Fat is the fuel for avian migration, but little quantitative information is available concerning the energy expenditure involved in these flights. Most studies of the energetics of avian migration have dealt with birds that use powered (flapping) flight. These investigations have been based on laboratory analyses of flight metabolism, field studies of changes in body mass, or inquiries into the quantities of fat necessary to fuel long-distance migratory flights that are less than a week or so in duration. In the present study the authors combine these approaches to analyze the energetics of a different type of migration, the long-distance, long-term soaring flight of Swainson's Hawk (Buteo swainsoni ) and the Broad-winged Hawk (B. platypterus ). Such a flight ought to involve large expenditures of metabolic energy. The migration, however, may be accomplished largely without feeding.