Egg-laying and nesting behavior of common eider at Stora Karlsö

Sammanfattning: The population of common eider (Somateria mollissima) has suffered a dramatic decline in several breeding locations since the 1990s. Predation is described as one of the main reasons, along with other suggested explanations such as food shortage, environmental pollutants, and thiamine shortage. Data about the common eider in Swedish waters is missing to a large extent. To better understand breeding behavior and predation, my study has investigated basic breeding parameters through camera surveillance at Stora Karlsö from late April until mid-June 2022. My study also aimed to investigate whether camera surveillance is a possible method. In total, 14 females were investigated. The hatching success was determined to be 79 %. Predation was carried out by hooded crows (Corvus cornix) and herring gulls (Larus argentatus). Out of > 400 observed nest days and 14 observed females, predation was only observed nine times. This study concluded that predation may not be a big threat to the species during incubation at Stora Karlsö. The first egg was observed on average on May 4th. The incubation started on average May 8th and lasted an average of 25 days, 12 hours, and 6 minutes. During the incubation period, the females left the nest on average 0.87 times per 24 hours, with a median leaving time of 22:10. These incubation intermissions had an average duration of 31 minutes and did not change over time. The average duration did significantly vary between females.  Further studies are needed to clarify if the females are foraging during the incubation intermissions. Males were observed in the nest area, with most observations between May 3rd and May 8th. This study concluded that camera surveillance is a possible method to investigate the basic breeding behavior of the common eider. The recording should start in mid-April and end in mid-June and an improved camera system is necessary to gain more reliable data.