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.