It has been proposed that immune functioning in wild animals is shaped by the trade-off between a probability of encountering pathogens and an availability of resources for maintaining immune system in active state. Due to resources’ seasonality one can expect that immune functioning, e.g., absolute and relative counts of white blood cells, WBC (leukocyte profile) also follows an annual cycle. However, dynamics of the seasonal changes of leukocyte profile are controversial and specific inquiries so far addressed only a portion of annual cycle. To study the seasonal dynamics of the leukocyte profile and to test its possible endogenous basis we experimentally simulated annual cycle in a migratory passerine bird, chiffchaff (Phylloscopus collybita abietinus). We report here a clear seasonal pattern of leukocyte profile in that species. The highest WBC counts were observed during pre-alternate moult and the lowest during the subsequent spring migration; the seasonal dynamics of HL ratio showed the opposite tendency. Surprisingly, HL ratios during autumn migration were relatively low, indicating little to no stress. Observed seasonal changes in controlled experimental conditions suggest an existence of time-keeping programme underlying these variations. Additionally we found negative relationship between the body condition index and WBC counts and positive relationship with HL (heterophil to lymphocyte) ratio; these findings are controversial with the data obtained with other bird species in wild populations. Understanding the origins of variation of leukocyte profile per se and in relation with other parameters of physiological condition can be a useful tool in studying physiological response to environmental changes.