Epsilon Archive for Student Projects

Abstract

The Common Eider (Somateria mollissima) population in the Baltic Sea has suffered from a huge decline during the last decades. Nest predation on eggs from Hooded Crow (Corvus cornix) and Herring Gull (Larus argentatus) has been proposed as one of the possible reasons for the decline. Other explanations are quality changes in the eiders’ main food Blue Mussels (Mytilus edulis) and predation on adults and ducklings. There is a lack of studies examining nest predation in eider colonies in Swedish waters. Therefore, this study has investigated the early nest predation on eider nests at Stora Karlsö in the Baltic Sea. Possible factors contributing to egg predation has been examined, along with the consequences for the eider breeding. Camera surveillance was conducted between mid-April until mid-June 2024, monitoring a total of 17 eider nests. The predation pressure on eider eggs was high, with 42 % of the total clutch size being predated by Hooded Crow and Herring Gull, thus reducing the nest success to 65 %. One of the conclusions was that nest predation had more effect on the nest success than on the number of chicks per nest, possibly caused by female eiders in a poor body condition being more affected by egg predation and thus abandoning their nest. Furthermore, the risk of egg predation was highest in morning and mid-day during the egg laying days, explained by activity patterns of predators and egg guarding patterns of females. Another conclusion was therefore that there is a possible trade-off between foraging and egg guarding for eider females at Stora Karlsö, making eider females in a poor condition particularly vulnerable to early egg predation.