Development, pre-fledging physiological state, and post-fledging performance in the European starling, Sturnus vulgaris.

Body size (mass, wing length) is typically used as a metric of developmental state and physiological quality in juvenile birds. However, our understanding of non-linear patterns of development (e.g., mass-overshoot recession) in relation to actual physiological state is poor, especially approaching fledging. We explored covariation between pre-fledging morphological development (body mass, tarsus, and wing length) and physiological state at fledging (aerobic capacity, energy state, oxidative status) in the European starling (Sturnus vulgaris). Developmental trajectories consistent with pre- fledging mass recession and compensatory wing growth were negatively associated with energy state and aerobic capacity, respectively. In contrast, no measure of body size covaried with any single physiological trait, although fledgling mass was positively correlated with principal component scores that comprised aerobic capacity and energy state. Next, we used a weight-treatment experiment to test how putative facultative adjustments in body mass prior to fledging affect growth and physiological state, using radiotelemetry data to subsequently analyze post-fledging performance. Only body mass and oxidative status differed between treatments, suggesting mass-independent development of some physiological traits but oxidative costs for maintaining developmental trajectories despite experimental perturbations. Both morphology (mass recession, wing growth and length) and physiological state (haemoglobin, reactive oxygen metabolites) were associated with metrics of post-fledging performance in control birds, although no trait predicted performance in the treatment group. To further test the apparent insensitivity of physiological development to environmental perturbations, we conducted a brood size manipulation and cross-fostering experiment. Results showed structural and physiological development that was robust to environmental variation in two growth phases. Again, this apparent canalization was associated with oxidative costs in low-quality environments, which carried over between growth phases even when conditions improved. Finally, we explored the role of behaviour in pre-fledging development by quantifying two pre-fledging 'exercises': wing extensions and wing flapping. We tested for covariation between these exercises and development, showing independent relationships with chick age, morphology, and provisioning rate, perhaps suggestive of separate adaptive functions. Collectively, this work suggests that some measures of physiological state independently vary with environmental context and performance, and this variation might affect juvenile performance independent of absolute body size.