Physiological maturity at a critical life‐history transition and flight ability at fledging

Developmental maturity (e.g. body condition, body mass) at major life-history transitions is known to affect fitness across a wide range of taxa. Fledging (leaving the nest), a major life-history transition in birds, is associated with high post-fledging mortality and is widely assumed to be related to poor initial flight ability of fledglings, which, in turn, might be related to developmental maturity at fledging. We investigated individual variation in developmental maturity of both somatic and physiological traits at this critical life-history transition in different ecological contexts (year, first or second broods) to determine the importance of physiological traits related to oxygen-carrying capacity (haematocrit, haemoglobin) for individual variation in initial flight ability at fledging. Haemoglobin concentration and haematocrit at fledging had much higher variance than somatic traits and were more variable across ecological contexts. Furthermore, fledgling haemoglobin concen...