Molt and Migratory Condition in Blue Tits: A Serological Study

Migration and molt are both energy demanding activities and can be expected to conflict with each other in migratory birds. We studied autumn migrating juvenile Blue Tits (Parus caeruleus) and investigated whether molting birds showed signs of diseases or physiological stress to a larger degree than non-molting individuals. There was no evidence that molting birds were more stressed than non-molting individuals, as hematocrit, red blood cell sedimentation rate and the size of subcutaneous fat stores were similar for birds in both groups. However, birds with high levels of subcutaneous fat had low sedimentation rate levels and high hematocrit values, indicating that the level of fat of an individual is associated with its state of health. Birds with large fat stores were more brightly colored than birds with smaller fat stores, suggesting a link between nutritional conditions experienced during molt and the size of fat stores during the subsequent migration. Taken together, our data indicate that fat deposit size in migrating birds might be affected by their current health status, as well as conditions experienced during molt.