Timing and seasonal changes in Eider Somateria mollissima spring migration in the northern Öresund, south Sweden, 1975–2001

This paper describes the spring migration of Eiders Somateria mollissima through the Öresund, south Sweden, during the years 1975 to 2001. The birds are leaving their wintering grounds in the Kattegat to breed in the Baltic. The passage takes place in the second half of March and the first half of April with the major migration period between 22 March and 13 April (median date 4 April). Over the study years, there is a significant trend that the migration takes place earlier in the season. This is both in terms of median date and onset of the migration period. The change in the timing of the migration appears to be related to milder winter weather and earlier springs during the end of the 20th century. It is shown that the spring passage of Eiders for individual years is significantly correlated with the ice indices for Swedish seas the preceding winter.