Hypotheses of hemoparasite-mediated sexual selection and reproductive costs rely on the assumption that avian blood parasite infections are harmful to their hosts. To test the validity of this assumption, we examined the health impact of Haemoproteus blood parasites on their great tit (Parus major) host. We hypothesised that if blood parasites impose any serious health impact on their avian hosts, then infected individuals must differ from uninfected ones in respect to hemato-serological general health and immune parameters. A 3-year study of two great tit populations, breeding in contrasting (urban and rural) habitats in south-east Estonia, revealed that Haemoproteus blood parasites affected the health state of their avian hosts. Infected individuals had elevated lymphocyte hemoconcentration and plasma gamma-globulin levels, indicating that both cell-mediated and humoral immune response mechanisms are involved in host defence. The effect of parasites on cell-mediated immunity was both age- and sex-specific, as infection status affected peripheral blood lymphocyte counts only in males, and among these, the magnitude of response was greater in old individuals than yearlings. Heterophile hemoconcentration and plasma albumin levels were not affected by infection status, suggesting that blood stages of Haemoproteus infection do not cause a severe inflammatory response. Parasitism was not related to hematocrit values, indicating that Haemoproteus infection does not cause anemia. In two years, infected individuals were heavier than uninfected ones in the urban but not in the rural study area. This suggests, that under certain circumstances (possibly related to reproductive tactics), breeding great tits may avoid losing body mass in order to save resources for an anti-parasite immune response.
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