Phase-Based Frequency Tracking and Analysis of Vocalisations in the Genus Saxicola

Abstract

Acoustic analysis provides valuable tools for studying bird vocalisations, but quantitative characterisation of call structure remains challenging when frequency and amplitude vary jointly over time. We present a phase-based frequency tracking method that represents amplitude as a function of instantaneous frequency and use it to characterise alarm calls in the genus Saxicola. The method enables the extraction of acoustic features describing the frequency trajectory and temporal structure of individual calls. We apply the method to a dataset of 801 calls from 49 recordings representing 15 species, and use principal component analysis (PCA) and robust PCA to investigate patterns of acoustic variation across species, recordings and geographic regions. The results demonstrate the feasibility of the approach and reveal structured patterns of acoustic similarity and variation among the calls analysed. Robust PCA concentrates the variance of the low-rank representation into fewer principal components than conventional PCA applied to the original feature matrix, providing a complementary representation of the dominant acoustic structure. The observed patterns illustrate the potential of quantitative acoustic characterisation for comparative studies of vocal variation. However, uneven species representation and uncertainty in individual identity and taxonomic labels limit the biological conclusions that can be drawn from the present dataset. The method provides a foundation for further acoustic research, in which broader sampling and integration with independent genetic and morphological evidence could help investigate variation within and among species.