| Summary: | Penguins are among the top predators of the Antarctic ecosystem. They are strongly adapted to the harsh conditions and temporal cycles of this region. Therefore, they are considered as indicators of changes of biotic and abiotic conditions in the environment. One possible response to climate variability is a temporal shift in the breeding phenology. A colony of Pygoscelis penguins on Ardley Island, next to the southwestern part of King George Island, South Shetland Islands, has been studied in four consecutive breeding seasons (2014/15 - 2017/18). About 120 nests of gentoo penguins ( Pygoscelis papua ) and about 30 of Adélie penguins ( P. adeliae ) were chosen to represent all topographic characteristic (altitude, slope, etc.) and biotic factors (nest group size). Nests were marked and controlled in intervals of three days. Dates of important breeding events, e.g. peak of egg laying and peak of hatching, were recorded. The resulting breeding phenology is representative for the temporal breeding pattern of the entire colony during the season. Thus, differences in the breeding phenology between the species and between seasons could be assessed. The breeding phenology of Adélie penguins was nearly consistent throughout the four observed seasons. In contrast, gentoo penguins show a much higher annual variability in their temporal breeding pattern. Their breeding process started two weeks earlier during season 2016/17 than in the two previous seasons. The peak of hatching was observed on December 4th, whereas in the other two previous seasons the peak was around December 18th. Accordingly, the whole phenology was preponed by about two weeks. The likely reason for the early start of the 2016/17 breeding cycle was the average snow height in spring, which was much lower than in the two previous years and allowed early access to snow-free breeding grounds. Furthermore the depth of snow is an indicator for the temperature. Higher temperature in spring has a high impact to the time of clutch initiation of the gentoo ...
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