The effect of parasites on survival and reproduction
There are at least two ways in which parasites as a selective force can enter models of life history evolution. First, they may have effects on reproduction or survival in adult birds. In such cases parasites enter life history models as a mediator of reproductive costs; i.e. a reduction in future reproductive success as a result of current reproductive effort. Second, if parasites are more likely to negatively affect the survival of juvenile birds (up until first reproduction), another trade-off in life history theory becomes more important namely the trade-off between number and quality of offspring. The condition of the juvenile bird during development would impact the functioning of the immune system. Thus, conditions during the early life of the birds may have consequences even long after the birds have become independent. Furthermore, selection pressure from parasites may vary spatially between different host species and populations or temporally between different years and seasons. The reason for this variation is potentially interesting, as it will create cohort effects potentially related to reproduction and survival.