1) are consistent with restricted, spatially heterogeneous access pathways at the time of observation. While rerouting through the eastern corridor cannot be excluded, the observed spatial concentration of individuals suggests that access constraints were non-uniform and likely imposed differential movement costs, particularly in terms of increased travel time and energetic expenditure, across the iceberg perimeter^11,29.

Consequently, delays in locating or navigating limited-access corridors during early chick rearing could reduce provisioning frequency to newly hatched chicks^42,43. Such topographically induced delays are particularly consequential during this period, when provisioning schedules are tightly constrained, and energetic margins are narrow. Following hatching, emperor penguin chicks can survive only for a limited period on esophageal secretions (penguin milk) provided by the incubating male^44,45. Even modest increases in travel time may therefore translate into a disproportionate increase in chick mortality risk^8,11.

Taken together, these results indicate that the impact of the grounded iceberg cannot be attributed to a single mechanism, but rather to the combined effects of increased commuting distance and navigation disruption. While access to open water was not entirely prevented, the asymmetric configuration and limited openings likely reduced the efficiency and predictability of movement between the colony and foraging areas. This interpretation highlights the importance of local geometric constraints in shaping ecological outcomes and provides a basis for comparison with previously documented iceberg-related disturbances.

Comparison with previous events and mechanisms

The 2025 reduction in breeding output at Coulman Island differs from previously documented declines at this colony and elsewhere^16,34,36. Long-term monitoring has recorded substantial interannual variability in chick numbers, including declines of approximately 46% in 1993 and over 50% in 2010 Fig. 3;^34,36. The 2010 event was accompanied by reduced adult attendance and interpreted as reflecting adult decisions to forego breeding, attributed to insufficient energetic reserves¹⁶. The rapid recovery of adult numbers by 2011 supports the view that this episode primarily reflected a temporary breeding suspension rather than large-scale mortality.

In contrast, the 2025 pattern suggests that many adults initiated breeding but experienced early-stage reproductive loss. Evidence includes continued adult presence at the colony alongside a relatively low chick-to-adult ratio (0.44). This pattern is more consistent with disrupted chick rearing than with pre-breeding abstention¹⁶, indicating an extrinsic physical cause rather than intrinsic life-history trade-offs.

A comparison with previously documented iceberg-related events further clarifies this mechanism²⁹. At Cape Crozier, reduced output resulted from the destruction of fast-ice breeding habitat. At Beaufort Island, habitat remained intact, but access routes were obstructed, requiring extended detours that disrupted provisioning. The 2025 Coulman Island event most closely resembles the Beaufort Island mechanism, with one important difference: the semi-enclosed coastal geometry (see Fig. 1) likely restricted alternative access routes. Unlike multi-year iceberg systems such as B15-A and C16, which exerted prolonged and large-scale ecological impacts, the event described here was shorter-lived and spatially confined. Accordingly, its impact should be interpreted as a localized, event-driven disturbance rather than a persistent, system-scale forcing. The iceberg’s asymmetric configuration, with a gently sloping ocean-facing surface, likely directed returning adults toward an impassable southern escarpment, compounding movement costs (Fig. 5b–d).

Evidence collected during the 2025 field season suggests that the demographic consequences of the iceberg event were concentrated on chick productivity. Flipper length and body mass were measured in 37 surviving chicks during December and showed no significant differences relative to chicks at Cape Washington, consistent with adequate provisioning among individuals that survived the early post-hatching period. No adult carcasses were observed during ground visits, although adult mortality could not be directly assessed. Access to the colony, while severely constrained by the iceberg, was not entirely blocked. Taken together, these observations indicate that the most evident demographic consequence of the 2025 event was reduced chick productivity.

One plausible explanation is that survival reflected heterogeneity in parental access and foraging success. Some adults may have located and repeatedly used alternative routes earlier than others, allowing them to continue provisioning their chicks despite the access constraints. Because surviving chicks did not differ markedly in body mass or flipper length from chicks at Cape Washington, the available data do not support a strong developmental-stage or body-condition bias among survivors.

Future perspectives: event-driven risks in a changing Ross Sea

The 2025 Coulman Island event is better interpreted as a localized, event-driven disturbance rather than as evidence of persistent risk to emperor penguins across the Ross Sea. While the Ross Sea region has historically exhibited relatively stable sea-ice conditions^19,21, recent variability suggests caution in generalizing its role as a refuge. This event highlights a class of event-driven disturbances that may not be fully represented in demographic models focused on large-scale mean-state variables, such as regional sea-ice extent.

The drift and grounding pathway of the 2025 iceberg highlights the importance of timing and spatial configuration in shaping ecological outcomes. Unlike iceberg C-33, which transited the Coulman Island sector in October 2016 and did not ground⁴⁶, the 2025 iceberg remained in proximity to the island and subsequently became grounded (Supplementary Fig. 6). To our knowledge, this represents the first documented case, based on satellite-era observations, of an iceberg grounding immediately north of the colony, despite multiple icebergs transiting the region without grounding. The processes controlling this behavior are not fully resolved and may involve interactions among sea ice, ocean forcing, and local bathymetry.

Increasing instability of Antarctic ice shelves over recent decades has been linked to iceberg calving events^47,48,49, potentially associated with climate-driven change such as ocean warming^30,31. Large icebergs that remain grounded near the coast for extended periods can disrupt access routes to foraging areas and influence breeding outcomes at the colony scale. However, individual iceberg events are inherently stochastic, and our findings should be interpreted as evidence of a potential mechanism rather than a dominant driver of population change.

