The coordination of ventilatory and locomotor rhythms has been documented in many birds and mammals. It has been suggested that the physiological significance of such coordination is a reduction in the cost of ventilation which confers an energetic advantage to the animal. We tested this hypothesis by measuring the external work required to ventilate birds mechanically during simulated flight. Patterns of wing motion and breathing were produced in which the relationship between wing motion and breathing was in phase and out of phase with the relationship seen during normal flight. Differences between the energetic costs of in-phase versus out-of-phase synchronization were particularly large (26 %) in instances where locomotion and respiration frequency were synchronized at one breath per wingbeat. The saving (9 %) obtained from in-phase versus out-of-phase coordination at the 3:1 coordination ratio seen normally in free-flying Canada geese was smaller but still supported the hypothesis that there is a significant net saving obtained from reducing the mechanical interference between locomotion and ventilation by locomotor­respiratory coupling.