Skua

Abstract: Several different techniques have been used to study the diet composition of skuas and gulls in polar regions. In this study, we assessed the diet of great skuas using five techniques (pellets, prey remains, spontaneous regurgitates, observed feeds and water off-loading). The estimates of diet derived from each sampling technique during a single breeding season were generally similar. Although it is easy to collect large samples of pellets and remains, these over-estimate the amount of indigestible material in the diet. Prey remains provide a high degree of taxonomic accuracy, but may under-estimate prey swallowed whole. Water off-loading provides unbiased estimates of chick diet but only during a short period in the breeding season. It may also stress birds, and is labour-intensive. Spontaneous regurgitates are useful for assessing chick and adult diet but are difficult to collect systematically, and differing prey digestibility may bias results. Observed feeds are time consuming to record and over-represent easily identifiable prey. Technique-dependent biases highlight that sampling methods should be selected on the basis of sample sizes, time, taxonomic detail and age of study birds. Biomass may be estimated using pellets with correction factors, and also spontaneous regurgitates and water off-loading, but prey remains and observed feeds may be more inaccurate.

Abstract: Life history theory predicts a trade-off between current reproduction and survival to maximize fitness. In long-lived seabirds, one might expect adults not to jeop- ardize survival by excessive reproductive investment when environmental conditions for breeding are adverse. Some environmental factors have reduced the fecundity of Black- legged Kittiwakes (Rissa tridactyla) breeding in Shetland, UK, a metapopulation that has declined substantially during the last two decades. Two hypotheses are proposed to explain this decrease: (1) reduced availability of sandeels (the main prey for kittiwakes), which has occurred since the late 1980s; and (2) increased predation by Great Skuas (Catharacta skua), the main predator of kittiwakes. Detailed data on food supply (sandeel stock size, by age groups), kittiwake body condition and breeding success, and diet and productivity of Great Skuas allowed us to assess the influence of each of these factors on adult kittiwake survival from 1986 through 1997. Capture-recapture models were constructed to examine correlations between local adult survival rate and environmental parameters. Body condition at the end of the breeding season and availability of 0-group (juveniles ? 1 yr old) sandeels positively influenced adult survival, whereas survival was lower when Great Skua pro- ductivity was high. Adult survival varied greatly during the study, from 0.53 + 0.04 to 0.98 ? 0.01 (mean + 1 SD), and was independent of sex. Both hypotheses were supported. Contrary to predictions of life history theory, kittiwakes did not refrain from breeding in years of poor food supply or high predator activity to buffer their survival at the cost of not rearing chicks. Although 0-group sandeel availability affected breeding success of kittiwakes, its influence on adult survival was also significant. The discovery that food supply affects adult survival rate has important implications for fisheries management to minimize impacts on seabirds.

Abstract: (1) Great skuas Catharacta skua Brunnich have among the highest tissue mercury concentrations of British seabirds, and many of the birds breeding on Foula, Shetland in 1988 and 1989 had concentrations well in excess of those known to affect reproduction adversely in some terrestrial bird species. Comparison of mercury concentrations in feathers of great skuas collected before 1900 and in 1988-89 indicates a significant increase during this century, which is assumed to be due to anthropogenic activities (Thompson 1989). (2) Mercury in muscle tissues of great skuas was entirely methyl mercury, whereas approximately half the mercury in liver and kidney tissue was inorganic. Feather mercury concentrations correlated with total mercury concentrations in soft tissues of birds culled during incubation. (3) Mercury concentrations in feathers of adult great skuas on Foula, Shetland were found to be higher than those in feathers of chicks, but were independent of adult age and sex. (4) There was no evidence of accumulation of inorganic mercury in soft tissues with age. This suggests that dietary variation and specialization are more important than age as determinants of mercury concentrations in this species. (5) There was no relationship between diet during the breeding season and feather mercury concentration, either comparing mercury concentrations in different years covering a period of major change in diet at the colony, or comparing different birds with markedly different diets in the same year. Great skuas disperse widely during the winter, and differences in exposure to mercury during this period may be more important than differences in diet during the breeding season. (6) Despite evidence of increased mercury pollution (Thompson 1989), there was no evidence of a relationship between the mercury concentrations of individuals and their breeding performance or survival.