Humpback whale song hierarchical structure: Historical context and discussion of current classification issues

Consistent and well-defined criteria for the classification and measurement of humpback whale song features are essential for robust comparisons between investigators. Song structure terminology has been well-established and used by many authors, though at times inconsistently. This review discusses the development of the nomenclature describing humpback song and explores the potential significance of the often-overlooked variation in song patterns. Within the hierarchical definition of humpback song, the most problematic issues arise from the inconsistent delineation of phrase types, and the use of the metric of song duration without regards to variability in thematic sequence. With regards to the former, a set of guidelines is suggested to facilitate consistent delineation of phrases. With regards to the latter, current research demonstrates that the song duration metric has resulted in the disregard of variability at this level, which is more widespread than traditionally reported. An exemplar case is used to highlight the problem inherent in defining and measuring song duration. Humpback song is evaluated within the framework of avian songbird research, and a shift in analysis paradigm is recommended, towards phrase-based analyses in which sequences of phrases are treated as a salient feature of song pattern.