Emperor penguin responses to environmental change are likely nonlinear, emerging from interactions among sea ice, prey availability, episodic disturbance, and colony-level movement responses^18,50. Although inter-colony connectivity and behavioral flexibility may buffer populations at the metapopulation scale^12,51,52, this flexibility does not preclude severe local reproductive collapse when accessibility abruptly declines at critical stages of the breeding cycle.

Future assessments of emperor penguin breeding prospects would benefit from accounting for the potential influence of stochastic iceberg–sea-ice interactions, including calving timing, drift pathways, grounding probability, and residence time near breeding sites^11,52. These processes represent low-frequency but high-impact disturbances that may affect local breeding outcomes under otherwise favorable regional sea-ice conditions. Additional factors, including coastal configuration and variability in fast-ice conditions, may further modulate colony vulnerability, although their specific contributions to the 2025 event remain uncertain. Addressing these processes will likely require continued long-term, cross-disciplinary monitoring, rather than direct incorporation into existing modeling frameworks, given their complexity.

Study area and colony context

The study was conducted at the emperor penguin colony located on annual landfast sea ice along the northern margin of Coulman Island (73°28′ S, 169°45′ E), North Victoria Land, Ross Sea, Antarctica (Fig. 1b). This colony is situated within Glacier Strait, a semi-enclosed coastal embayment bounded by Cape Jones to the south and open waters extending northward toward the Ross Sea polynya system (see Fig. 5a). The breeding aggregation forms on annual landfast sea ice along the island’s northern coast and spans several kilometers during the breeding season.

Emperor penguin census surveys

Two field surveys were conducted on 13 November 2025 and 5 December 2025 at the Coulman Island emperor penguin colony. The November survey formed part of the Korea Polar Research Institute (KOPRI) annual monitoring program. The December survey was conducted to complement the November survey and to assess habitat conditions, limited chick morphometrics, and iceberg topography. Both surveys used a helicopter-mounted platform for aerial photogrammetry at an altitude of approximately 600 m above ground level, consistent with KOPRI monitoring protocols since 2017. This ensured comparability in image resolution and survey coverage across years.

Chick counts were derived from high-resolution imagery through independent manual interpretation by four observers. Inter-observer variability was low (coefficient of variation = 2.04%), comparable to values reported for high-resolution imagery-based censuses typically ≤2.5%;⁵⁰. Survey coverage (approximately a 10–15 km radius from the colony; see Fig. 5a) included the full spatial extent of the breeding area and adjacent fast-ice margins to minimize omission of peripheral groups. At the time of the November survey, the breeding aggregation was distributed among multiple subgroups, which were delineated, imaged, and counted separately before aggregating totals (Supplementary Fig. 7). During the December survey, additional peripheral subgroups were identified, increasing the total chick count to 6,658. Identical counting criteria were applied across both surveys to ensure consistency with previous datasets (2017–2024).

During the December survey, limited ground landings were conducted to obtain morphometric measurements from 37 chicks. Body mass was measured with a portable digital scale, and flipper length was measured with a standard caliper.

Satellite data and iceberg tracking

Multi-sensor satellite data were used to reconstruct iceberg calving, drift, grounding, and final configuration relative to the Coulman Island colony. Datasets included MODIS Terra true-color imagery, Sentinel-1 synthetic aperture radar (SAR), Landsat-8/9 Operational Land Imager (OLI), and very-high-resolution WorldView imagery.

The calving date from the Nansen Ice Shelf was identified using MODIS imagery. Despite partial cloud cover, imagery acquired on 12 March 2025 showed separation at the ice-shelf front, and subsequent scenes confirmed a detached iceberg, indicating calving on this date. Higher-resolution optical imagery (WorldView and Landsat) did not provide adequate coverage of the detachment period.

Post-calving drift was tracked using Sentinel-1 SAR imagery obtained from the Alaska Satellite Facility Data Search Portal. All available scenes from 11 March to 28 July 2025 were analyzed. Iceberg outlines were manually digitized, and centroid positions were used to reconstruct drift trajectories. Grounding was inferred from reduced displacement, onset of rotation, and subsequent positional stability adjacent to Coulman Island from 28 July 2025 onward.

Landsat-8/9 OLI imagery (30 m) from the USGS EarthExplorer archive was used to identify the final location of the grounded iceberg and to assess routes to the colony, using imagery acquired on 11 November 2025.

Very-high-resolution Maxar WorldView imagery (WorldView-3: 20 November 2024; WorldView-2: 6 November 2025) was used to evaluate surface biological indicators at the breeding site.

Iceberg morphology and elevation analysis

A high-resolution Digital Surface Model (DSM) of the grounded iceberg was constructed using aerial photogrammetry. For contextual reference, the 2 m-resolution Reference Elevation Model of Antarctica (REMA) v2.0 mosaic was used⁵³. Horizontal coordinates were defined in WGS84/Antarctic Polar Stereographic (EPSG:3031), and elevations were converted from ellipsoidal heights (EPSG:4979) to orthometric heights using the EGM2008 geoid.

The aerial DSM was generated from 1071 photographs acquired from a helicopter platform. Most images were collected at approximately 1200 m above ground level to capture overall geometry, with additional lower-altitude (approximately 600 m) flights to improve coverage of steep surfaces. Imagery was captured with a Canon EOS R5 camera and a 100 mm lens. Three-dimensional reconstruction was performed in Agisoft Metashape Professional v2.2.2 using structure-from-motion workflows, and the resulting products were processed in Global Mapper to produce elevation products at an effective spatial resolution of approximately 1.2 m. Ground control points were not available. The analysis focused on relative elevation differences between iceberg surfaces at scales of tens of meters.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.