The breeding biology and distribution of Adelie penguins: adaptations to environmental variability

Adelie penguins are long-lived, highly philopatric seabirds that dominate the bird biomass of the Western Antarctic Peninsula region, and serve as focal animals for our Long Term Ecological Research (LTER) study of the effects of environmental variability on animal populations in the Antarctic marine ecosystem. The major physical factors affecting the breeding success, distribution and demography of Adelie penguins in the Southern Ocean are variability in sea ice cover, ocean circulation patterns and terrestrial topography. We analyzed Adelie distributions in the Antarctic Peninsula region and concluded that Adelie penguins have discrete subpopulations in the northeastern and southwestern regions of the area. These subpopulations are separated by a 400 km gap in their respective distributions, but each is within several hundred kilometers of predictable pack ice areas in the Weddell and Bellingshausen Seas, respectively. We propose that these pack ice areas are the wintering grounds for each subpopulation, and that access to these pack ice areas, early in the season following courtship fasting, is the key to successful breeding in Adelies. We further analyzed the colony distributions within each subpopulation and found highly clumped distributions of Adelie penguins that were strongly correlated to physical factors such as bathymetry, currents and wind direction. We propose that these variables reduce the occurance of pack ice in the vicinity of breeding colonies of Adelie penguin populations, thereby assuring access to open water in the early season. Finally, we examined the influence of the interaction of local topography and weather on the size, location and persistence of breeding groups within Adelie penguin colonies. Snow accumulation, melt water runoff and solar radiation all impact the microclimate of breeding colonies and influence the selection of nesting sites among Adelie penguins. The abandonment of breeding areas by Adelie penguins, following two to three years of failure at "poor" sites, suggests that changes in the population distribution of Adelie penguins may be very rapid in response to changing environmental conditions, such as increased snow deposition. Adaptations to environmental variability are seen in every aspect of the natural history of the Adelie penguin, from the distribution of subpopulations around Antarctica, to the sizes and distributions of colonies within regions, to the choice of breeding sites within colonies.