A decrease in the frequency of two southeast Pacific blue whale song types was examined over decades, using acoustic data from several different sources in the eastern Pacific Ocean ranging between the Equator and Chilean Patagonia. The pulse rate of the song units as well as their peak frequency were measured using two different methods (summed auto-correlation and Fourier transform). The sources of error associated with each measurement were assessed. There was a linear decline in both parameters for the more common song type (southeast Pacific song type n.2) between 1997 to 2017. An abbreviated analysis, also showed a frequency decline in the scarcer southeast Pacific song type n.1 between 1970 to 2014, revealing that both song types are declining at similar rates. We discussed the use of measuring both pulse rate and peak frequency to examine the frequency decline. Finally, a comparison of the rates of frequency decline with other song types reported in the literature and a discussion on the reasons of the frequency shift are presented.

Humpback whale song is comprised of well-structured distinct levels of organization: combinations of sounds, repetition of combinations, and a sequence of repetitions, which have no clear silent intervals. This continuous sound output can be hard to delimit, rather, it could be interpreted as a long series of states of a system. Recurrence plots are graphical representations of such series of states and have been used to describe animal behavior previously. Here, we aim to apply this tool to visualize and recognize structures traditionally used in inferences about behavior (songs and themes) in the series of units manually extracted from recordings of humpback whales. Data from the Abrolhos bank, Brazil were subjected to these analyses. Our analytical tool has proven efficient in identifying themes and songs from continuous recordings avoiding some of the human perception bias and caveats. Furthermore, our song extraction is robust to errors coming from both manual and automated transcriptions, constructing a level of description largely independent of the first stage of analysis. HIGHLIGHTS• The complex hierarchical and cyclical structure of humpback whale song can be visually represented in recurrence plots • Recurrence plots of humpback whale singing provide insights into the intraindividual variation in song structures of a male • Our tool can help extracting structures, from long recordings, removing some human perceptual caveats

Pulsed sounds are an interesting example of complex biological sounds, frequent in cetaceans' vocalizations. A two-group classification of these sounds is proposed: tonal or non-tonal. Through two simple mathematical models, it is shown that this classification can lead to better techniques for measuring the pulse rate. This classification is thus useful for improving measurement accuracy, but can also help in formulating hypotheses regarding mechanisms of sound production. This method of classification is applied to south Pacific blue whale vocalizations and it is found that the pulse rate corresponds to the fundamental frequency (not expressed in the spectrum) of the song. Thus, the hypothesis that the sound is produced by only one organ and then filtered by the body of the giant is reinforced.