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.