The Breeding Ecology of Great Skuas Catharacta skua in Shetland over a Period of Declining Sandeel Stock

A discriminant function was generated to determine the sexes of great skuas on Foula, Shetland from external measurements. The accuracy of the function was assessed and its possible future applications were discussed. Changes in the breeding ecology of great skuas on Foula were examined over a sixteen year period covering a major reduction in stocks of sandeels around Shetland. Results were discussed with particular reference to the predictions of life history theory. Ring recovery data and annual returns of colour-ringed adults to the great skua colony on Foula were used to examine possible changes in survivorship over the period of declining sandeel stocks. Data from breeding adults of known age were used to assess whether breeding performance improved with age during a period of food shortage, and to test predictions of life history theory concerning age-specific differences in reproductive effort. Several hypotheses were tested concerning individual, seasonal and year to year variation in the aggression displayed by great skuas during brood defence. Samples of feathers taken from breeding adults were used to examine whether accumulation of mercury by great skuas on Foula effected their breeding performance or success during a period of food shortage. The results were discussed in terms of possible implications for future work on the effects of pollutants upon breeding performance