Seasonal decline in health status of Great Tit (Parus major) nestlings

Seasonal variation in offspring survival and recruitment rates in birds may be mediated by immune function, as it defines the ability of individuals t o protect themselves against parasites and infectious diseases. To investig ate the relationship between hatching date and health status of Great Tit ( Parus major) nestlings, two blood parameters (leukocyte level and haematocr it) were estimated. Leukocyte level decreased as the season progressed with in first but not second broods, while haematocrit showed no seasonal variat ion within either brood type. However, nestlings from first broods had both higher leukocyte levels and higher haematocrit than nestlings from second broods. Nestling body condition (defined as the residual of body mass on ta rsus length calculated from linear regression) was not related to hatching date within brood type, while a significant difference between brood types was found. We suggest that the commonly observed decline in juvenile surviv al rate as the season progresses may be at least partly attributed to seaso nal changes in health status.