The early birds and the rest: do first nesters represent the entire colony?

Climate change studies have detected earlier spring arrival of breeding birds. However, first nest dates (date first nests were found), which commonly provide the metric for earlier arrival, can be biased by population size or sampling effort. Our aims were to determine if: 1) first nest dates and median nest date (date when at least 50 % of all females have nested) were equivalent predictors for the spring arrival and 2) first nest date or median nest date were related to nest numbers. We recorded first and median nest dates and nest numbers at the common eider (Somateria mollissima) colony at Rif, Iceland, during 1992–2013. First nest date was advanced by 11 days during the study, but median nest date was advanced by only 4 days. First nest date and median nest date were correlated, but this relationship was only a small improvement over the null model (Nagelkerke R 2 = 30 %). We found a relationship with nest count for both first and median nest dates once the analysis had accounted for inter-annual variability. First nest date may not represent the colony as a whole but rather the physically fittest or the most determined individuals, which may be more prone to nest early than the general population. Nesting birds must decide how much to advance breeding based on nest numbers and other non-temporal cues which necessitate earlier breeding. We argue that nest numbers affect the birds in a biological sense and that the advancement was not explained solely by increased nest numbers.This study was supported by the University of Iceland. We sincerely thank Auður Alexandersdóttir, Árni Ásgeirsson and Thordur Örn Kristjánsson for their support through the duration of this study. We thank the editor and three anonymous reviewers for helpful comments that greatly improved earlier drafts of this manuscript. Sigmundur H. Brink kindly provided the map for Fig. 1. This study complies with the current laws of the Republic of Iceland and all regulations pertaining to the treatment of study animals, including the banding permits of JEJ and SJL.Peer Reviewe

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

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