Fitness consequences of variation in natural antibodies and complement in the Barn Swallow Hirundo rustica

1. The immune system and its component parts have evolved and are maintained due to the fitness benefits that hosts acquire in terms of parasite resistance. However, there are relatively few studies of the fitness consequences of natural variation in immunity in free-living populations, mainly due to the complexity of the immune system and logistic problems of assessing immunity and fitness components. 2. We quantified two aspects of humoral immunity in a population of Barn Swallows Hirundo rustica, natural antibodies and complement that constitute the first line of defence against parasites. 3. The magnitude of both immune components increased during the breeding season. 4. Very old adults had reduced levels of natural antibodies, providing evidence consistent with immuno-senescence, while that was not the case for lysis. However, there was no evidence that survivors from one year to the next had stronger agglutination or lysis than nonsurvivors. 5. Females with a second clutch had higher levels of natural antibodies than females that only had a single clutch, while males showed no differences. Annual fecundity of females decreased with natural antibody levels, that was not evident in males. Therefore, in females, natural antibody and complement levels are important predictors of vital rates. 6. Tail length of male partners, but not of females, explained the variation in female natural antibodies and complement, while there was no similar effect in males. This pattern may have arisen from differential parental investment by females mated to the most attractive males. 7. The abundance of four different ectoparasites was not predicted by either immune component.