Quantitative Tools for Examining the Vocalizations of Juvenile Songbirds

The singing of juvenile songbirds is highly variable and not well stereotyped, a feature that makes it difficult to analyze with existing computational techniques. We present here a method suitable for analyzing such vocalizations, windowed spectral pattern recognition (WSPR). Rather than performing pairwise sample comparisons, WSPR measures the typicality of a sample against a large sample set. We also illustrate how WSPR can be used to perform a variety of tasks, such as sample classification, song ontogeny measurement, and song variability measurement. Finally, we present a novel measure, based on WSPR, for quantifying the apparent complexity of a bird’s singing. 1. Introduction A bird’s song can be a powerful marker of identity, used by other birds—and humans—to identify the singer’s species or even to identify a single individual. In many species this song is innate, but for the Oscine songbirds, every bird must acquire its own song [1, 2]. With one such bird, the zebra finch (Taeniopygia guttata), it is the males that sing, and juvenile males learn their song from nearby adults such as their father [3]. The learning process has two overlapping but distinct parts: in the first, the animal hears the songs of other birds and somehow commits to memory a model of the song it will sing; in the second, the animal learns how to produce a version of this memorised song through practice [1]. As adults, zebra finches sing in bouts during which they perform their single song motif a variable number of times. The song motif of a zebra finch is on the order of one second long and is composed of multiple syllables, elements separated by silence or a sharp drop in amplitude. Syllables can often be broken down further into notes, segments of distinct sound quality. These notes may demonstrate pronounced frequency modulation and complex harmonics. Adult zebra finches typically exhibit a very high degree of stereotypy in their song, with one performance of the song’s motif being very similar to any other. Two typical examples are shown in Figure 1. Figure 1: (a) Spectrogram of a bout of singing from an adult zebra finch. Noted in the figure are the following song parts: introductory notes, underlined in red; syllables, underlined in green; the silent interval between syllables, underlined in yellow. The blue lines mark the repetitions of the bird’s motif. Note that each performance of the motif appears much like the others, except for the truncated final motif. (b) Spectrogram of a bout of singing from a different zebra finch. Although its song is also highly