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Haematophagous mites are frequently found on nestling house sparrows (Passer domesticus), but their effects are poorly known. In this study we investigated whether natural levels of infection by two mite species Pellonyssus reedi and Ornithonyssus sylviarum have any fitness consequences on their hosts, including some physiological indices of chick health, body condition and fledging success. Among the haematological variables, thrombocytes and heterophils, but not lymphocytes and eosinophil granulocytes showed positive correlations with mite loads. There was also a significant decrease in the haematocrit values of the nestling sparrows with increasing mite intensity. We found no significant effect of ectoparasites on shortterm indices of nestling fitness, such as body mass or fledging success. These results suggest that the blood-feeding mites of the studied house sparrow population mainly affected the haematological parameters of their hosts: They generated a non-specific immune response, with inflammatory processes and anaemia. On the other hand, parasite infestation seemingly has only weak influence on feather and skeletal growth of nestling sparrows, and no effect on fledging success and body mass.