Variability in susceptibility of nestling passerines to parasites

Young birds face a diversity of nest-dwelling, blood-feeding ectoparasites. Size hierarchies among nest mates increase the variation within broods, and susceptibility to parasites may vary with this hierarchy. I investigated how nestling susceptibility to parasites differs both among and within broods. Experimental manipulation of nestling susceptibility to parasites showed that nestlings with reduced susceptibility benefitted morphologically and physiologically, while within broods, larger-sized nestlings benefitted more than smaller-sized nestlings. Experimental manipulation of nestling haemoglobin concentration, a common physiological effect of blood-feeding parasites, did not show strong support that reduced haemoglobin concentration could be the mechanism that causes changes in nestling phenotype in response to increased parasitism. Lastly, nestlings that received an anti-parasite treatment were more likely to survive a harsh weather event than non-experimental nestlings. Therefore, I demonstrated that nestling susceptibility to parasites does vary among and within broods and plays an important role in determining nestling phenotype and survival.