Song Units Research Articles

Blue whale song sequencing off Southern California

Blue whales off Southern California produce a unique song consisting of pulsed A unit and tonal B unit calls. These units are either sequenced in an alternating AB pattern, or a single A unit is followed by multiple B units (here denoted ABB). To investigate whether there is geographic variation in the occurrence of these different sequence types, data were analyzed from four High-frequency Acoustic Recording Packages deployed at two sites each inshore and offshore of the Channel Islands from September 2009 to June 2010. Additionally, fine-scale behavior associated with song and its sequencing was analyzed from acoustic tags deployed on blue whales in Southern California since 2000. A higher proportion of song bouts detected in offshore recordings were type ABB, whereas the dominant song type observed inshore was AB. This pattern may indicate geographic variability in song function. Most song units recorded on tags were produced during surface-travel dives, with the whale less than 30 m deep. Song bouts detected on tags occurred primarily at night, and whales did not interrupt their calling sequence with breathing intervals. The observed differences in calling and song preference off Southern California may be useful in identifying regionally distinct behavioral contexts for blue whales.

Zebra Finch Song Phonology and Syntactical Structure across Populations and Continents-A Computational Comparison.

Learned bird songs are often characterized by a high degree of variation between individuals and sometimes between populations, while at the same time maintaining species specificity. The evolution of such songs depends on the balance between plasticity and constraints. Captive populations provide an opportunity to examine signal variation and differentiation in detail, so we analyzed adult male zebra finch (Taeniopygia guttata) songs recorded from 13 populations across the world, including one sample of songs from wild-caught males in their native Australia. Cluster analysis suggested some, albeit limited, evidence that zebra finch song units belonged to universal, species-wide categories, linked to restrictions in vocal production and non-song parts of the vocal repertoire. Across populations, songs also showed some syntactical structure, although any song unit could be placed anywhere within the song. On the other hand, there was a statistically significant differentiation between populations, but the effect size was very small, and its communicative significance dubious. Our results suggest that variation in zebra finch songs within a population is largely determined by species-wide constraints rather than population-specific features. Although captive zebra finch populations have been sufficiently isolated to allow them to genetically diverge, there does not appear to have been any divergence in the genetically determined constraints that underlie song learning. Perhaps more surprising is the lack of locally diverged cultural traditions. Zebra finches serve as an example of a system where frequent learning errors may rapidly create within-population diversity, within broad phonological and syntactical constraints, and prevent the formation of long-term cultural traditions that allow populations to diverge.

Automated analysis of song structure in complex birdsongs

Understanding communication and signalling has long been strived for in studies of animal behaviour. Many songbirds have a variable and complex song, closely connected to territory defence and reproductive success. However, the quantification of such variable song is challenging. In this paper, we present a novel, automated method for detection and classification of syllables in birdsong. The method provides a tool for pairwise comparison of syllables with the aim of grouping them in terms of their similarity. This allows analyses such as (1) determining repertoire size within an individual, (2) comparing song similarity between individuals within as well as between populations of a species and (3) comparing songs of different species (e.g. for species recognition). Our method is based on a particular feature representation of song units (syllables) which ensures invariance to shifts in time, frequency and amplitude. Using a single song from a great reed warbler, Acrocephalus arundinaceus, recorded in the wild, the proposed algorithm is evaluated by means of comparison to manual auditory and visual (spectrogram) song investigation by a human expert and to standard song analysis methods. Our birdsong analysis approach conforms well to manual classification and, moreover, outperforms the hitherto widely used methods based on mel-frequency cepstral coefficients and spectrogram cross-correlation. Thus, our algorithm is a methodological step forward for analyses of song (syllable) repertoires of birds singing with high complexity.

Humpback whale (Megaptera novaeangliae) song unit and phrase repertoire progression on a subarctic feeding ground.

The songs of the male humpback whales have traditionally been associated with breeding activities at low latitude breeding grounds during winter. This study provides the first detailed analysis of humpback whale songs recorded in the subarctic waters of Iceland using passive acoustic recorders. Recordings were collected during three winter seasons: 2008-2009, 2009-2010, and 2011 during which singing was detected in all seasons. Peak song occurrence was during January-February in all years; this coincides with the timing of the peak breeding season of humpback whales in the Northern hemisphere. A total of 2810 song units from all years were measured and statistically divided into 14 groups, which constructed 25 phrases. The song unit repertoires included stable song unit types that occurred frequently in songs during all years while the occurrence of other song unit types varied more between years. Around 60% of the phrases were conserved between the first two study seasons, while the majority of phrases found during the last study season had not been observed before. This study indicates the importance of a subarctic feeding ground for song progression and song exchange and possibly as an opportunistic mating ground for migrating or overwintering humpback whales.

