Trade-offs between life history traits and immune defence in the collared flycatcher Ficedula albicollis

Abstract: This thesis examines the effects of reproductive effort on immune function and parasite resistance in breeding collared flycatchers, Ficedula albicollis. It also focuses on the role of the immune function in the trade-off between number and fitness of offspring.The reproductive effort of collared flycatchers was altered by manipulating the number of offspring in their broods. Increased effort affected negatively immune function and parasite resistance. The total immunoglobulin and leukocyte levels decreased, as did the specific immune responses to Newcastle disease virus vaccine, whereas the intensity of Haemoproteus infections increased. Increased effort reduced the probability of survival. Recapture of females subjected to experimental manipulation the previous year showed that increased effort had long-term negative effects on reproduction and parasite resistance. Prevalence of haematozoan infections increased, breeding onset was delayed and clutch size reduced. The data suggest a trade-off between investment in reproduction and immune defence, and increased susceptibility to infectious disease via immune suppression as a pathway for the life-history cost of reproduction.During the mating/oviposition period, females had more than twice as high prevalence of Trypanosoma infections compared to males, whereas males tended to be more frequently infected by Haemoproteus. During the subsequent nestling feeding period, the total haematozoan prevalence was male-biased. Leukocyte and erythrocyte levels were also sexually dimorphic. It is suggested that, as a result of differential selection pressures during reproduction, males and females have different strategies for allocating resources to immune defence and reproductive effort.Nestling immunocompetence was assessed by measuring the ability to degrade/remove a subcutaneously applied immunogen. Two acute phase proteins were quantified as a means to assess the effect of experimental brood size on the susceptibility to natural infections. Poor immunocompetence and increased levels of acute phase proteins were associated with low body mass and reduced survival. Increased brood size affected nestling growth rate negatively and reduced their immunocompetence. However, no detectable effects on infection susceptibility were observed. Three explanations are suggested: i) negligible effects of reduced immunocompetence, ii) protection by maternally acquired antibodies, iii) adaptive reaction norms. Under poor rearing conditions, offspring may be prepared to cut down on an already poor growth process to secure immune function and thereby survival.