Summary1. The aim of this study was to examine the mechanisms by which parasites can affect the expression of ornamental traits. 2. Levels of an intestinal coccidian parasite, Isospora lacazei, were manipulated in captive male greenfinches (Carduelis chloris) by suppressing the natural infections with a coccidiostatic sulphonamide drug. Subsequently, half the birds were experimentally infected, while another half continued receiving medication. 3. Over the course of the experiment the effect of our treatments upon 14 mainly haemato-serological condition indices was recorded. Additionally, changes in colour and carotenoid content of yellow tail and breast feathers, which serve as sexually dimorphic ornamental traits, were measured. 4. Eighty-nine per cent of birds hosted chronic isosporan infection before the experiment, yet experimental inoculation with mixed parasite strains resulted in drastic but transient decreases in serum carotenoid, vitamin E, triglyceride and albumin concentrations, and reduced body mass, indicating serious pathology and probable nutrient malabsorption due to damaged intestinal epithelium. 5. Laboratory-grown tail feathers of infected birds contained 52% less carotenoids and also had smaller values of chroma and hue than those of medicated birds. 6. These results suggest that coccidian infection reduced the expression of plumage coloration by creating a deficiency of carotenoids available for deposition in ornamental feathers.

Reactive oxygen and nitrogen species produced by metabolism and immune defenses can cause extensive damage to biomolecules. To counteract this damage, organisms rely on exogenous and endogenous antioxidants, although their relative importance in maintaining redox balance is unclear. We supplemented captive greenfinches with dietary antioxidants--carotenoids and vitamin E--and injected them with an inflammatory agent, phytohemagglutinin. Compared to controls, immune-challenged birds circulated more lipid peroxidation products but also increased total plasma antioxidativity. Carotenoid (but not vitamin E) supplementation generally reduced lipid peroxidation, but this did not compensate for the effects of immune activation. Levels of an endogenous antioxidant--uric acid--strongly contributed to plasma antioxidativity. We found no evidence that dietary antioxidants are immunostimulatory. These results demonstrate the antioxidant function of carotenoids in birds and show that simultaneous assessment of oxidative stress-driven damage, antioxidant barrier, and individual antioxidants is critical for explaining the potential costs of immune system activation.

SUMMARY Costs accompanying immune challenges are believed to play an important role in life-history trade-offs and warranting the honesty of signal traits. We performed an experiment in captive greenfinches (Carduelis chlorisL.) in order to test whether and how humoral immune challenge with non-pathogenic antigen [sheep red blood cells (SRBC)] affects parameters of individual condition including intensity of coccidian infection, estimates of total antioxidant protection, plasma carotenoids and ability to mount a cell-mediated immune response. We also asked whether the potential costs of immune challenge can be alleviated by dietary carotenoid supplementation. None of the treatments affected intensity of coccidiosis. Humoral immune challenge suppressed the cell-mediated response to phytohemagglutinin (PHA), suggesting a trade-off between the uses of different arms of the immune system. Immune challenge reduced body-mass gain, but only among the carotenoid-depleted birds, indicating that certain somatic costs associated with immune system activation can be alleviated by carotenoids. No evidence for oxidative stress-induced immunopathological damages could be found because immune activation did not affect total antioxidant protection or carotenoid levels. Carotenoid supplementation inclined birds to fattening, indicating that lutein interfered with lipid metabolism. Altogether, our results support the hypotheses of biological importance of carotenoids and exemplify the overwhelming complexity of their integrated ecophysiological functions.

The main tenet of immunoecology is that individual variation in immune responsiveness is caused by the costs of immune responses to the hosts. Oxidative damage resulting from the excessive production of reactive oxygen species during immune response is hypothesized to form one of such costs. We tested this hypothesis in experimental coccidian infection model in greenfinches Carduelis chloris. Administration of isosporan coccidians to experimental birds did not affect indices of antioxidant protection (TAC and OXY), plasma triglyceride and carotenoid levels or body mass, indicating that pathological consequences of infection were generally mild. Infected birds had on average 8% higher levels of plasma malondialdehyde (MDA, a toxic end-product of lipid peroxidation) than un-infected birds. The birds that had highest MDA levels subsequent to experimental infection experienced the highest decrease in infection intensity. This observation is consistent with the idea that oxidative stress is a causative agent in the control of coccidiosis and supports the concept of oxidative costs of immune responses and parasite resistance. The finding that oxidative damage accompanies even the mild infection with a common parasite highlights the relevance of oxidative stress biology for the immunoecological research.

The question why different host individuals within a population differ with respect to infection resistance is of fundamental importance for understanding the mechanisms of parasite‐mediated selection. We addressed this question by infecting wild‐caught captive male greenfinches with intestinal coccidian parasites originating either from single or multiple hosts. Birds with naturally low pre‐experimental infection retained their low infection status also after reinfection with multiple strains, indicating that natural infection intensities confer information about the phenotypic ability of individuals to resist novel strains. Exposure to novel strains did not result in protective immunity against the subsequent infection with the same strains. Infection with multiple strains resulted in greater virulence than single‐strain infection, indicating that parasites originating from different host individuals are genetically diverse. Our experiment thus demonstrates the validity of important but rarely tested assumptions of many models of parasite‐mediated selection in a wild bird species and its common parasite.