Blood-feeding ectoparasites as developmental stressors: Does corticosterone mediate effects of mite infestation on nestling growth, immunity and energy availability?

How resources are distributed to growth and self-maintenance early in life is likely to impact survival and reproduction. Early resource allocation decisions may be particularly critical in altricial birds, as they have rapid developmental trajectories, and may be highly susceptible to environmental factors that can perturb development. The aim of this study was to determine if blood-feeding ectoparasites act as developmental stressors in European starling (Sturnus vulgaris) nestlings, driving a trade-off between growth and immunity. We hypothesized that because ectoparasites compete for resources they would induce growth-immunity trade-offs in parasitized nestlings. We also tested the hypothesis that changes in plasma corticosterone mediate the effects of ectoparasites on growth and immunity. Throughout development we assessed between-nest variation in ectoparasite density, measured growth, and a variety of blood parameters, including plasma corticosterone. We also assessed immune function across development. We found that nestlings from nests with high levels of ectoparasites were smaller, had elevated blood glucose, lower hematocrit levels, and appeared to engage in compensatory growth prior to fledging. They also had elevated innate immune responses early, but reduced responses later relative to nestlings from nests with low levels of ectoparasites. Plasma corticosterone was not affected by ectoparasite load, but did increase with nestling age. Overall, we find evidence that ectoparasites are developmental stressors that affect growth-immunity trade-offs, but their effects do not appear to be mediated by changes in circulating levels of corticosterone.