 When air temperatures rise above , thermal gradients driving heat passively by convective dissipation from around the eyes, shoulders, and feet are reduced or eliminated, requiring heat dissipation mainly by evaporation and exhalation.In cold climates, hummingbirds retract their feet into breast feathers to eliminate skin exposure and minimize heat dissipation.The dynamic range of metabolic rates in hummingbirds requires a parallel dynamic range in kidney function.