RELATIONS BETWEEN THE LEUKOCYTE PROFILE AND THE ENERGY STATE, HEMATOCRIT AND BLOOD METABOLITES IN THE GREAT TIT (PARUS MAJOR)
The state of animal populations can reflect ongoing changes in their environment. Various physiological parameters can be used to assess individual and population conditions. These parameters may be interrelated or independent, allowing for organisms to flexibly respond to environmental change. We investigated the dynamics of the leukocyte profile (total number and relative proportions of various types of leukocytes) in an urban population of the Great tit (<i>Parus major</i>) during two stages of the life cycle: autumn migration and wintering. We also analyzed the relations between the leukocyte profile and the energy state, hematocrit, and the levels of blood metabolites (glucose and β-hydroxybutyrate). We found the total number of leukocytes and lymphocytes to be higher during autumn migration compared to wintering. We argue that such a pattern indicates a more active state of the immune system in Great tits during migration. An increased number of lymphocytes (specific immunity) can reflect high probability rates of meeting with already familiar infections during relatively short-distance migrations typical of this species. The ratio of heterophils to lymphocytes (H : L ratio, a measure of long-term physiological stress) was higher in winter than during autumn migration. We suppose that this may be caused by low temperatures and an increased competition for food in wintering birds. The total number of leukocytes correlated positively with energy reserves and negatively with the blood glucose level, this possibly indicating a coordinated response of these physiological parameters to environmental cues. Our results also show that birds that have adequate energy reserves and low blood glucose concentrations (i.e. they are in good condition) maintain a more active immune system. Reduced energy reserves, for example, because of unfavorable feeding conditions, higher energy costs of both locomotor activity and thermogenesis in a cold weather caused increased blood glucose concentrations and a decreased number of leukocytes. The H : L ratio failed to correlate to other physiological parameters, this showing that the level of long-term physiological stress was related to neither an individual energy state nor the level of metabolism in Great tits under the observed environmental conditions. Overall, our results show a low level of long-term physiological stress in the study population of Great tits.