Validating the reliability of passive acoustic localisation: a novel method for encountering rare and remote Antarctic blue whales

Since its near extirpation during the period of industrial whaling in the early and mid 20th century, the once common Antarctic blue whale Balaenoptera musculus intermedia re mains extremely rare. While annual systematic surveys around Antarctica from 1978 to 2009 re corded only 216 visual encounters of this species, their loud and distinctive calls were detected frequently throughout the Southern Ocean. We describe and assess a new method for locating these whales by acoustically detecting their vocalisations, tracking the location of their calls, and finally locating the whales visually. This methodology was employed during an Antarctic research voyage from 140° E to 170° W, between January and March 2013. The loudest song unit (a 26 Hz tone) was detected at all 298 recording sites south of 52° S. Acoustically derived bearings from these whales enabled visual observers to eventually sight the whales, often hundreds of kilometres from initial acoustic detections. Received sound pressure levels of detections increased with decreasing range to several hotspots where both song and non-song calls were detected. Within hotspots, short- range acoustic localisation yielded 33 visual encounters of Antarctic blue whales (group size: 1 to 5 whales) over a 31 d period south of 60° S. These results demonstrate that acoustic tracking pro- vides the capacity to locate Antarctic blue whales widely dispersed over many hundreds of kilo- metres, as well as the capacity to acoustically track individual whales for days at a time irrespec- tive of most weather conditions. Thus, passive acoustic localisation is a reliable and efficient method to track Antarctic blue whales, and this technique should be considered for future studies of these iconic animals.

Non-song vocalizations of pygmy blue whales in Geographe Bay, Western Australia.

Non-song vocalizations of migrating pygmy blue whales (Balaenoptera musculus brevicauda) in Western Australia are described. Simultaneous land-based visual observations and underwater acoustic recordings detected 27 groups in Geographe Bay, WA over 2011 to 2012. Six different vocalizations were recorded that were not repeated in a pattern or in association with song, and thus were identified as non-song vocalizations. Five of these were not previously described for this population. Their acoustic characteristics and context are presented. Given that 56% of groups vocalized, 86% of which produced non-song vocalizations and 14% song units, the inclusion of non-song vocalizations in passive-acoustic monitoring is proposed.

Humpback whale songs during winter in subarctic waters

The songs of the male humpback whales (Megaptera novaeangliae) have traditionally been associated with mating at tropical and subtropical mating grounds during winter. However, songs also occur out of mating season, both on feeding grounds in spring, late summer and fall. This study provides the first report of humpback whale singing behaviour in the subarctic waters of Northeast Iceland (Skjalfandi Bay) using long-term bottom-moored acoustic recorders during September 2008–February 2009 and from April to September 2009. Singing started in late November and peaked in February, within the breeding season. No songs were detected from spring to fall, despite visual detections of humpback whales. Non-song sound signals from humpback whales were detected during all recording months. Songs were partly composed of fundamental units common with other known mating grounds, and partly of song units likely unique to the study area. The variety of song unit types in the songs increased at the end of the winter recordings, indicating a gradual change in the songs throughout the winter season; as has been shown on traditional mating grounds. The relative proportion of songs compared with non-song signals was higher during dark hours than daylight hours. The short light periods of the winter, and where food is available, likely influence the daily occurrence of humpback whales’ songs in the subarctic.

Localization of humpback whale signals with two directional frequency analysis and recording sonobouys

For many years, the navy has used directional frequency analysis and recording (DIFAR) sonobouys to record and track ships. DIFAR sonobouys compute acoustic particle velocity for two bimodal perpendicular hydrophone elements, which, with a magnetic compass, gives a directional bearing to the sound recorded on a third omnidirectional hydrophone. Use of DIFAR represents an easily deployable alternative method to standard time-difference of arrival localization with towed or bottom-mounted hydrophone arrays. In this study, we used two tethered DIFAR sonobouys to record and track playbacks of humpback whale song. GPS positions of playbacks were recorded. Digital signals were recorded at the sonobouy before multiplexing. Song units were recorded on both sonobouys, and then matched between sonobouys using time from the sonobouy GPS. To localize the signals, azimuths from the DIFAR sonobouys were combined with the time difference of arrival parabola to create a three-dimensional likelihood surface that gives the probabilities that a signal originated at a specific point in space. The locations with maximum likelihood were used to estimate source level of whale calls, and singing whales were tracked over time. Comparisons will be made between DIFAR methods and GPS locations from the playback boat, to ground-truth this localization methodology.

