Assessing the similarity of song-type transitions among birds: evidence for interspecies variation

In many species of songbird, individuals sing multiple song types, some of which are shared with their neighbours. Individuals may also share syntactical rules that govern the transitions between different song types, but few studies have attempted to study this kind of sharing. Progress has been inhibited by a lack of statistical tools to compare song-type transitions among individuals. We present a straightforward method for comparing song transitions based on Markov transition matrices. The method calculates the number of mutually preferred song-type-to-different-song-type transitions found in the song sequences of two birds, then assesses whether that number is significantly greater than would be expected if the two birds ordered their songs independently of one another. We applied this method to song sequences from five songbird species. All pairwise comparisons among male Cassin's vireos, Vireo cassinii, showed significant similarity in song transitions, as did a minority of comparisons among Adelaide's warblers, Setophaga adelaidae, and one pair of marsh wrens, Cistothorus palustris. In contrast, dyads of rock wrens, Salpinctes obsoletus, and rufous-and-white wrens, Thryophilus rufalbus, did not share song-type transitions at levels exceeding chance. Interterritory distance was not significantly related to our measure of song transition similarity in any of our study species. These results provide evidence that interindividual similarity in song-type transitions is a trait that varies considerably among species. We discuss the potential drivers of similarity in song transitions, but note that assessing its evolutionary breadth will require a larger sample of species. The application of our method to additional species will provide a more comprehensive understanding of signal use and vocal interaction in songbirds.