A developing organism faces a dilemma: whether to allocate available resources to building its body structures (growth) or to the development of its immune system. The outcome of this tradeoff is likely to be modified by parasites. We manipulated the abundance of ectoparasitic Hen Fleas (Ceratophyllus gallinae) on nestling Blue Tits (Cyanistes caeruleus) by microwaving nests and subsequently adding 200 Hen Fleas (15 infested nests) or not (16 reduced-infestation nests). In addition, we manipulated the host nestlings' food resources by supplementary feeding 10–15% of daily energy needs to half the nestlings in a nest during the key developmental period (days 2–12). Feather growth (tail and wing length) and hematocrit were reduced by the presence of Hen Fleas, indicating negative effects on nestling development. In comparison to the control nestlings, food-supplemented nestlings aged 16 days were larger (tarsus, residual body mass), but only in reduced-infestation nests (interaction between both treatments). Body size of fed male offspring increased in relation to that of females, but only in the absence of ectoparasites. We hypothesized that supplemented resources are allocated to immune defense when ectoparasites are present, but humoral immune function (total immunoglobulin concentration) and cell-mediated immune defense (phytohemagglutinin response) were not affected by either treatment. Either the nestlings allocated additional resources away from growth (into an unknown developmental component) when parasites were abundant, or the ectoparasites preferentially fed on supplementary-fed host nestlings and thereby equalized the development of soma and immune defense of nestlings despite provision of additional resources.