VARIATIONS IN NUMBERS OF NONBREEDING GREAT SKUAS ATTENDING A COLONY

Diurnal and seasonal variations in numbers of nonbreeding Great Skuas Catharacta skua on Foula, Shetland, were monitored during the 1975, 1976, 1988 and 1989 breeding seasons. The accuracy and use of daytime and night-time counts are compared. All nonbreeders spend the night at clubs and most go to sea to feed in early morning. Hence, night-time counts represent total numbers of nonbreeders attending the colony, whilst daytime counts are more variable, although consistently timed afternoon counts can provide useful diurnal and seasonal comparisons. The mean number of nonbreeders attending the colony between May and August was significantly less in 1988 (732 individuals) and 1989 (662 individuals) than in 1975 (approximately 1949 individuals). This decline in numbers of nonbreeders since the mid-1970's coincided with a decline in the breeding population over the same period, although nonbreeding numbers decreased much more (breeders by 8% from 1975 to 1989, nonbreeders by 57%). Numbers of nonbreeders at the colony correlated with seasonal changes in diet that appeared to indicate changes in food availability. A reduced afternoon attendance by nonbreeders in 1989 (40%), compared with 1976 (67%), reflected unusually poor Great Skua breeding seasons on Foula in 1988 and 1989. Numbers of nonbreeders attending a colony appear to be good indicators of environmental changes that could usefully be added to current seabird monitoring programmes.