Abstract
Compromise of immune function during reproduction may form a link between parental effort and the cost of reproduction, but the role of environmental variation in structuring intra-individual life-history trade-offs has been poorly investigated. We manipulated the need for parental effort in Eurasian kestrels, Falco tinnunculus, by food-supplementing broods for three nestling periods, during which the natural main food supply (voles) varied, and found that parental parasitaemia was inversely related to yearly vole densities. The level of parasitaemia in females was, however, reduced by food supplements. No effect on males was expected, as earlier work has shown that only females responded to the supplements by changing their behaviour. We show directly that the likelihood of female parasitaemia was diminished by spending less time in flight-hunting, which was related to reproduction during a good vole year, to our supplementary feeding, and to being mated to a male with high parental effort. Our results represent a novel direct benefit for females in resource - providing species, linked to female, as well as offspring, well-being, and they provide insight into why the appearance of reproductive costs may be linked to gender or environmental conditions.