A choice between two adjacent islands: is switching nest sites related to weather or nest density in the Common Eider (Somateria mollissima)?

Abstract

Birds change nest sites between years in response to breeding success in previous years, activities of other breeders, or weather conditions. Sixteen years of banding data and nest counts of the Common Eider in Rif, western Iceland, were used to evaluate probability of switching nest islands. The two man-made islands studied had different substrates (grass vs. rock) and in some years, different nest-initiation dates. Nest numbers were not related to weather. The apparent survival was not related to annual variation, weather or nest den-sity. Conversely, the probability of switching islands (movement probability) was plausi-bly affected by three covariates: (1) winters with higher precipitation decreased the prob-ability of switching from the grassy to the rocky island; (2) as winters became milder from 1993 to 2008, the probability of switching islands decreased; and (3) as the nest numbers increased from 155 to 606 during 1993–2008, the probability of switching islands de-creased. Winter weather may have affected nest switching primarily via precipitation, which partly determined water levels in the colony. An inverse relationship between nest density and the probability of switching islands is consistent with conspecific facilitation in colonially-nesting bird species.