Physiological constraints on heat adaptation in birds.

Rising global temperatures and compound climatic extremes outpace avian thermoregulatory capacity. Yet, heat tolerance has been studied in only 1-2% of birds, mostly from the southern subtropics. The field requires research in two main areas. First, temperature and humidity must be considered in unison, and we provide a guiding, biophysical, framework for how these factors interact across climates. Second, we need experiments integrating ecology, evolution, and physiology. To that end, we introduce a life-history model relating thermoregulatory effort and the accumulation of heat-related damage to lifetime fecundity. Moving forward, we advocate multidisciplinary approaches to study heat tolerance across phylogenetic breadth and in understudied biomes. This is essential for identifying species at risk and forecasting bird population dynamics in a changing world.