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Blood parasites mediate morph‐specific maintenance costs in a colour polymorphic wild bird

Parasites can mediate profound negative effects on host fitness. Colour polymorphism has been suggested to covary genetically with intrinsic physiological properties. Tawny owl colour polymorphism is highly heritable with two main morphs, grey and brown. We show that experimental medication acts to reduce blood parasites and that medicated grey females maintain body mass during breeding, whereas medicated brown females decline in body mass similar to control females of both morphs. We find no effect of medication on general immunoglobulin levels, antigen‐specific humoral response or H/L ratio. In the descriptive data, both morphs have similar blood parasite infection rates, but blood parasite infection is associated with decreased body mass in brown but not in grey females. We conclude that blood parasite infection primarily has somatic costs, which differ between the two highly heritable tawny owl colour morphs with more pronounced costs in the grey (little pigmented) morph than in the brown (heavily pigmented) morph. Because our descriptive results imply the opposite pattern, our findings highlight the need of experimental manipulation when studying heritable variation in hosts’ response to parasitism.