The vocal repertoire of mallard ducklings, Anas platyrhynchos, consists of only two call types, both of which are made up of trains of repeated notes. Recognition of separation-induced distress calls is known to depend on the degree to which a number of acoustic features match species-typical values. The present study extends these findings by examining the importance of spectral characteristics for recognition of contentment calls. Measurements were made of the dominant frequency at the temporal mid-point of contentment notes, created estimates of the population mean and variance for this feature. Synthetic calls were then created that either matched the mean dominant frequency, or differed from it in increments of two standard deviations. Ducklings received repeated computer-controlled playback presentations of one of these sounds, or of a control natural call. Stimuli were timed to occur just as the ducklings began a distress call, and the duration of the vocal inhibition elicited was used as an assay of call recognition. Ducklings were reliably less responsive to synthetic calls that had frequency characteristics that differed from the population mean by more than two standard deviations and were hence improbable in natural calls. Comparisons with song playback experiments suggest that such 'probability-based' recognition of conspecific acoustic signals may be a common perceptual process in birds. The effects of increasing and decreasing dominant frequency were similar, implying that processing of spectral information is based on absolute, rather than relative, frequency characteristics.