From the Cover: Abrupt recent shift in δ13C and δ15N values in Adélie penguin eggshell in Antarctica

Abandoned colonies of Adelie penguins (Pygoscelis adeliae) have been located in many coastal, ice-free regions of Antarctica. When excavated, these sites often provide abundant well preserved remains of penguin tissue (bone, feathers, dried skin, and eggshell) and hard parts from prey remains (fish bone, otoliths, and squid beaks) in guano. Radiocarbon dates on these tissues range from hundreds to tens of thousands of years old, providing considerable information on the occupation history and population movements of Adelie penguins from the late Pleistocene through Holocene in the Antarctic Peninsula (1–3), Ross Sea (4–6), and East Antarctica (7). Although these studies have been expanded in recent years to include analysis of ancient DNA (8, 9), until now stable isotope analyses have not been applied except for characterization of living specimens. Since the 1980s, carbon (δ13C) and nitrogen (δ15N) stable isotope values of seabird tissues have been used to address questions on diet and foraging behavior (10–14). These studies are particularly useful when traditional diet studies (observations of chick feedings and stomach lavage) are difficult to carry out on far-ranging species such as many seabirds. For penguins in Antarctica stable isotope analyses allow assessment of seasonal differences in diet among widely dispersed populations. Modern Adelie penguins are known to feed primarily on krill (Euphausia superba) during the chick-rearing period in the Antarctic Peninsula, although Antarctic silverfish (Pleuragramma antarcticum) and squid (Psychroteuthis antarctica) can also be important (15). However, because most dietary studies have been conducted during the chick-rearing period, little is known of winter diet (16). Isotope ratios stored in seabird tissues can provide information on diet during brief to long periods, depending on the tissue used, and can provide important new information on diet outside the breeding season. Moreover, fossil eggshell can provide information on dietary shifts with climate change over millennia (17). We determined δ13C, δ15N, and δ18O values of modern and fossil eggshell recovered from eight active and 28 abandoned colonies of Adelie penguins in Antarctica to determine whether dietary shifts through time could be discerned and compared with environmental and climate change documented in the geological record (ice cores and marine sediments). We chose eggshell for this analysis because it is almost always well preserved in ornithogenic soils (bird-formed soils characteristic of abandoned penguin colonies), whether these soils are hundreds or tens of thousands of years old, whereas other tissues (bone and feathers) are more variably preserved. Because the soils of Antarctica are particularly cold and dry, diagenesis in fossil specimens is unlikely and the original eggshell isotope values should be well preserved. Stable isotope values of eggshell can provide valuable information on the diet of breeding females during a critical stage in the breeding cycle [see Schaffner and Swart (18) for a review of this topic]. Eggshell formation, when initiated, is completed within a 24-h period (19), thus recording the bird's most recent dietary signal. Most female Adelie penguins begin laying eggs in October/November each year (15). Consequently, analyses of eggshells will characterize penguin diet during a brief interval in the annual cycle. To characterize any regional differences in penguin diet, we also examined samples of eggshell from abandoned and active colonies from three major regions of Antarctica: the Ross Sea, East Antarctica, and the Antarctic Peninsula. Samples were collected over several field seasons between 1999 and 2004. All sites were excavated in 5-cm arbitrary levels to the lower boundary of ornithogenic sediments as recognized by a distinct change in soil texture and color [see Emslie et al. (5) for excavation methods].