Energetically demanding life-history stages in birds (reproduction, migration) might be associated with sex- and age-specific physiological changes expressed in suppressed immune function or declined body mass. Here, we examined the numbers of red and white blood cells (RBC and WBC), the ratio of heterophils and lymphocytes (H/L)], and body mass of reed warblers (Acrocephalus sirpaceus) and sedge warblers (A. schoenobaenus) during the breeding and migration periods. Higher WBC counts and H/L ratio in adults (both warblers) and lower RBC counts (only the reed warbler) during the prelaying/laying period as compared with those during the incubation/ chick-rearing period suggest greater energetic demands during the latter period. In adults, higher H/L ratio and WBC counts in males of both species as compared with those in females during the prelaying/laying period indicate higher energy expenditures of males, probably associated with mate guarding and chasing intruders. Lower HL ratio and WBC counts in adults of both species as compared with those in immatures during the migration period suggest lower stress level of adults, possibly due to their greater experience and/or higher social status.
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