Publication Date




Embargo Period


Degree Type


Degree Name

Doctor of Philosophy (PHD)


Biology (Arts and Sciences)

Date of Defense


First Committee Member

William A. Searcy

Second Committee Member

J. Albert C. Uy

Third Committee Member

Michael McCullough

Fourth Committee Member

Rindy Anderson


The development and evolution of low amplitude "soft song" was investigated in thrushes of the genus Turdus. Sharing of soft song was studied in the white-throated thrush (T. assimilis) and compared to sharing of broadcast song in the same species, in order to draw inferences about how both song modes are acquired. It was found that most of the song repertoires of each song mode are unique to particular white-throated thrushes, suggesting that this species relies more on invention than imitation during song development. The spatial pattern of sharing between local males differed between song modes. Broadcast song sharing peaks with neighbors located at intermediate distance while soft song sharing has no association with geographical distance. The implication of these patterns for the song learning strategy of soft song are discussed and a potential role for sexual selection is highlighted. In addition, the possible function of soft song to avoid eavesdropping and its adaptations to increase habitat-induced degradation and attenuation were analyzed by means of a field sound propagation experiment. The effect of basic acoustic features on excess attenuation, blur ratio and signal-to-noise ratio was evaluated and compared between song modes while controlling for amplitude differences. The acoustic structure of soft syllables undergoes significantly more habitat-induced degradation than does the acoustic structure of broadcast syllables. These results are consistent with the eavesdropping avoidance hypothesis and suggest that soft signals design is adapted to limit eavesdropping by third party receivers such as predators or conspecifics. Finally, the correlated evolution of body size, beak morphology and habitat structure (i.e. closed vs open habitats) with the acoustic structure of both song modes was investigated in a comparative study of 26 species of Turdus. Based on the assumption that morphology effects are independent of signal communication range and the acoustic adaptation hypothesis, it was predicted that both song modes are correlated with morphological traits but only broadcast with habitat structure. Results showed that both song modes are indeed correlated with morphological traits, but each song mode has evolved different correlated responses. Novel and interesting associations between soft song structure and beak morphology were found. Contrary to expected, broadcast song was not correlated with habitat. Soft song showed a correlation with habitat but in the opposite direction to that expected under the acoustic adaptation hypothesis. Since soft song is a signal mainly used for short-range communication, selection favoring habitat-induced degradation in closed habitats is a plausible explanation.


animal communication; behavioral ecology; evolution; signal design; bioacoustics; low-amplitude signals