Heat dissipation limit theory and the evolution of avian functional traits in a warming world

Summary It is generally assumed that animal energy expenditure is limited by energy acquisition. In a series of publications, Speakman, Król and colleagues argue that the capacity to dissipate metabolic heat may also limit maximum rates of energy expenditure in endotherms (heat dissipation limit theory – HDL theory). The implications of the HDL theory for the evolution of avian functional traits are substantial and open fascinating research perspectives. Notably, the HDL theory leads us to (i) link elevated bird body temperatures with their capacity to achieve higher rates of heat loss and of energy expenditure, (ii) reconsider the evolution of avian plumage patterns and speculate upon the capacity of white birds to achieve higher field metabolic rates than darker relatives, (iii) hypothesize that the avian brood patch also functions as a thermal window allowing birds to shed excess heat and (iv) revise our current view of the adaptive significance of limited plumage thermal insulation in great cormorants. Such features have important implications for the capacity of birds to cope with global warming and for the design of mechanistic models of animal energetics aiming at predicting their responses to changing environmental conditions.