Three races ofQuelea quelea,inhabiting East, West and southern Africa, each perform a migration at the beginning of the wet‐season, but the distances involved differ greatly. The quantity of pre‐migratory fat accumulated differs in the three populations and correlates with the respective migration lengths. The timing of fattening is in some way associated with the onset of heavy rain. Though winged termites usually provide the food supply for fattening they do not appear to stimulate the process. Flight muscle hypertrophy accompanies fattening; it is suggested that the extra protein is required for body maintenance during the fasting period of the migrat