Female characteristics and parental care mode in the crèching system of eiders, Somateria mollissima

Abstract

Eider females may abandon their young, care alone, or join in multifemale crèches. We studied the characteristics of female eiders adopting these strategies in 1996–1999. Female condition at hatching varied significantly between years. Over all years, 31% of all females abandoned, 23% tended alone and 46% crèched. In the year when average female condition at hatching was lowest, abandonment rate peaked (67%). Crèching birds were further identified as true crèchers staying more than 2 weeks with the original crèche, and transient crèchers leaving the crèche. The condition of females shortly before hatching showed a decreasing trend, with lone tenders being in best condition, followed by true crèchers, transient crèchers and abandoners. Clutch size, date of hatching relative to the population median, and female body size did not differ between groups. Individual females switched between care modes between years. Females weighed significantly less when abandoning than when tending, with no significant weight difference when the females remained as tenders between 2 years. This is consistent with the energetic salvage strategy hypothesis, which states that females in poor body condition should be more prone to abandon their brood. Our results support an adaptive approach to offspring care behaviour in eiders, driven by female condition.