Mallard ducklings (Anas platyrhynchos) freeze upon hearing the maternal alarm call, which the hen issues when there are disturbances (e.g., potential predators) near the nest. Our previous work indicates that ducklings that have been devocalized embryonically and reared in auditory isolation exhibit a significant reduction in the incidence of freezing, as compared to vocal ducklings or to devocal ducklings that have been exposed to duckling sounds throughout the perinatal period up to the time of testing at 12 hr posthatch. The main purpose of this paper is to assess whether there is a prenatal or postnatal sensitive period for such auditory experience. Two groups of 30 devocalized ducklings were either (a) stimulated with duckling sounds prenatally and tested to the alarm call at 12 hr posthatch (Expt. 1), or (b) stimulated from the time of hatching to the time of testing at 12 hr (Expt. 2). These experiments rendered no conclusive evidence of either a prenatal or postnatal sensitive period, but they suggested that auditory experience might be important in maintaining freezing. To test this hypothesis, a third group of 30 ducklings was stimulated from hatching to 12 hr and tested at 24 hr (Expt. 3). Only those ducklings receiving auditory experience contiguous with the time of testing (Expt. 2) showed high levels of freezing. Whenever a gap occurred between the offset of stimulation and the onset of testing (Expts. 1 and 3), the incidence of freezing dropped. Thus, auditory experience is important in maintaining the freezing response. This effect contrasts markedly with traditional conceptions of sensitive periods.