Prominent sex difference in migratory locomotor activity in captive reed buntings is not reflected in concomitant variation of their leukocyte profile

Migratory birds adopt different strategies to reach their destinations. In particular, males of many passerines arrive to the breeding grounds earlier than females (protandry). Each strategy involves specific behaviors and underlying physiological adjustments. To understand possible physiological consequences of protandry, we investigated leukocyte profile in captive migratory reed buntings, Emberiza schoeniclus (Linnaeus, 1758), a species with pronounced protandry in the wild. Since males exhibited increased locomotor activity and reduced energetic reserves then females in spring, we expected a higher heterophil-to-lymphocyte ratio and a lower total leukocyte counts in males. However, both sexes showed similar leukocyte profile parameters. Therefore, in captivity, sex differences in behavior and condition do not influence these indices of immune state or long-term physiological stress. This may reflect immune system adjustments for pathogen response in spring, regardless of energy expenditure. Alternatively, captivity conditions (unlimited food, no competition) may obscure natural variations. Compared to autumn, total leukocyte counts were lower in spring in both sexes. Additionally, contrary to most field observations, we found a negative correlation between body fat reserves and total leukocyte counts. Overall, our study enhances understanding of physiological mechanisms underlying migratory strategies in passerines.