Abstract
The allocation into T-cell-mediated immunocompetence was experimentally increased in 68 out of 139 nestlings by supplementary feeding methionine to half of the nestlings in 15 blue tit nests. Methionine-supplemented nestlings had an increased T-cell-mediated immunocompetence, but a reduced growth compared with control siblings. Nestlings that had low initial weights and nestlings that were supplemented with methionine had an increased mortality risk. The investment that nestling blue tits make in immunocompetence appears to be tightly controlled by survival costs paid through a trade-off between immunocompetence and growth.