Swennen C. (1983) Reproductive output of Eiders Somateria m. mollissima on the southern border of its breeding range. ARDEA 71 (2): 245-254

The Dutch West-Frisian Islands represent the most southern offshoot of the breeding range of the Eider in Europe. In the Dutch breeding colonies, reproduction parameters of the Eider were determined during a number of years in the period 1959-1982. These data were compared with those of colonies more to the north. The mean clutch size was 4.74 eggs (s.d. 0.16), range 4.45-4.95 (12 years, 3,070 clutches). This mean clutch-size does not differ markedly from records of colonies more to the centre of the breeding range. The yearly differences in mean clutch size are even smaller than in the other colonies. The mean hatching success in the protected breeding places in The Netherlands was high, amounting to more than 90% in the years 1959-1979. Only during the period 1962-1969 was the nesting success lower, as was survival of adult females in the nesting period as a consequence of pollution of the foraging grounds by chlorinated hydrocarbons. Unhatched eggs were only rarely found in nests which produced young. Nest and hatching success were at least as high as in the most successful colonies elsewhere in the breeding range. Duckling mortality in the nursery area (Wadden Sea) was, however, variable and usually very high. In the Vlieland colony of 1,700-2,200 nests, the mean number of fledglings produced per female per year was only 0.29 (range 0.00-0.85, 8 years). The production per female in the 5 colonies studied was unequal, but the fluctuations from year-to- year ran parallel. The average fledgling production over all colonies was 0.47 per female per year. The proportions of the eggs laid that resulted in a fledgling were much lower than in the colonies in the centre of the breeding range. Thus, high duckling mortality may be the key factor determining the southern border of the distribution range of the Eider in Europe.