Abstract
The song of the northern mockingbird, Mimus polyglottos, is notable for its extensive length and inclusion of numerous imitations of several common North American bird species. Because of its complexity, it is not widely studied by birdsong scientists. When they do study it, the specific imitations are often noted, and the total number of varying phrases. What is rarely noted is the systematic way the bird changes from one syllable to the next, often with a subtle transition where one sound is gradually transformed into a related sound, revealing an audible and specific compositional mode. It resembles a common strategy in human composing, which can be described as variation of a theme. In this paper, we present our initial attempts to describe the specific compositional rules behind the mockingbird song, focusing on the way the bird transitions from one syllable type to the next. We find that more often than chance, syllables before and after the transition are spectrally related, i.e., transitions are gradual, which we describe as morphing. In our paper, we categorize four common modes of morphing: timbre change, pitch change, squeeze (shortening in time), and stretch (lengthening in time). This is the first time such transition rules in any complex birdsong have been specifically articulated.
Highlights
Among songbirds, the mockingbird has an extraordinarily variegated song that is much more complex than most species’ songs
We argue here that the variegated modes of mockingbird phrase-transition morphing are closer to those musical strategies than the stereotyped syllable morphing in mallard quacks, blackbird warning calls, or the like, which is more in line with efficiencybased explanations
Example Quantitative Analysis: Testing Whether Syllable Similarity Is Higher in Adjacent vs. Random Phrases To provide a quantitative way of telling whether morphing— i.e., transitioning from one phrase of sound elements to a phrase of related sound elements—is a true feature of mockingbird song, as opposed to a human heuristic created by biases of our own auditory system, we investigated whether measurable acoustic similarity of adjacent phrases is higher than expected by chance
Summary
The mockingbird has an extraordinarily variegated song that is much more complex than most species’ songs. Mockingbirds have enormous repertoires of song elements (Derrickson, 1987, 1988; Kershenbaum et al, 2015) which are arranged in a particular way: individual “syllables” (which can be a single sound, as in phrase 3, or a small group of sounds, as in phrase 1) are repeated to form short phrases, which, in turn, are strung up into long songs that can go on for minutes (Figure 1A) As indicated by their name, mockingbirds are famous for their ability to mimic the sounds of other species or their acoustic environment (Whittle, 1922; Laskey, 1944; Baylis, 1982; Farnsworth et al, 2011; Gammon, 2013; Gammon and Corsiglia, 2019). A second video shows a longer clip of a single mockingbird singingb, revealing a longer series of the transitions we categorize in the paper
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