A long‐term study of the population dynamics of Common Eiders Somateria mollissima: why do several parameters fluctuate markedly?

Abstract

Capsule The population dynamics of an Eider population has been studied from1958 to 2006 to evaluate the causes of changes in abundance and breeding potential. Aims To evaluate the changes in clutch size, survival rate of adult ducks, the extent of non‐breeding, and recruitment to the breeding population of Common Eiders nesting on Coquet Island, Northumberland. Methods Survival rates, recruitment and size of the population have been determined by an annual mark-recapture study, while the numbers of nests and clutch size have been directly recorded each year. Results There was marked annual variation in the number of ducks alive, their mortality rates, extent of non‐breeding, clutch size and recruitment of adult ducks. Overall, the annual survival rate and clutch size decreased, and numbers of adult ducks, and presumably the population, peaked on two occasions. Conclusions No density‐dependent effects were identified and the size of the population was mainly determined by changes in the adult survival rates. Because of non‐breeding, nest counts were an unreliable measure of the population size. It is suggested that food shortage is the main factor that has produced the long‐term changes in the population and there is an indication that adverse conditions for Eiders have extended over much of northern Europe in recent years. Identification of why the duckling survival rates were so low and whether these can be increased offers the most likely means of active conservation of this species and would involve management of feeding sites rather than breeding areas.