Abstract 1. Interclavicular air sac pressures were measured in intact, unrestrained mallard ducks. All vocalizations were accompanied by increases in positive pressure in the interclavicular air sac. 2. Opening the sac to the atmosphere reduces the intensity of calling in adults and ducklings. Reducing interclavicular air sac pressures increases the harmonic complexity of the calls and the effect is most pronounced in adult females and ducklings. These results are discussed in terms of normal maturational events during call development in ducklings. 3. In adult males, the effect of reducing interclavicular air sac pressures is minimal. There were no significant changes in the harmonic structure of the call. These differences are related to differences in syringeal structure between adult males and females.