Movements of Eiders Somateria mollissima on the east coast of Britain

Abstract

Movement patterns, sex differences in natal dispersal and breeding dispersal, and interchange of birds between colonies were studied in the population of Eiders Somateria mollissima breeding on the east coast of Britain. First‐winter Eiders reared at the Sands of Forvie, Grampian, remain at or close to the colony, while most adults move about 100 km south to winter on the Firths of Forth and Tay. A proportion of the Forvie population is sedentary. Eiders which breed in Northumberland either move north to winter on the Tay and Forth estuaries or remain close to their breeding areas. Eiders breeding in Fife are sedentary. Recoveries of British‐ringed Eiders in Scandinavia indicate that some British‐born males join the Baltic breeding population, probably by pairing with Scandinavian females wintering in Britain. There is extensive natal dispersal of males from Forvie, with more than twice as many Forvie‐bred females as males returning to the colony to breed. The breeding dispersal of males is also twice that of females. Dispersal of males from the relatively sedentary Forvie wintering population is less than that from the breeding population. Previous work suggested that at Forvie sedentary birds nesting close to the estuary were genetically isolated from migratory ones nesting along the coast. This situation is less clear cut than had been supposed previously, with many migrants nesting close to the estuary. It is unlikely that the genetic differences between females nesting in different parts of the Forvie colony will remain stable in the long term, due to the natal and breeding dispersal of males.