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Title: Population dynamics of the eider (Somateria mollissima) in north-east Scotland
Author: Baillie, Stephen R.
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 1981
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Long term data on the population of Eiders (Somateria mollissima ) breeding at the Sands of Forvie, Scotland, have been analysed to investigate changes in numbers and regulation of the population. The importance of immigration and emigration and the ability of the population to maintain itself from its low reproductive output were also investigated. Most of the adult population migrates to the Tay and Forth estuaries in winter, but several hundred birds remain on the Ythan estuary throughout the winter. First year birds usually disperse to coastal areas within 50 km of Forvie. The movement patterns of the population are described in detail. The size of the summer and winter populations increased slowly over the last 20 years. The average annual rate of increase of the winter population was 4%. Tests for density dependence indicated that both summer and winter populations were regulated. The size of the winter population declined between October and March in a density dependent manner. It is suggested that this is related to a decline in the abundance of the Eider's principal prey species, Mytilus edulis, over the same period. Some female Eiders bred when they were two years old, and most did so by the time they were four. Young females laid later, had smaller clutches and were lighter in weight at the start of incubation than older birds, but egg size did not vary with age. The number of ducklings fledged was very low in 16 out of the 21 years studied. Thus although losses due to breeding failure were the largest mortality factor in the annual cycle, changes in the summer population were not related to variations in duckling production. Egg production and breeding success were not density dependent. Losses of adults between summers were density dependent, and were largely responsible for changes in population size. In important component of these changes was the apparent failure of part of the population to attend the colony in some years. First year mortality was density dependent. Average survival rates were estimated to be 67% for first year birds and 96% for adults. About half the males reared at Forvie emigrated, and some joined the Baltic breeding population. Most Forvie bred females returned to the colony to breed. On average the population was able to maintain itself from its own reproductive output, but limited immigration did take place in some years.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available