What determines the body temperature of birds and mammals?

Energy regulation is fundamental to temperature regulation for both birds and mammals. Mammals can induce sweat to keep them cool, but birds must resort to more creative means to keep them warm or get out of them. Birds lie low during the hottest parts of the day and seek food or other essentials during the cool morning and evening hours. Some birds take to the skies on hot days to find relief in cooler temperatures at higher altitudes.