Repeatability and individual correlates of microbicidal capacity of bird blood

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 7 years 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.