Trophic structure of arthropods in Starling nests matter to blood parasites and thereby to nestling development

Nestling development and long-term survival in many bird species depend on factors such as parental feeding, time of breeding and environmental conditions. However, little research has been carried out on the effect of ectoparasites on nestling development, and no research on the impact of the trophic structure of arthropods inhabiting the nest (combined effects of ectoparasitic mites and predatory mites feeding on ectoparasites). We assess nestling development of European Starlings (Sturnus vulgaris) in relation to the number of parasitic mites Dermanyssus gallinae (DG, a blood-sucking mite) and their predators, i.e. Androlaelaps casalis (AC), both dominant species of nidicolous arthropods in Starling nests. DG densities were not associated with nestling body mass or tarsus length during development (10 and 17 days of age), which contradicts our expectation that parasitic mites negatively influence growth. Furthermore, an increase in AC densities was associated with a significant decrease in body mass (not tarsus length) later during nestling development (at day 17—a proxy for nestling age—but not at day 10). The latter seems counterintuitive, but not when the inherent density-dependent delays in Lotka-Volterra predator–prey interactions are taken into account: a high density of predatory mites (AC) always arises after an increase of prey mites (DG). Thus, the high density of predatory mites indicates a preceding peak density of parasitic mites. Clearly, this explanation requires insight in the trophic structure of mites inhabiting Starling nests and bird nests in general. We conclude that multitrophic interactions (between predator, parasite and host) in nests should not be ignored when assessing nestling development.