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 4 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. Furthermore, 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.