1. A cornerstone concept of ecological immunology is that immune function, interacting with various aspects of individual health state, plays a central role in the life‐history trade‐offs between conflicting demands of survival and reproduction. In order to develop this research, more knowledge about the applicability and usefulness of different health state assays is needed. 2. Eleven, mostly hemato‐serological, health state indices are described and their suitability for sensing the condition of breeding Great Tits (Parus major L.) in terms of measurement precision, constancy in time, diurnal variation, and sex‐ and site‐related differences, is examined. 3. Measurement errors for the plasma albumin content, residual body mass, heterophile/lymphocyte ratio and total plasma protein content were relatively small compared with the total variation, suggesting these indices to be most adequate for ecological research. Measurement precision was lowest for the heterophile count and ‘buffy coat’ layer height (relative amount of leucocytes in total blood volume). Buffy coat layer height correlated weakly (r=0·21) with total leucocyte count estimated from blood smears and therefore appeared inappropriate for estimation of the leucocyte number. 4. Body mass (residual in respect to size) and intensity of Haemoproteus blood parasite infection were the least variable state indices during the nestling period (for both, the correlation between the values measured on the 8th and 15th days of the nestling period=0·71). Haematocrit, heterophile count and albumin/globulin ratio showed no individual constancy across the nestling period, while other traits revealed moderate but statistically significant correlations between 8th‐ and 15th‐day values. 5. Leucocyte (both lymphocyte and heterophile) counts were higher among females captured at night compared with those captured during the day. 6. Females had higher intensities of Haemoproteus infection, higher heterophile counts and higher heterophile/lymphocyte ratios than males. Contrary to published information, females had higher haematocrits than males. 7. Haematocrit values in both sexes, as well as total plasma protein and albumin concentrations in males, differed significantly between Great Tits breeding in urban habitat and rural woodlands, respectively.

The costs of exploiting an organism's immune function are expected to form the basis of many life-history trade-offs. However, there has been debate about whether such costs can be paid in energetic and nutritional terms. We addressed this question in a study of wintering, free-living, male great tits by injecting them with a novel, non-pathogenic antigen (sheep red blood cells) and measuring the changes in their basal metabolic rates and various condition indices subsequent to immune challenge. The experiment showed that activation of the immune system altered the metabolic activity and profile of immune cells in birds during the week subsequent to antigen injection: individuals mounting an immune response had nearly 9% higher basal metabolic rates, 8% lower plasma albumin levels and 37% higher heterophile-to-lymphocyte ratios (leucocytic stress indices) than sham-injected control birds. They also lost nearly 3% (0.5 g) of their body mass subsequent to the immune challenge. Individuals that mounted stronger antibody responses lost more mass during the immune challenge. These results suggest that energetic expenditures to immune response may have a non-trivial impact upon an individual's condition.

Summary 1.Many birds sequester carotenoid pigments in colourful patches of feathers to advertise or compete for mates. Because carotenoids can be scarce in nature and serve valuable physiological functions, only the highest-quality individuals are thought to acquire or allocate more pigments for use in sexual displays. 2. A critical but rarely tested assumption of carotenoid-based signals is that the colour of pigmented feather patches directly reveals the total amount of carotenoids contained within them. 3. We studied the relationship between carotenoid-based coloration (hue, chroma and brightness) and the pigment content of tail feathers in wild-caught and captive male greenfinches ( Carduelis chloris [Linnaeus]). Greenfinches incorporate two main carotenoids − canary xanthophylls A and B − into feathers to develop yellow patches of colour in their tail. 4. Variation in feather carotenoid content explained 32-51% of variation in chroma and hue of the yellow parts of tail feathers, while feather brightness was not significantly related to carotenoid concentration. Hence, chroma and hue appear good candidates to indicate feather carotenoid content. 5. Birds with the most colourful feathers deposited significantly more of both canary xanthophylls into plumage. Thus, there does not appear to be a specific biochemical strategy for becoming colourful in greenfinches; males instead follow the general decision rule to deposit as many xanthophylls as possible into feathers to become yellow.

Carotenoid-based sexual coloration has been hypothesised to be prevalent across many vertebrate taxa because it reliably reflects individual phenotypic quality in terms of foraging efficiency or health status due to the trade-off between signal colour and use of carotenoids for immune function and detoxification. We investigated the ventral, yellow coloration of breeding adult great tits (Parus major L.) in relation to sex, age, breeding habitat, local survival and infection status with respect to Haemoproteus blood parasites. The extent of plumage coloration (estimated as hue and lutein absorbance) was generally higher in rural than in urban birds. Males had higher values of hue than females. In both male and female yearlings, the plumage of unparasitised individuals had a greater hue of yellow than parasitised ones, while older males revealed the opposite pattern. The survival of infected yearlings was worse than that of uninfected yearlings, while the opposite was true for old breeders. Survivors had generally higher values of hue than non-survivors. These results are consistent with predictions of functional hypotheses, suggesting that carotenoid-based plumage coloration serves as a signal reflecting individual quality in terms of health status and local survival.