Regional and annual variability in common eider nesting ecology in Labrador

Nesting densities are often used to estimate breeding population size and with other measures of reproductive performance can be useful indicators of population status. These aspects of breeding biology often show considerable spatial and temporal variation. Between 2000 and 2003, we surveyed nesting common eiders ( Somateria mollissima ) on 172 islands in three archipelagos (Nain, Hopedale, Rigolet) on the Labrador coast. Rigolet was the largest archipelago (2834 km²) followed by Nain then Hopedale, and island density varied inversely with archipelago size. Overall means were: nest density 52.0 ± 141.9 (SD) nests/ha; nest initiation 12 June ± 12 days; clutch size 3.7 ± 1.2 eggs/nest; egg volume 98.8 ± 10.4 cm³; and clutch volume 392.3 ± 135.0 cm³. Rigolet had the highest average egg volumes and nest densities, the highest single island nest density of 1053 nests/ha, and the earliest average nest initiation date. We found significant differences in nest densities among archipelagos and across years; significant archipelago and year interactions were detected for nest initiation date and clutch size. Significant differences were found among individual islands for all response variables except egg volume. For egg volume, within-archipelago island differences were not significant, but between-archipelago differences were significant. Thus egg volume may be a useful diagnostic to identify population affiliation.