Flight energy is an important factor in the lives of birds. Many strategies and adaptations serve to minimize energy cost and to allow a range of performance consistent with a bird's ecological needs. Theoretical methods can produce good estimates of flight energy that suggest why flight adaptations occur; but remarkably little is known of the physiological adaptations required by flight, or of how these change, a I believe they must, in relation to ecology and flight behavior. More data on the metabolic power consumption of birds in natural flight would be valuable, but it is more important to determine the changes in internal metabolic processes associated with different levels of flight activity. Muscle efficiency in flight, in particular, may have substantial implications for our understanding of the energetic performance of birds. This is but one of a variety of unknown quantities, and only when the mechanisms that determine these are more deeply investigated can flight adaptations be completely understood.