NONBREEDERS AS A BUFFER AGAINST ENVIRONMENTAL-STRESS - DECLINES IN NUMBERS OF GREAT SKUAS ON FOULA, SHETLAND, AND PREDICTION OF FUTURE RECRUITMENT

1. The number of breeding great skuas Catharacta skua on Foula, Shetland, was determined by counting apparently occupied territories (AOTs) during the 1988 and 1990 breeding seasons (2400 and 2340 AOTs, respectively, representing 20% of the northern hemisphere population). 2. A decrease in the availability of the preferred food, sandeels Ammodytes marinus, has been accompanied by a slight fall in AOTs, a considerable increase in foraging effort of breeding adults and a 75% fall in breeding success, indicating that food shortage has put the population under stress. 3. Two methods are presented which allow the number of non-breeding great skuas frequenting Foula to be estimated, based on studies of individually colour-ringed birds. Totals of 3303 (SE = 243) and 4013 (SE = 479) non-breeding great skuas were estimated to frequent Foula in the 1989 and 1990 breeding seasons, respectively. These totals are far greater than indicated by individual counts, due to each bird staying in the colony an average of only 16-21 days. 4. Both the number of breeders and the peak number of non-breeders have declined since 1977, although the peak number of non-breeders decreased much more (non-breeders by 80%, breeders by 21.5%). The rapid decrease in numbers of non-breeders appears to be due to increased recruitment as a consequence of a reduction in adult survival rate, since age at first breeding and breeding success did not change before 1987. 5. Because breeding success since 1987 has been only 25% of the previous average, we predict that the dramatic reduction in the number of non-breeders at Foula since the mid-1970s will soon be followed by a rapid reduction in recruitment, and a further and much faster drop in breeding numbers, over the next few years. 6. This study supports the suggestion of Porter & Coulson (1987) that numbers of non-breeders at seabird colonies may provide an early and sensitive indication of environmental changes having adverse effects on the population, since non-breeders act as a buffer to such stress.