Global change is causing a redistribution of biodiversity worldwide. Humans continue to introduce species to areas far beyond their natural geographical ranges while changing climates force organisms to adapt, or to shift in location to stay within preferred environmental conditions. Understanding what underlies species’ geographical distributions is a longstanding question in ecology and evolution, given new impetus by the conservation challenges global change brings. Here, we use birds to test which fundamental mechanisms and processes that underly species’ capacity to sustain temperatures outside of their thermoneutral zones. Whereas most ecological forecasting currently relies on extrapolating realized niches, we will quantify species’ fundamental (thermal) niches to tackle the problem of how endothermic organisms balance the need to both generate and dissipate heat. By combining experimental studies on (sub)tropical birds introduced to Europe and on native European passerines that either migrate to the tropics or stay to overwinter in the north, we will elucidate what underlies bird species’ capacity for thermoregulation. By integrating this information with spatial environmental data under a single biophysical framework, we will leverage information on thermoregulation to generate, and evaluate, ‘mechanistic’ predictions of invasion success and range distributions.  This way, the project will foster the uptake of ecophysiological frameworks in predictive ecology.