Fitness in common buzzards at the cross-point of opposite melanin-parasite interactions

1. Melanin is one of the major pigments of vertebrates and has many suggested functions. Although it is supposed to have an antipathogenic effect, its relationship to infection with ecto- and endoparasites largely remains to be examined. 2. The common buzzard Buteo buteo occurs in three eumelanin morphs which differ in their lifetime reproductive success. We examined the distribution of the blood-sucking ectoparasite Carnus haemapterus and the blood parasite Leucocytozoon toddi among buzzard nestlings and related parasite infection to the morph of the host and its parents. We controlled the effect of melanin morph on parasites for the potentially confounding total leukocyte numbers, heterophil : lymphocyte ratio (H : L ratio), age, sex, hierarchical rank within the brood and annual food availability. 3. Blood parasite prevalence was lower in offspring of intermediately coloured males. This pattern was apparent during high and low food availability, and although this result did not remain under intermediate vole densities, it could contribute to observed fitness differences in male buzzards. In contrast, carnid fly score increased with darkness of the host plumage. However, among chicks infected with the blood parasite, Leucocytozoon infection intensity decreased with plumage darkness. 4. The two parasite species may exert selection pressures in opposite directions on melanism of their host. Therefore, buzzards with intermediate amounts of melanin may prosper as they are sufficiently protected against endoparasites, but not too attractive for ectoparasites. Parasites could help explain the persistence of colour polymorphism in their buzzard host. © 2008 The Authors.