Factors affecting duckling survival of Eiders Somateria mollissima in northeast Scotland

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Abstract
Mortality factors affecting Eider ducklings Somateria mollissima and their impact with respect to age were studied on the Ythan estuary, Aberdeenshire, Scotland. Survival from hatching to fledging averaged 10.4% over 13 years; numbers declined steeply during the first three to four weeks and slowly thereafter. Growth of tagged young varied considerably between broods. Mortality was not correlated with numbers of ducklings or adults.Predation in 1974 averaged 13.6% per day during the first two weeks, and accounted for most mortality. Herring Gulls Larus argentatus were the principal predators. Predation was 2.5 times as high in rainy, windy weather as in fine, calm weather. Weather, primarily mean rainfall and wind speed, during early life accounted for over 90% of all variation in survival during eight of nine years. Predation was considered the proximate factor in most weather‐related mortality. High survival in 1971 was not explained by weather. An estimated 25–40% of mortality during the second week of life was from renal coccidiosis Eimeria somateriae. Reproductive output of the Ythan population probably limits the rate of population growth, but numbers of breeding birds seem to be regulated by other factors acting after fledging.