Structure and evolution of song form in the wrens Thryothorus sinaloa and T. felix

Summary The songs of two species of wrens, Thryothorus sinaloa and T. felix, are less similar than previously believed. We have found that song structure, bout patterning, and other aspects of singing behavior are widely divergent between the two species, which are sympatric over much of their ranges. The nature of these differences supports the hypothesis of contrast reinforcement in bird song. Further evidence is provided by essentially parallel differences in another pair of sympatric species, T. pleurostictus and T. maculipectus. A cluster analysis of song form of the above four species plus one other, T. ludovicianus, suggests that those species which are allopatric are the most similar in song form, while sympatric species are the most dissimilar. We relate various features of singing behavior in the two species to some aspects to their ecology, and examine relationships among various song parameters within the genus Thryothorus. The parallel differences in pairs of sympatric Thryothorus in structure of song units, temporal organization of song, and duetting, along with the close correlation of groups of song parameters in the five species of Thryothorus leads us to propose that strategies or adaptive complexes of singing characteristics may exist in the genus.