Neotropical bird species provide exceptional opportunities for advancing the understanding of avian communication systems. This dissertation focuses on the singing behavior of the White-breasted Wood Wren (Henicorhina leucosticta), which is not well understood even though it expresses behaviors such as female song and duetting that are at the forefront of research in avian communication. I present the background in this area of research and make the case that this species is of special interest to the study of avian communication because of its phylogenetic relationship to other duetting wrens and because its singing style is not easily categorized by singing behavior of other duetting species. In Chapter 2, I characterize the repertoire and singing behavior of this species and show that male repertoires are larger than those of females and can be classified into three groups based on spectral characteristics and song use. I describe unique high-frequency ‘introductory notes’ that directly precede songs within a bout but are facultatively included with the songs. In Chapter 3, I discuss playback experiments that suggest introductory notes act as a graded signal in certain song types during countersinging interactions. Differences in threat salience between four common song types was identified with one song type in particular that provoked lower song rates from subjects and was never song matched. These results suggest a separate function of this song type and together emphasize the potential role of structural cues in regulating singing interactions. In Chapter 4, I assess potential functions of duetting with two-speaker playback experiments and utilize a microphone sensor array developed at UCLA to localize the vocalizations of individuals. I show that in opposition to the mate-guarding hypothesis, the threat of unmated same sex intruders (solo playback) was not greater than the threat posed by paired intruders (duet playback). This supports territorial defense as the primary function of duets in this species. This research helps fill a taxonomic gap in our understanding of the singing behavior of tropical birds and suggests future directions of research for a better understanding of avian communication.

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.

Introduction

Methods

Results

Conclusion

We performed a sequence of tests on a partially water-saturated sand sample contained in an x-ray transparent aluminum pressure vessel that is conducive to x-ray computed tomography (CT) observation. These tests were performed to gather data for estimation of thermal properties of the sand/water/gas system and the sand/hydrate/water/gas systems, as well as data to evaluate the kinetic nature of hydrate dissociation. The tests included mild thermal perturbations for the estimation of the thermal properties of the sand/water/gas system, hydrate formation, thermal perturbations with hydrate in the stability zone, hydrate dissociation through thermal stimulation, additional hydrate formation, and hydrate dissociation through depressurization with thermal stimulation. Density changes throughout the sample were observed as a result of hydrate formation and dissociation, and these processes induced capillary pressure changes that altered local water saturation.

We performed a sequence of tests on a partially water-saturated sand sample contained in an x-ray transparent aluminum pressure vessel that is conducive to x-ray computed tomography (CT) observation. These tests were performed to gather data for estimation of thermal properties of the sand/water/gas system and the sand/hydrate/water/gas systems, as well as data to evaluate the kinetic nature of hydrate dissociation. The tests included mild thermal perturbations for the estimation of the thermal properties of the sand/water/gas system, hydrate formation, thermal perturbations with hydrate in the stability zone, hydrate dissociation through thermal stimulation, additional hydrate formation, and hydrate dissociation through depressurization with thermal stimulation. Density changes throughout the sample were observed as a result of hydrate formation and dissociation, and these processes induced capillary pressure changes that altered local water saturation.