G. Sorci et al., REDUCED IMMUNOCOMPETENCE OF NESTLINGS IN REPLACEMENT CLUTCHES OF THE EUROPEAN MAGPIE (PICA-PICA), Proceedings - Royal Society. Biological Sciences, 264(1388), 1997, pp. 1593-1598
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.