The population dynamics of the Eider Duck Somateria mollissima and evidence of extensive non‐breeding by adult ducks

The dynamics of an Eider Duck population have been investigated over 25 years, using census and capture‐mark‐recapture methods. During the study the population increased two and a half fold, with two periods of major increases in numbers, giving a stepped growth pattern. Mean clutch size showed considerable annual variation, the extremes being 5.40 and 3.78 eggs. The variation in clutch size was greater than that recorded in the Netherlands. Adult female Eiders had a high annual survival rate, averaging 0.895, and varying between 0.75 and 1.00 in individual years. The survival rate decreased markedly in the old ducks. There was no indication of any change in the survival rate during the study. Recruitment of ducks to the breeding group was irregular, with most years showing little recruitment and a few years high recruitment. However, recruitment, associated with good duckling survival, appears to have been the main factor associated with increase in the population. In many years, an appreciable proportion of the surviving ducks, which had already bred in a previous year, failed to nest. The extent of non‐breeding increased during the study and in one year, 1973, this reached 65%. Lower clutch size and adult survival were associated with years of high non‐breeding. The ‘red‐tides’ in 1968 and 1975 appeared to have little effect on the Eider. It is suggested that the Eider missed breeding in years in which its survival was potentially poor, in order to maximize its reproductive output during its life span. This is supported by the smaller clutch size laid by those females which nest in years when many females fail to breed. It is suggested that young ducks may also miss breeding in the year after first nesting but this is not associated with the non‐breeding in older ducks, although it may he related to body condition. It is suggested that non‐breeding by adults of long‐lived birds may be widespread. This has important implications in survey work based on nest counts.