Context-Dependent Effects of Amino Acid Supplementation on Nestling Growth and Baseline Innate Immune Function

Growth and immune development-fundamental parts of any organism's ontogeny-depend critically on nutrient availability, particularly on essential amino acids that support protein synthesis and physiological processes. However, environmental variation can reduce prey diversity and nutrient quality, producing suboptimal diets that lack essential amino acids, potentially constraining development. In this study, we experimentally supplemented great tit (Parus major) nestlings from forest and suburban environments with methionine, leucine, or tap water (control) from day 4 to day 7 post-hatching and investigated the effects on growth and baseline innate immune function across each habitat. We assessed nestlings' growth using body mass, tarsus length, and wing length and baseline innate immune function by quantifying complement activity (measured as lysis) and natural antibody titers (measured as agglutination). Supplementation significantly increased body mass gain in forest nestlings, particularly in smaller individuals, suggesting enhanced protein synthesis efficiency, although its effects on tarsus and wing length were limited. Methionine significantly improved lysis activity, suggesting enhanced innate immune function, whereas agglutination was not notably affected. In contrast, suburban nestlings showed limited responses to supplementation, suggesting broader nutritional constraints beyond individual amino acid deficiencies. Habitat and initial body mass significantly influenced growth rate, but their effects depended on the treatment. These findings highlight the complex, condition-dependent effects of amino acid supplementation on nestling development and emphasize the importance of considering habitat-specific nutritional limitations when assessing avian developmental plasticity.