The effects of changes in food availability on the breeding ecology of great skuas Catharacta skua in Shetland

Great skuas on Foula, Shetland have responded to a decline in the availability of sandeels since the late 1970s by increasing the proportion of other items in their diets. This change is correlated with the annual recruitment of sandeels in Shetland waters. Since 1983 there has been a 10-fold increase in predation by great skuas upon other seabirds, as Furness & Hislop (1981) suggested might occur in response to a low availability of sandeels. Changes in diet have been accompanied by a 50% reduction in adult territorial attendance as adults increased their foraging effort, such that between 1987 and 1989 breeding adults were probably working as hard as they were able to. Despite this, breeding success was less than 40% in 1987 and less than 15% in 1988 and 1989. The major cause of breeding failure was predation of unguarded chicks by adults from neighbouring territories. The willingness of adults to expose their chicks to high predation risk is probably maintained because of a positive correlation between chick pre-fledging growth and post-fledging survival, which is expressed up to the age of two years and which will place a strong pressure upon adults to feed their chicks as well as possible. The high expenditure of effort by adults in 1987 and 1988 did not affect the weights of those birds incubating eggs in 1988 and 1989, but there was a slight (3%) decrease in egg size between the late 1970s and the late 1980s. Changes in the age structure of the breeding population and the absence in 1989 of 28% of adults colour-ringed during incubation in 1988 suggest an increase in the rate of egress since the 1970s. These changes probably represent an increase in the long-term costs of reproduction to adults at this colony.