Cumulative heatwave stress disrupts thermal homeostasis and plumage structure in a Mediterranean passerine

Heatwaves are becoming increasingly frequent across the Mediterranean and pose critical challenges for small passerines, yet the physiological and morphological limits to their resilience remain poorly understood. We exposed Great tits ( Parus major ) to four consecutive simulated heatwaves and quantified thermoregulatory responses, oxidative stress and feather development under controlled conditions. Heat‐exposed birds maintained persistently elevated body temperatures (+0.2°C) while exhibiting a lower probability and reduced within‐hour expression of hyperthermia events relative to controls. Circadian modulation of body temperature and heterothermy was context dependent across successive heatwaves, indicating altered precision of thermal regulation rather than a uniform loss of rhythmicity. These thermophysiological adjustments were accompanied by sustained oxidative damage and reduced antioxidant capacity, whereas body mass and feather corticosterone levels remained unchanged. Feathers grown under heat stress exhibited reduced barb and barbule density and brighter, more UV and red‐shifted coloration, consistent with altered feather microstructure and potential changes in insulative and signalling properties. Our results demonstrate that recurrent sublethal heat exposure rapidly engages thermoregulatory and morphological plasticity but entails measurable physiological costs. The limited buffering capacity of phenotypic flexibility highlights the need to identify trait‐specific thresholds and potential carry over effects when forecasting avian resilience to climate change. Read the free Plain Language Summary for this article on the Journal blog.