Studies on the biology of sea birds with particular reference to the Eider (Somateria mollissima)

Abstract

The thesis has been divided into three parts, Part A is concerned with the biology of the Eider, Part B deals mainly with the distribution and movements of the Eider around the Farne Islands and the coasts of Northumberland and S. E. Scotland. The mortality rates of Black headed gulls and Gannets and their causes of mortality have been studied in Part C. The biology of the Eider was studied under the following headings: pre-laying behaviour, breeding season, laying frequency and time of laying, clutch size, incubation period, desertion, hatching and hatching success, activity, tidal rhythm and sex ratio. In Part B it has been shown that there is a local distribution as well as a moult migration in the Eider, particularly in the males, which move to the Firth of Forth from the Farne Islands to moult. From the data examined in Part C it has been found that the cause of mortality in Black headed gulls varies throughout their life history, young birds being more prone to shooting than adults. The mortality rate was found to be 17.8 per cent. Gannets also were found to have varying cause of mortality throughout their life history, though in this case young birds were more likely to be caught in fishing nets. Using the same method to calculate the mortality rate an impossible result was obtained and this was thought to be due to biased ringing. Recoveries from the South African Gannet were used in the calculations and a new mortality rate of 5.3 per cent was found which is a more realistic value. An appendix of the numbers of Eiders counted during the twelve month period July 1961, to June 1962, along the coasts of Northumberland and South East Scotland have been included at the back of the thesis, also further data on clutch size.