Does true syntax or simple auditory object support the role of skylark song dialect?

Parallels between birdsong and human language are numerous and include particular temporal arrangements of acoustic units and the existence of dialects. In animal communication, modifications of the temporal ordering of existing acoustic units have rarely been clearly linked with changes in information content, particularly in a natural environment. Here, we show that the organization of birdsong units (‘syllables’) in sequences supports interindividual relationships within skylark communities. We manipulated the temporal arrangement of song dialect variants (‘shared phrases’) in the skylark, Alauda arvensis, a songbird with a very large repertoire of syllables and complex song. When tested with playback experiments performed in the field, skylarks were able to perceive subtle differences in the ordering of syllables. Modifications of the syllable ordering within shared phrases changed the information content from ‘group member’ to ‘unfamiliar individual’ and induced more aggressive reactions than shared phrases with a preserved syllable arrangement. Shared phrases often varied between individuals in the number of successive repetitions of similar syllable types, but were very consistent in terms of syllable type ordering. Our results indicate that skylarks rely not simply on the composition in syllable types of shared phrases to recognize group members, but on syllable type ordering. Shared phrases could be perceived by birds as ‘auditory objects’ embedded within songs. Alternatively, birds might identify incorrect phrases using grammatical rules governing the succession of syllables composing the phrases shared by their group. The presence of between-individual variation in phrase length, associated with consistent syllable type ordering revealed by our analysis, suggests that the latter hypothesis is more likely. Our results show that birds perceive disruptions in the natural temporal pattern of song units, and that this temporal pattern is behaviourally salient and carries information.

Variation in the songs of humpback whales (Megaptera novaeangliae) wintering in the Northwestern and Main Hawaiian Islands

A study of the humpback whale song in the Northwestern Hawaiian Islands (NWHI) and the Main Hawaiian Islands (MHI) during the 2009 season suggests that humpback whale song may be more variable than previously suggested. Data from five autonomous acoustic recorders deployed at locations in the NWHI and MHI were analyzed to compare the frequency of occurrence of song units by whales in the island chain. There appears to be a gradient of differences in song units throughout the Hawaiian Island chain, rather than the previously assumed, more discrete differences between breeding populations. Recordings from each site were randomly selected. Song units were classified as one of 23 units and counted to compare between sites. Changes in the frequency of occurrence in a few of the most abundant units suggest a gradual change throughout the island chain. However, this may be confounded by changes that occur throughout the season throughout the ocean basin. Further work examining the amount of variation both between and within humpback whale breeding populations should be conducted.

Comparison of automated and aural/visual techniques to classify humpback whale (Megaptera novaeangliae), song units

Humpback whale song research has focused on analyzing the full song structure rarely describing individual song units. Even less progress has been made in automatically distinguishing and classifying these individual units. Two different techniques were employed to study their call units, visual/aural and automated/statistical. Humpback whale songs were recorded in the Hawaiian Islands both remotely with an autonomous acoustic recorder and by a snorkeler with a portable digital tape recorder. Humpback whale song units collected by the autonomous acoustic recorders were aurally separated into 23 distinct units in a companion study. Song units collected by a snorkeler using the portable recorder off Maui were analyzed using a specialized Matlab script that defined 48 frequency and temporal parameters for each unit. From the 48 parameters, the units were separated into distinct categories using a multivariate categorical analysis. The distinct units were compared between the different techniques to gage if automated methods could be used in future humpback whale studies. After this comparison was made, a principal component analysis (PCA) determined which of the aforementioned 48 parameters were important in statistically distinguishing between the distinct units furthering our understanding of frequency and temporal importance in categorizing song structure.

Characterizing and classifying humpback whale (Megaptera novaeangliae) song units.

Humpback whales, Megaptera novaeangliae, are one of the most recognizable and investigated marine mammals. However, little progress has been made in automatically distinguishing and classifying individual units of their song. A Matlab script has been developed to characterize the different song units and to apply the appropriate statistics to separate and categorize each unit. The Matlab program measures 48 parameters from each song unit. The songs were recorded by a swimmer snorkeling above vocalizing humpbacks in the waters off Maui, HI. A 16 bit, digital tape recorder with the automatic gain control disabled and a sample rate of 44.1 kHz was used to record songs from different whales. The swimmer determined the range of the whale using a portable handheld fathometer. Singing whales typically suspended themselves in the water column at depths varying from 15 to 30 m, which was contingent on the bottom depth. Song units were separated into distinct categories using a principle component analysis (PCA) based on the 48 parameters describing each unit. A classification algorithm was then developed based on the categories determined by the PCA. The classifier will be integrated into a modified version of the real-time Odontocete call classification algorithm (ROCCA) for future analyzes.

