Ardea
Official journal of the Netherlands Ornithologists' Union

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Swennen C., Duiven P. & Reyrink L.A.F. (1979) Notes on the sex ratio in the Common Eider Somateria mollissima (L.). ARDEA 67 (1-2): 54-61
In the years 1975-1977 the sex was established in a total of 3,217 newly hatched Eider ducklings in the breeding colony on the Island of Vlieland, The Netherlands. Not once during these three years did the sex ratio deviate appreciably from unity. In 1978, on the nursery grounds off Vlieland, all 1,174 ducklings present were captured and sexed at the time when they were ready to fledge. The males accounted for 54.2% of the total, a noticeable and statistical significant departure from the balanced sex ratio at birth. Observations on 3,217 Eider ducklings kept in captivity, revealed that during the first four weeks of their life female ducklings are more apt to contact certain fatal diseases than males. It seems reasonable to assume that this' difference also operated in ducklings in their natural habitat, and in fact explains the slight dominance of males at the fledgling stage. When the ducklings were set free no differences in mortality rate between males and females were found from the fourth to the fifteenth week after hatching. In the winter of 1977-1978 a number of photographs were taken from an aircraft showing various clusters of Common Eider, almost all adult, wintering in the Dutch Waddenzee. The total number of Eiders recorded on the photographs was 17,689 with 9,708 males (54.88%), a significant deviation from a balanced sex-ratio. Thus, our data on a differential mortality between males and females in the duckling stage tend to explain the sex ratio observed in adult Common Eider in the Waddenzee. However, the Eider population wintering throughout the Dutch Waddenzee, estimated at 110,000 individuals in 1977-1978, consists of the local birds and a large number of ducks from the Baltic region. Due to a segregation of the sexes, estimates of sex ratios in the population of mature birds seem unreliable, although a slight dominance (approx. 55-60%) of males is suggested by the available material for the combined Baltic-Waddenzee population wintering between southern Sweden and the western part of the Waddenzee. The sex ratio in the Eider population gathering offshore from the breeding grounds in April, some days before the first egg was laid, was even. This is in keeping with earlier recorded findings. However, Eiders gathering much farther offshore towards the centre of the Waddenzee during that same period showed a marked surplus of females. There is evidence of an earlier male surplus in the non-breeding population in the Waddenzee about 6 years before. That proportionately more male than female offspring is produced and that such an imbalance is offset due to an increase in male mortality as reported by Belopolskii, Goryainova & Tarnovskaya (1974) can, on the basis of our findings, not be affirmed.


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