Investigation of musicality in birdsong

Abstract

Songbirds spend much of their time learning, producing, and listening to complex vocal sequences we call songs. Songs are learned via cultural transmission, and singing, usually by males, has a strong impact on the behavioral state of the listeners, often promoting affiliation, pair bonding, or aggression. What is it in the acoustic structure of birdsong that makes it such a potent stimulus? We suggest that birdsong potency might be driven by principles similar to those that make music so effective in inducing emotional responses in humans: a combination of rhythms and pitches-and the transitions between acoustic states-affecting emotions through creating expectations, anticipations, tension, tension release, or surprise. Here we propose a framework for investigating how birdsong, like human music, employs the above "musical" features to affect the emotions of avian listeners. First we analyze songs of thrush nightingales (Luscinia luscinia) by examining their trajectories in terms of transitions in rhythm and pitch. These transitions show gradual escalations and graceful modifications, which are comparable to some aspects of human musicality. We then explore the feasibility of stripping such putative musical features from the songs and testing how this might affect patterns of auditory responses, focusing on fMRI data in songbirds that demonstrate the feasibility of such approaches. Finally, we explore ideas for investigating whether musical features of birdsong activate avian brains and affect avian behavior in manners comparable to music's effects on humans. In conclusion, we suggest that birdsong research would benefit from current advances in music theory by attempting to identify structures that are designed to elicit listeners' emotions and then testing for such effects experimentally. Birdsong research that takes into account the striking complexity of song structure in light of its more immediate function - to affect behavioral state in listeners - could provide a useful animal model for studying basic principles of music neuroscience in a system that is very accessible for investigation, and where developmental auditory and social experience can be tightly controlled.

Figure 1. Olavi Sotavalta’s nightingale song analysis (Sotavalta, 1956)

A, generalized musical structure of each phrase. B, excerpts from his catalog of all the phrases sung by his two study birds. C, analysis of sequence of phrases in the nightingale song, showing a loosely patterned progression through the repertoire.

Figure 2. Time structure of the song

A, One song motif with syllable outlines in dark red; B, phase plot of onset-to-onset time intervals for an entire performance. Each dot represents a syllable in its rhythm context, i.e. its temporal relation to the previous and the following syllable’s onset. We call this a rhythm unit. Clusters indicate specific rhythm units that are recurring throughout the bird’s performance C, each song can be plotted as a trajectory in this phase plot, propagating from one rhythm structure into the next. D. Continuous escalation of the rhythm during the first part of the motif, curve indicate onset-to-onset intervals for pairs of downsweeps while spectral bandwidth increases; E, subtle escalation of amplitude during a trill of clicks leading to the next motif whistle.

Figure 2. Time structure of the song

A, One song motif with syllable outlines in dark red; B, phase plot of onset-to-onset time intervals for an entire performance. Each dot represents a syllable in its rhythm context, i.e. its temporal relation to the previous and the following syllable’s onset. We call this a rhythm unit. Clusters indicate specific rhythm units that are recurring throughout the bird’s performance C, each song can be plotted as a trajectory in this phase plot, propagating from one rhythm structure into the next. D. Continuous escalation of the rhythm during the first part of the motif, curve indicate onset-to-onset intervals for pairs of downsweeps while spectral bandwidth increases; E, subtle escalation of amplitude during a trill of clicks leading to the next motif whistle.

Figure 2. Time structure of the song

A, One song motif with syllable outlines in dark red; B, phase plot of onset-to-onset time intervals for an entire performance. Each dot represents a syllable in its rhythm context, i.e. its temporal relation to the previous and the following syllable’s onset. We call this a rhythm unit. Clusters indicate specific rhythm units that are recurring throughout the bird’s performance C, each song can be plotted as a trajectory in this phase plot, propagating from one rhythm structure into the next. D. Continuous escalation of the rhythm during the first part of the motif, curve indicate onset-to-onset intervals for pairs of downsweeps while spectral bandwidth increases; E, subtle escalation of amplitude during a trill of clicks leading to the next motif whistle.

Figure 3. Songs depicted as trajectories through spectral space

A, an example of three consecutive songs produced by a thrush nightingale, each song including phrases of whistles and clicks. Below the sonogram, zooming in using spectral derivatives shows that the click trills include a complex fine structure, produced in sets of 4, 2 or 1, sometimes with low pitch whistles in between (second panel); B, to summarize an entire singing performance over about 1 hour of continuous recording, we present a scatter plot of syllable features, where each dot represents the pitch versus Wiener entropy of one syllable; C, a trajectory of one song in this space. D–E, same representation for a different bird.

Figure 4. Auditory responses in the zebra finch brain are stimulus specific

A, BOLD responses to different stimuli at the medial portion of right and left hemispheres in zebra finches; B, summary of BOLD effects across birds. TUT tutor’s song, BOS bird’s own song, CON a song of unfamiliar zebra finch, TONE a 2000Hz tone. From Voss et al., 2007.

Figure 5. Auditory responses depend on developmental experience

A, BOLD responses in colony raised birds show strong differences comparing the bird’s own song (BOS) to a repeated syllable, but not in isolate males. B, a summary across stimuli show stimulus specific responses only in colony raised males, but not in isolate males, whose responses are highly variables. From Maul et al., 2010.