Mallard duckling distress calls consist of a series of stereotyped notes and are usually given in calling bouts. When two or more ducklings are separated from their brood and begin to distress call, they alternate their calls, with each bird inhibiting its own vocalizations during those of another bird. In two playback experiments we reduced the acoustic variability found in distress calls and examined the effect this had on the alternation response of ducklings. In Exp. 1, one treatment involved an unedited tape-recording of a natural bout of 8 distress calls. Three other treatments involved edited tapes in which (a) the original 8 calls were presented, but at a constant intercall period, or (b) a single call was repeated 8 times at the natural (variable) intercall periods, or (c) a single call was repeated 8 times at a constant intercall period. The two tapes on which the same call was repeated 8 times elicited significantly weaker alternation responses from ducklings than the two tapes on which the original 8 calls were presented; the manipulation of the intercall period had no significant effect on the alternation response. In Exp. 2, analogous manipulations were performed to remove the variability between the notes within a single distress call or between the internote periods within that call. Ducklings alternated significantly better with the unedited distress call than with the distress calls in which variability had been removed. These results suggest that the variability found within many “Fixed Action Patterns” may serve an important antihabituation function.