Abstract
Objective identification and description of mimicked calls is a primary component of any study on avian vocal mimicry but few studies have adopted a quantitative approach. We used spectral feature representations commonly used in human speech analysis in combination with various distance metrics to distinguish between mimicked and non-mimicked calls of the greater racket-tailed drongo, Dicrurus paradiseus and cross-validated the results with human assessment of spectral similarity. We found that the automated method and human subjects performed similarly in terms of the overall number of correct matches of mimicked calls to putative model calls. However, the two methods also misclassified different subsets of calls and we achieved a maximum accuracy of ninety five per cent only when we combined the results of both the methods. This study is the first to use Mel-frequency Cepstral Coefficients and Relative Spectral Amplitude - filtered Linear Predictive Coding coefficients to quantify vocal mimicry. Our findings also suggest that in spite of several advances in automated methods of song analysis, corresponding cross-validation by humans remains essential.
Highlights
The fundamental step to any study of vocal mimicry is to distinguish between mimicked calls and species-specific calls in an objective manner
If we set a threshold of 80% accuracy for each call, i.e. correct matches in at least four of the five trials per call, according to this criterion, 15 of the 21 (71.4%) mimicked calls that were tested were matched correctly to their models in the first rank; this increased to 81% when we included second-ranked correct matches (Fig. 5 A, grey bars)
One call was not matched to its putative model in the first or second rank by both the computer-based methods and humans (Table 1: Oriental honey buzzard courtship call) and will be deleted from the mimicked calls data set. This is the first study of vocal mimicry in the racket-tailed drongo that has attempted to quantify mimicry using an objective approach
Summary
The fundamental step to any study of vocal mimicry is to distinguish between mimicked calls and species-specific calls in an objective manner This is usually done by listening to available sound libraries of a number of different species and identifying model species based on human psychophysical, often qualitative, perceptions of similarities between calls. This is commonly backed by visual inspection of spectrograms [1,2,3]. Hamao and Eda-Fujiwara [12] were, probably the first to attempt an objective definition of mimicry in their study of the black-browed reed warbler (Acrocephalus bistrigiceps) They were the first to study mimicry of a relatively large number of model species (eight)
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