Fat reserves and health state in migrant Goldcrest Regulus regulus

1. Although the importance of adequate fat deposits for migrating birds has long been recognized, Little is known about causes of intraspecific variation in the size of energy reserves. 2. We studied individual variation in visible fat reserves, body size, state of health and nutrition among autumn migrating Goldcrests Regulus regulus. 3. Birds showing signs of infectious/inflammatory diseases (high red blood cell sedimentation rate) or anaemia (low packed red blood cell volume) had significantly smaller fat stores than birds lacking these signs. However, no differences in fat reserves or health state were detected in relation to stage of post-juvenile moult. 4. These results suggest that the ability to deposit fat in the face of an energetically demanding migration may be affected by an individual's state of health.