Haemoparasites prevalence in birds can vary among species, age and sex groups and ecological conditions. To comprehend the host—parasite assemblages, it is important to investigate physiological mechanisms underlying the interactions in wild bird species. Here, using blood smears we investigated how leukocyte profiles in reed warblers (Acrocephalus scirpaceus) and sedge warblers (Acrocephalus schoenobaenus) vary in relation to the heamoparasite presence with respect to the host's age and sex. We sampled the birds during their autumn migration in 2008 at a stopover site in northern Poland. We found a significant relationship between the leukocyte profiles and parasite infection. Parasite infection was associated with elevated relative number of heterophils with simultaneous lowered relative number of lymphocytes. These opposite pattern resulted in an elevated H/L ratio in infected birds. Total leukocytes, lymphocytes and heterophils counts were elevated in birds infected with blood parasites as compared with those of non-infected ones. Leukocyte profiles of parasitized birds were similar regardless of individual age. However, non-infected adults had lower total leukocyte counts and values of H/L than non-infected immatures.