Studies on the breeding biology and population dynamics of the great skua (catharacta skua brűnnich)

Abstract

The history and breeding biology of the-North Atlantic Great Skua Catharacta skua skua Brűnnich was examined to determine its status in seabird communities, and to find reasons for current population increases. A variety of colony census methods are critically compared. Changes in colony sizes in Iceland, Faroe and Britain, which appear to be genetically isolated populations, are documented in detail. Evidence is presented suggesting that the Great Skua first colonised the northern hemisphere around 1500 AD. Ringing- recoveries are analysed to examine movements of known-age birds through the year. Causes of mortality are examined in relation to age and area of recovery. Attendance of prebreeders is described. A computer model is used to estimate energy requirements of Great Skua and other seabird populations. Model precision is examined using sensitivity analyses and a Monte Carlo method. Seasonal variations in Great Skua diet were examined by analyses of pellets and regurgitates. Time budgets of breeders and prebreeders were compared, and food preferences determined. The relative importance of predation and kleptoparasitism as feeding techniques, and their effects on other species are discussed. Methods were developed to classify eggs into first or second laid, and to estimate hatching dates from egg densities. Rates of chick growth were studied in relation to hatching status and date. Conversion tables were prepared to age chicks and to determine an index of body condition from measurements. Pesticide and pollutant concentrations were examined in adults and eggs, as were the influences of these, of age, experience, nest density and food-availability, on breeding success. Effects of interactions with nesting Arctic Skuas were investigated, and factors influencing success of kleptoparasitic chases by both species were determined. Adult and first year survival rates were estimated and rates of colony increase compared. Future population changes are discussed speculatively.