Three experiments examined mallard ducklings' perception of conspecific distress calls, as measured by their tendency to alternate their own vocalizations with tape-recorded test calls. This alternation response involved a duckling inhibiting its own calls during the tape-recorded stimuli, and then calling during the interstimulus intervals. 1. In Experiments 1 and 2, when ducklings were tested with calls having note periods that were substantially shorter or longer than the normal note period for distress calls, they showed a significant decrement in their call alternation response. The effects of shortening and lengthening note period, however, were not symmetrical: shortening note period by a given amount had a greater effect on responding than lengthening note period by that same amount. This response asymmetry may be due to a selection pressure operating to enable ducklings to discriminate between distress calls and another conspecific vocalization, contentment calls, which have shorter note periods than distress calls. 2. Experiment 3 examined the effect on ducklings of simultaneously manipulating two acoustic features known to be important for duckling recognition of distress calls: the temporal patterning of the notes within the call, and the frequency sweep found at the end of each distress note. Ducklings tested with a call having both a shortened note period, and the terminal frequency sweep removed from each note, displayed a significantly worse alternation response than ducklings tested with calls having either shortened note periods, or deleted frequency sweeps, but not both. The ducklings in the latter two treatments, in turn, had a significantly worse alternation response than ducklings tested with a normal call. Thus, the effects of manipulating these two acoustic features summated. 3. These results are consistent with a model of auditory perception in mallard ducklings in which recognition of a species-typical call depends on the summation of outputs from a set of perceptual filters each tuned to a specific acoustic feature of the call. Experiments are currently in progress to examine whether this model can be applied to mallard duckling perception of a second species-typical vocalization, the duckling contentment call.