Decoupling of growth, physiological state, and subsequent performance in a developmentally manipulated songbird

It is generally assumed that larger juveniles are more physiologically mature, hence their overall condition and subsequent performance are higher. Yet, some taxa face extreme workload transitions during development (e.g., nest departure in birds), which may select for non-linear growth profiles that decouple relationships between body size and physiological state, making associations between development and juvenile performance uncertain. We manipulated perceived mass in European starlings approaching fledging using 4.0 g weighted backpacks, measuring subsequent growth trajectories (mass, wing length) and physiological state (aerobic capacity, energy state, oxidative status) to test whether body size and physiology are developmentally coupled during a non-linear growth phase (pre-fledging mass recession). Nanotag radio transmitters were then used to track post-fledging performance (activity, activity-slope, dispersal). Weighted nestlings had greater pre-fledging mass recession and lower oxidative status than controls, but equal aerobic capacity and energy state. Haemoglobin and dROMs, as well as metrics of growth and body size, predicted post-fledging performance in control birds, whereas weighted birds showed no correlation between fledgling state and performance. Our data suggest mass-independent development of some physiological traits in species with non-linear growth profiles, potentially with a context-dependent oxidative cost, and that physiology (not just body size) may predict post-fledging performance.