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Data from: Chicks of cavity-nesting birds do not ‘exercise’ prior to fledging

Williams, Tony1; Earle, Kate1; Allen, Josh1; Hodinka, Brett2

Published Oct 30, 2025 on Dryad. https://doi.org/10.5061/dryad.ffbg79d7m

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Abstract

Fledging represents a key life-history transition involving a rapid increase in workload associated with a rapid transition from sedentary nestling to volant, active fledgling. Here, we tested the idea that chicks might prepare for fledging through increased voluntary activity (‘exercise’) and whether this would impact somatic and physiological development. European starling (Sturnus vulgaris) chicks, in cavity nests, increased levels of putative exercise (wing flapping) and more general active behaviours (e.g., perching, standing) in the five days up to fledging. However, facultative mass loss and wing growth between days 15 – 20 were independent of time spent wing flapping, standing, or perching, and – counterintuitively – we found a weak negative relationship between hematocrit (a measure of aerobic capacity) and time spent wing flapping or standing. Thus, although exercise is commonly associated with an increase in hematocrit in other species, this does not appear to be a mechanism for increasing pre-fledging hematocrit in chicks. Despite widespread anecdotal observations of flight preparation (e.g., wing flapping) in larger seabirds and raptors, our data suggest that exercise, or increased activity in general, does not contribute to improved development just prior to fledging: starling chicks do not ‘exercise’ enough to show somatic or physiological effects.