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Abstract: With the rapid development of the field of ecological and evolutionary immunology, a series of new techniques to measure different components of immune function is becoming commonplace. An important step for the interpretation of these new measures is to understand the kind of information about the animal that they convey. We showed that the microbicidal capacity of Stonechat (Saxicola torquata) blood, an integrative measure of constitutive immune function, is highly repeatable when tested against Escherichia coli and not significantly repeatable when tested against Candida albicans. The low repeatability against C. albicans results from relatively low variation among individuals, providing only low resolution to identify if this interindividual variation is consistent. In addition, we explored the effect of sex and age on microbicidal capacity, and found that over a range of ages from 1 to 7years the blood of older birds had a better capacity to kill microbes. We concluded that, over a time period of weeks, microbicidal capacity of avian blood is an individual-bound trait, that shows consistent interindividual variation partly related to age, and unaffected by sex. This knowledge is important when interpreting the possible evolutionary mechanism underlying immunological differences, for example among individuals, environments and seasons. [Copyright &y& Elsevier]

Some equatorial environments exhibit substantial within‐location variation in environmental conditions throughout the year and yet have year‐round breeding birds. This implies that breeding in such systems are potentially unrelated to the variable environmental conditions. By breeding not being influenced by environmental conditions, we become sure that any differences in immune function between breeding and non‐breeding birds do not result from environmental variation, therefore allowing for exclusion of the confounding effect of variation in environmental conditions. This create a unique opportunity to test if immune function is down‐regulated during reproduction compared to non‐breeding periods. We compared the immune function of sympatric male and female chick‐feeding and non‐breeding red‐capped Calandrella cinerea and rufous‐naped larks Mirafra africana in equatorial East Africa. These closely‐related species occupy different niches and have different breeding strategies in the same grassland habitat. Red‐capped larks prefer areas with short grass or almost bare ground, and breed during low rainfall periods. Rufous‐naped larks prefer areas of tall grass and scattered shrubs and breed during high rainfall. We measured the following immune indices: nitric oxide, haptoglobin, agglutination and lysis, and measured total monthly rain, monthly average minimum (Tmin) and maximum (Tmax) temperatures. Contrary to our predictions, we found no down‐regulation of immune function during breeding; breeding birds had higher nitric oxide than non‐breeding ones in both species, while the other three immune indices did not differ between breeding phases. Red‐capped larks had higher nitric oxide concentrations than Rufous‐naped larks, which in turn had higher haptoglobin levels than red‐capped larks. Tmax was higher during breeding than during non‐breeding for red‐capped larks only, suggesting potential confounding effect of Tmax on the comparison of immune function between breeding and non‐breeding birds for this species. Overall, we conclude that in the two year‐round breeding equatorial larks, immune function is not down‐regulated during breeding. [ABSTRACT FROM AUTHOR]