REDUCED IMMUNOCOMPETENCE OF NESTLINGS IN REPLACEMENT CLUTCHES OF THE EUROPEAN MAGPIE (PICA-PICA)

Laying date is one of the most important determinants of reproductive success and recruitment probability in birds. Late breeders usually fl edge fewer chicks than individuals with earlier breeding dates, and fl edglings produced late in the season have high mortality rates. Food a vailability and nestling mass have been evoked as the principal mechan istic links between laying date and offspring survival. Here we sugges t that aa another factor may actually account for the difference in su rvival rate between early and late offspring: immunocompetence. We pre dicted that nestlings produced later in the season or in replacement c lutches should have lower immune responses when challenged with an ant igen, than early nestlings or nestlings produced in first clutches. Th is hypothesis was tested in a population of magpies (Pica pica), in wh ich we experimentally induced breeding failure in a group of nests and compared the immune response of nestlings in replacement clutches wit h the immune response of first clutch nestlings. Cellular immune respo nse, as measured by wing web swelling (a correlate of T-lymphocyte pro duction after injection of phytohaemagglutinin-P), significantly decre ased with hatching date and was significantly lower in nestlings of re placement clutches. Furthermore, coefficients of intraclutch variation in immune response were higher in nestlings of replacement clutches. This experiment demonstrates an inverse relationship between immune re sponsiveness and breeding date, and reduced recruitment probability of late nestlings may-be a direct consequence of their inability to cope with parasites.