Are avian blood parasites pathogenic in the wild? A medication experiment in blue tits (Parus caeruleus)

The Hamilton and Zuk hypothesis on haemoparasite-mediated sexual selection and certain studies of reproductive costs are based on the assumption that avian blood parasite infections are detrimental to their hosts. However, th ere is no experimental evidence demonstrating harmful effects of blood para sites on fitness in. wild populations, it even having been suggested that t hey may be non-pathogenic. Only an experimental manipulation of natural blo od parasite loads may reveal their harmful effects. In this field experimen t we reduced through medication the intensity of infection by Haemoproteus majoris and the prevalence or infection by Leucocytozoon majoris in blue ti ts (Parus caeruleus), and demonstrated detrimental effects of natural level s of infection by these common parasite species on host reproductive succes s and condition. The fact that some of the costs of infection were paid by offspring indicates that blood parasites reduce parental working capacity w hile feeding nestlings. Medicated females may be able to devote more resour ces to parental care through being released from the drain imposed upon the m by parasites and/or through a reduced allocation Co an immune response. T herefore, this work adds support to previous findings relating hosts' life- history traits and haematozoan infections.