Song types as fundamental units in vocal repertoires

We investigated whether song types function as fundamental units of song variation in song sparrows, Melospiza melodia. As the size of a male song sparrow's repertoire increases, so does the mean similarity of his song types, as measured by the sharing of minimal units of production (MUPs). It follows that if MUP similarity is important perceptually, then small repertoires (of dissimilar song types) may be functionally equivalent to large repertoires (of similar song types). We performed two experiments to test whether MUP similarity is important perceptually to male song sparrows. Both experiments used a habituation/recovery design, in which recovery in response at a switch in stimuli is used to gauge the subject's perception of the similarity of the stimuli. The results of both experiments indicate that the level of perceived similarity between pairs of songs does not depend on their level of MUP similarity, within the range of MUP similarities found between song types. Songs with high enough MUP similarity to be judged as variants of the same song type are, however, perceived to be much more similar than are any two song types. The results are compatible with a categorical model of song type perception. Copyright 1999 The Association for the Study of Animal Behaviour.