Abandoned penguin colonies and environmental change in the Palmer Station area, Anvers Island, Antarctic Peninsula

Six abandoned colonies of Adelie penguin (Pygoscelis adeliae) were excavated near Palmer Station, Anvers island, Antarctic Peninsula, to investigate the occupation history of this species. Sediments from each site yielded abundant fish bones and otoliths and squid beaks that represent prey remains deposited by penguins during the nesting period. Radiocarbon analyses indicate that colony occupation began prior to the Little Ice Age (LIA; 1500-1850 AD), with the oldest site dating to 644 yrs before present (sp; average reservoir-corrected date with Is range, 603-679 yr sp). Food remains indicate that the non-euphausiid prey of penguins consisted primarily of a mesopelagic squid (Psychroteuthis glacialis) and two species of fish (Pleuragramma antarcticum and Electrona antarctica). The relative abundance of the first two prey taxa varied significantly among six sites (chi(2) > 34.6; df = 10; P < 0.001) with colonies dating prior to the LIA having greater representation of squid, and less of silverfish, than those occupied during the LIA. Data from control excavations at three modem colonies indicate a diet similar to that of the pre-LIA sites. These results suggest that Adelie penguins may have changed their diet in response to warming and cooling cycles in the past. In addition, only Adelie penguins are known to have nested in the Palmer Station area prior to the 1950s; gentoo (Pygoscelis papua) and chinstrap (P. antarctica) penguins now breeding in this region have expanded their ranges southward in the Peninsula within the past 50 yrs, in correlation with pronounced regional warming.