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By switching great skuas Catharacta skua from one isotopically distinct diet to another, we measured diet-tissue discrimination factors and tested the assumption that dietary nitrogen and carbon isotope signatures are incorporated into blood and feathers at similar rates. We also examined the effects of metabolic rate and looked for evidence of isotopic routing. We found that blood δ[sup15]N and δ[sup13]C signatures altered after the diet swish at similar rates (14.4 d and 15.7 d, respectively). Qualitative analyses imply that the same was true with feathers. Mass balance calculations suggest that only a small amount of lipid is likely to be involved in the synthesis of Mood and feathers. Differences in diet-tissue discrimination factors before and after the diet switch may mean that toward the end of the experiment, some of the nutrients for blood synthesis had been coming from stores. Repeated measures mixed models provided evidence that increases in metabolic rate might accelerate fractional turnover rates in blood. There is a need for more laboratory-based experimental isotope studies in order to address further questions that this study has raised. [ABSTRACT FROM AUTHOR]

Populations of scavenging seabird species in the North Sea may fluctuate with an artificial food source: the availability of fishery waste. To document this impact, it is necessary to assess the birds' nutritional status during periods with decreased fishing activity. Reference data for this purpose was collected from 22 herring gulls investigated during laboratory fasting. After 6 d of food deprivation and body mass losses exceeding 15%, the first birds entered starvation phase 3. Comparatively, this is a rather weak fasting capacity. Plasma levels of total protein and thyroid hormones decreased and &beta-hydroxybutyrate increased with fasting duration. The leucocyte proportions were shifted from lymphocytes to heterophils. After 3 d of refeeding, most of the fasting changes were reversed. Plasma enzyme activities increased and hematocrit, hemoglobin, and erythrocyte numbers decreased in both fasting and control birds, most likely as a result of experimental stress and repeated blood sampling. Glucose, cholesterol, monocytes, basophils, and glycosylated hemoglobin remained fairly constant. Triglycerides, free fatty acids, uric acid, and urea varied significantly, but changes were not as clearly a result of fasting. Therefore, total protein, &beta-hydroxybutyrate, triiodothyronine, thyroxine, and lymphocyte and heterophil percentages may be the most reliable indicators of the nutritional status and the condition of free-living herring gulls. [ABSTRACT FROM AUTHOR]

Parasites and disease have been increasingly recognized as agents of ecological and evolutionary importance. Because the endocrine and immune systems are related, it is also likely that hormonally controlled traits, such as secondary sexual characters, may be influenced by responses to disease. Testosterone appears to be associated with a suppressed immune system in many vertebrates, and evidence from captive Red Jungle Fowl suggests that the sexes differ in blood parameters involved in immunity and that comb length is negatively correlated with lymphocyte number in males. Immune system parameters may provide a useful gauge of condition in captive and field-caught animals. Key words: disease ecology; endocrine-immune interactions; Red Jungle Fowl; sex differences; sexual selection., INTRODUCTION Over the past decade, ecologists have become increasingly aware of the effect of parasites and disease on the population biology of their hosts (Dobson and Hudson 1986, Toft and [...]