Song syntax and singing behavior of Cassin's Vireo (Vireo cassinii)

Songbirds display immense variability in the complexity of their songs, ranging from simple, repeated trills to intricate sequences. Explaining the evolutionary origins of song complexity has proven challenging because the role of song complexity in communication is poorly understood. This may, in part, arise from a disproportionate focus on the role of repertoires in communication, while ignoring other characteristics of song. In this thesis, I study the songs of Cassin’s Vireos, taking a broader view of song organization that encompasses both repertoires and the syntax governing the organization of repertoires into sequences. Chapter 1 describes the repertoire sizes and patterns of sequential organization in males at my study site. Repertoires are comprised of about 50 phrase types delivered in sequences that show evidence of temporal structure and repeated patterns. These sequences contain small-world structure, as has been demonstrated in the songs of other bird species. Chapter 2 more formally examines the syntax governing these sequences. I tested the fit of three competing syntactic models of varying complexity on the song sequences of Cassin’s Vireos. Analysis revealed that song sequences were too complex to be described by a zero- or first-order Markov model, meaning that the true complexity is higher, being well described by a model intermediate in complexity between a first- and second-order model. Sequences proved to be predictable with >55% accuracy, and syntax appears to change little, if at all, during adult life. In Chapter 3, I conducted playback experiments to examine the role of repertoires and syntax during counter-singing interactions. Birds presented with playback of a phrase type often responded with the phrase type(s) that would typically follow the stimulus phrase type in their normal sequences, as though they had delivered the stimulus phrase themselves. This implies that both repertoires and syntax play a role in counter-singing exchanges, suggesting that a complete understanding of vocal interactions will require an understanding of syntax in addition to accurate designation of repertoires. This thesis helps elucidate the nature of song complexity in Cassin’s Vireo and its role in communication, and provides methodological and theoretical advances that may be applicable to other species and the study of animal communication more generally