Immune function and survival of great tit nestlings in relation to growth conditions

Life history theory predicts a trade-of between number and quality of offsp ring. Reduced quality with increasing broad size may arise from a decrease in body condition or in immunocompetence that would be important in fightin g off virulent: parasites by immunologically naive offspring. We tested the effect of rearing conditions on immune function of nestling great tits (Pa rus major) by reducing or increasing broods by two hatchlings. In the middl e of the nestling period (on day 8), nestlings from enlarged broods develop ed lower T cell responses [as measured from the cutaneous swelling reaction to injection with phytohaemagglutinin (PHA)] and tended to have lower tota l leukocyte and lymphocyte concentrations in their peripheral blood than ne stlings from reduced broods. Brood size manipulation affected the PHA respo nse of nestlings most strongly in small clutches, suggesting that nestling immune function was dependent on their parents' condition, as estimated by original clutch size. Intra-brood differences in nestling mortality were un related to immune parameters, but nestlings in broods without mortality had a stronger PHA response, higher concentration of lymphocytes and higher bo dy mass on day 15 than nestlings in broods with mortality. These results su pport the prediction that the immune function of altricial birds is affecte d by rearing conditions, and that growth and immune parameters are related to inter-brood differences in nestling survival.