Data from: Forest degradation and weather jointly affect early-life development in a tropical understory bird

Tropical forest birds face mounting pressures from habitat loss,
degradation, and climate warming, yet their combined effects on early-life
development remain unclear. Using over a decade of morphological and
behavioural observations from Kenya’s Taita Hills and four years of
nestling corticosterone measurements across eight forest patches differing
in size and degradation, we examined how forest quality and climate shape
nestling condition in the understorey insectivore placid greenbul
(Phyllastrephus cabanisi placidus). Nestlings in smaller or more degraded
patches showed lower body condition. Provisioning rates did not vary with
forest quality, suggesting that poor condition in degraded habitats may
result from lower prey quality rather than reduced parental effort.
Unexpectedly, corticosterone levels were higher in larger forest patches,
and nestlings with elevated corticosterone also showed more advanced wing
development, indicating that corticosterone here may reflect developmental
stage (readiness to fledge) rather than condition per se. In addition,
nestling condition declined in hot weather. However, although body
extremities were shorter in small or degraded patches at low temperatures,
in hot weather nestlings developed extremities similar in length to those
in larger, less degraded forest patches, consistent with short-term
thermoregulatory plasticity. By contrast, in high-canopy areas tarsus
growth decreased with increasing temperature, highlighting that multiple
developmental mechanisms may operate simultaneously. High canopy cover
also buffered body condition under dry weather, whereas nestlings in
low-canopy habitats were particularly vulnerable during drought. Our
findings demonstrate how forest degradation and climate interact to shape
nestling growth, physiology, and potential thermal plasticity. Maintaining
intact forest canopies and large forest patches is thus critical for
preserving the microclimatic buffering needed to support offspring
development in a warming climate. Because multiple, and sometimes
opposing, developmental mechanisms may act simultaneously, integrating
morphological, behavioural, and physiological indicators provides a
powerful multi-metric approach to disentangle how habitat and climate
jointly shape early-life development.