Impacts of early-life thermal challenge on development, survival, and physiology in wild great tits.

Rapid climate change is increasing thermal extremes that may challenge developing endotherms. In altricial birds, early postnatal life is a sensitive period because thermoregulatory and physiological systems are still maturing. We experimentally manipulated nest temperature in a wild population of great tit to test whether mild, ecologically realistic heat (+2.3°C) and cold (-1.3°C) challenges affected development and physiology. Treatments were applied from days 2-8 post-hatching, and we measured growth, survival, hydration status (plasma osmolarity and uric acid), mitochondrial density, and immunoglobulin levels. Contrary to our predictions, thermal manipulation had no detectable effects on survival, morphology, or physiological traits at either day 8 or day 14. In contrast, all physiological markers varied significantly with age, reflecting normal osmoregulatory, metabolic, and immune development. These findings suggest that moderate, short-term thermal deviations during early development fall within the tolerance range of this population, or that parental behaviour buffered nestlings against temperature changes. However, stronger or prolonged extremes may exceed buffering capacity, and delayed effects cannot be excluded. Long-term studies are needed to assess latent fitness costs under increasingly variable climates.