Population ecology of Arctic skuas Stercorarius parasiticus, on Foula

• Main Author: Phillips, Richard A.
• Format: Thesis
• Language: unknown
• Published: University of Glasgow 1995
• Subjects: Ecology.; Arctic; Arctic skua; Arctic tern; Stercorarius parasiticus; Sterna paradisaea
• Online Access: https://dx.doi.org/10.5525/gla.thesis.71712; http://theses.gla.ac.uk/id/eprint/71712

1. The taxonomy, breeding distribution, current status and role of plumage polymorphism in the population dynamics of Arctic skuas are discussed. Information on recent changes in sandeel recruitment in Shetland waters is provided, and some general methods used throughout the thesis are outlined. 2. Both Arctic skua chick growth and fledging success were depressed during the years of low sandeel Ammodytes marinus availability, particularly from 1987 to 1990. Adults spent considerably longer foraging in 1987 than they did in 1979, or from 1992 to 1994. However, they were able to breed with moderate success up until at least 1986, in sharp contrast to Arctic terns which failed from 1983 to 1990. 3. The relationships between both chick growth and number of chicks fledged per pair of Arctic skuas, and fisheries-based and avian indices of food availability were better described by threshold effects rather than linear functions. 4. The size of the Arctic skua breeding population in each year and the level of net recruitment (the difference between the observed numbers in a given year and the expected number of breeders surviving from the previous year) correlated with the size and productivity of the Arctic tern breeding population on Foula and an independent fisheries-based index of 0-group and 1-group sandeel abundance in Shetland waters. 5. There was significant annual variation in mean hatching date, clutch size and egg volume, but not hatching success or chick survival to a week. Mean clutch size and egg volume were lowest in 1979 in comparison with 1987 and 1992-1994. Hatching date showed a strong positive relationship with the net recruitment index. It was suggested that these results were a consequence of a change in the age structure of the Arctic skua population since the late 1970s. 6. Analysis of kleptoparasitic interactions of Arctic skuas Stercorarius parasiticus foraging within sight of Foula indicated that the skuas were able to switch hosts if particular species (notably Arctic terns Sterna paradisaea) were breeding unsuccessfully in a given year. 7. Breeding Arctic skua adults appeared to be in poorer body condition in 1988, the second year of particularly low sandeel recruitment, and there was also strong evidence that many established pairs deferred breeding in that year and in 1990. Changes in behaviour may therefore act as a buffer allowing Arctic skuas to withstand some degree of reduced prey availability but this appears to involve at least some longer-term costs. (Abstract shortened by ProQuest.).