Formation movement, particularly during swimming and flying, has been hypothesized to reduce an individual animal's energy expenditure. Although a number of aeroand hydrodynamic models have been proposed to estimate energy savings of animals travelling in formation, little empirical data are available that test this presumption. An examination of the metabolic energetics of formation swimming was undertaken with mallard ducklings (Anas platyrhynchos) trained to swim behind a decoy in a flow tank. Oxygen consumption was measured for ducklings in clutches of one, two or four individuals, with the decoy in the water or suspended above the surface. Metabolic rate per individual duckling decreased with increasing clutch size by 7·8-43·5%. Following in the wake of a decoy significantly decreased metabolic swimming effort by ducklings and this effect was most pronounced at three days of age and in small clutches. These findings suggest that the flow pattern generated by the formation and in the wake of an adult reduces the resistance of the water to locomotion and allows an individual duckling to conserve energy.