Pagophilic rather than sea ice obligate: effects of sea ice and colony size on Adélie penguin molt location

Strongly pagophilic, the Adélie penguin Pygoscelis adeliae has long been designated a sea ice obligate species, which included the requirement to find pack ice on which to molt. However, observations have accumulated over the years to question this attribute; therefore, we investigated this aspect of the species’ natural history strategy through a synthesis of published, unpublished, and historical records obtained Antarctic-wide and with 12 years of time-lapse camera observations at Cape Royds (Ross Island, Antarctica). We describe patterns of molt habitat use at Cape Royds and evaluate whether interannual variation in regional sea ice conditions (which include at Cape Royds both fast ice and pack ice) during the month immediately preceding the molt season (January) is associated with the number of penguins observed molting at the colony (February). At Cape Royds, automatic camera data, 2008–2024, showed interannual variability in both the timing and number of molting penguins. Peak numbers in view of the camera ranged from 75 to 590 individuals (7%–30% of the breeding population); timing of the peak ranged from February 23 to March 2, although the date of the last molting penguin was consistent among years (March 16 and 17). Based on the results from a negative binomial generalized linear model, peak on-land molt counts varied negatively with mean January sea ice concentration (SIC) within the colony’s foraging area and with breeding population size (breeding pairs; more breeders, fewer molters). Though a longer time series might supply more definitive indications, there was a ~2.2% decrease in the number of penguins molting per 1% increase in SIC. Supplying context to Cape Royds patterns, a review of the literature revealed marked geographic heterogeneity in molt habitat, affected by whether sea ice is present during the molt season (February–March), colony size, and access to prey: in some colonies, most individuals return to molt on land (including landfast ice) or coastal glaciers, while at others only a fraction do so, and at several large colonies molt appears to occur primarily in distant pack ice. Earlier qualitative interpretations in the literature suggested that greater SIC promotes on-land molting, but at Cape Royds higher January SIC brought fewer molting penguins. Together with the broader circumpolar evidence, these findings suggest that Adélie penguin molt habitat is more context-dependent and variable than implied by a simple obligate sea ice paradigm. This plasticity, which applies as well to other life history patterns, likely contributes to the species’ ecological success and persistence across annual- to millennial-scale contrasting sea ice regimes.