Ecological factors influencing disease risk in Eagle OwlsBubo bubo

Abstract

In this study we assessed whether local habitat features and host population density influenced disease risk in Eagle OwlBubo bubofledglings. Measures of immune defence (concentrations of circulating white blood cells), prevalence of three parasite types (a blood parasiteLeucocytozoon ziemanni, an insectCarnus haemapterus, and a tickRhipicephalussp.) and total number of parasite species were used to quantify disease risk. We tested the hypotheses that disease risk in fledglings was higher in nests located in areas with higher length of and proximity to watercourses (as a higher abundance and viability of parasites and vectors occur in wetter areas), higher cover of forest (as forest moistness and humidity can favour higher vector and parasite proliferation), higher habitat diversity (as environmental heterogeneity increases the pool of potential vectors and parasites) and higher local owl population density (as disease transmission might be density‐dependent). The clearest relationship was with the proximity of freshwater, although the other hypotheses were also partially supported. Concentrations of white blood cells, the number of parasite species and, weakly, the prevalence ofCarnus haemapteruswere all higher in nests closer to watercourses. The prevalence of blood parasites increased with the cover of forested areas. Fledglings from nests located in more diverse habitats had higher white blood cell concentrations and showed higher prevalence of blood parasites. Finally, local host population density was positively correlated with the prevalence of blood parasites. The results suggest the existence of complex and interrelated links between ecological parameters and three different measures of disease risk, and highlight the importance of immunological approaches to assess disease risk at an intraspecific level.