SYNOPSIS. Some birds can fly for more than 1000 kilometers without feeding. Are these distances compatible with the fuel reserves and the power requirements that flying birds are thought to have? The fuel for flight is primarily fat, which can make up 50% of the total body mass of a bird prior to a long distance flight. As the bird uses up fuel during the flight and becomes lighter, the power requirements of flight probably decrease. However, a constant power requirement can be assumed throughout the flight without introducing serious errors into the estimate of maximum flight distance at a given flight speed. Various methods that have been used to estimate the power requirements of flight are reviewed. Estimates based on indirect calorimetry indicate that the maximum flight distances of birds, when a given proportion of body mass is used as fuel, are directly proportional to body mass raised to the 0.227 power. Calculated values of range suggest that birds have small margins of safely in long, over-water flights unless they are aided by winds or vertical air currents.