Knowledge gaps regarding the capacity of organisms to adjust their ecophysiological traits limit the potential of ecophysiology for predicting the impacts of global change on biodiversity. In this thesis, I therefore test hypotheses about how small songbirds respond to diverse climatic conditions. I use two common bird species as study systems: a tropical species (common waxbill, Estrilda astrild) and a temperate species (great tit, Parus major). My research shows that both species have the capacity to adjust to different temperatures, mainly through changes in both their basal and summit metabolic rates. Tropical species have evolved in warm, stable climates and are therefore assumed to possess only a limited capacity for ecophysiological adjustments. However, my research demonstrates that tropical common waxbills reduce their energy expenditure at colder temperatures. Such energy-saving mechanisms may contribute to their success as an invasive species, including in Europe. Great tits from colder regions appear, as expected, to be able to produce more heat due to their considerably higher summit metabolic rates. Contrary to expectations, however, there was no strong relationship between basal and maximal metabolic rate, in either great tits or common waxbills. Further research is therefore needed to fully understand the mechanisms that determine how birds regulate their energy balance.