A comparison of the song of humpback whales (Megaptera novaeangliae) wintering in the Northwestern and Main Hawaiian Islands.

An extensive study of the population structure of North Pacific humpback whales (SPLASH) has revealed that many of the whales feeding in the Aleutian Islands and the Bering Sea are not observed in known wintering and breeding areas in the Pacific. Recent findings suggest that the Northwestern Hawaiian Islands (NWHIs) represent a previously undocumented wintering area that may be distinct from the Main Hawaiian Islands’ (MHIs’) wintering ground. To examine this issue further, data from Ecological Acoustic Recorders (EARs) deployed at five locations in the NWHIs and MHIs were analyzed to compare the structure of songs produced by whales in both areas. Previous research suggests that songs from different breeding groups differ from one another, providing a method to evaluate whether whales in the NWHIs are part of the same breeding group as those in the MHIs. Recordings from each site were randomly selected, and song units were classified and counted to compare between sites. Preliminary results indicate that there are differences between the NWHIs and MHIs. These findings suggest that further work using photo ID should be undertaken to confirm whether whales in the NWHIs are indeed those feeding in the Aleutians and the Bering Sea.

A real-time method for autonomous passive acoustic detection-classification of humpback whales

This paper describes a method for real-time, autonomous, joint detection-classification of humpback whale vocalizations. The approach adapts the spectrogram correlation method used by Mellinger and Clark [J. Acoust. Soc. Am. 107, 3518-3529 (2000)] for bowhead whale endnote detection to the humpback whale problem. The objective is the implementation of a system to determine the presence or absence of humpback whales with passive acoustic methods and to perform this classification with low false alarm rate in real time. Multiple correlation kernels are used due to the diversity of humpback song. The approach also takes advantage of the fact that humpbacks tend to vocalize repeatedly for extended periods of time, and identification is declared only when multiple song units are detected within a fixed time interval. Humpback whale vocalizations from Alaska, Hawaii, and Stellwagen Bank were used to train the algorithm. It was then tested on independent data obtained off Kaena Point, Hawaii in February and March of 2009. Results show that the algorithm successfully classified humpback whales autonomously in real time, with a measured probability of correct classification in excess of 74% and a measured probability of false alarm below 1%.

Geographic variation in songs of the Tree Pipit (Anthus trivialis) at two spatial scales

ABSTRACT. We studied geographic as well as individual variation of songs in five Czech populations of the Tree Pipit (Motacillidae: Anthus trivialis), a passerine bird with a medium-sized syllable repertoire and complex song. We focused on repertoire composition at the level of syllables (the smallest invariant song units) and of structures that are potentially important in recognition among individuals (i.e., bi-syllables, or regular repetitions of two different syllables) and among populations (introductory phrases). We tested the hypotheses that the repertoires reflect local dialects (i.e., different male origins) and that within-site similarities of male repertoires are distance dependent. The populations studied differed significantly in all analyzed characteristics. Male origin explained ∼18% of the variation in individual syllable repertoires. However, no clear within-site geographic structure in repertoire similarity was observed, either between closest neighbors or on larger scales. We observed a...

An analysis of Humpback whale songs for individual classification

Acoustics based tracking systems are in use for studying cetacean behaviour. Such noninvasive techniques can also prove efficient for population assessment of vocally active species. One problem in this regard is to distinguish already counted reappearing cetaceans, even if reappearance occurs after a brief interval. In this connection we propose development of individual identification system for humpback whales (Megaptera novaeangliae) based on unique acoustic features underlying a song. Earlier we analyzed recognition performance of cepstrum based voice signature in seven humpback whales. The results were based on training and test data sampled from the same song. In this work, we test our hypothesis using data from different songs (i.e., recording timings of training and test data sets do not overlap) for two different individuals. Recognition rates for two individuals are above 80% and 95%, respectively. However an eight years old song gives degraded recognition rate of around 58%. The results give an insight into the effects of temporal song evolution on accuracy of our system. Our observations are followed by harmonic analysis of song units in two individuals. Cepstral coefficients and support vector machine (SVM) were used for classification and signal band‐crossing rate was used for segmenting song units.

Implications of shallow water propagation for ranging by marine mammals

Marine mammals rely heavily on acoustic communication to locate one another in low‐visibility underwater environments. How these animals localize sounds sources is poorly understood. The speed of sound, attenuation, and reverberation in shallow water environments differ from those in terrestrial environments such that intensity cues alone may not be sufficient for ranging. The purpose of this study was to examine sound propagation in shallow, coastal waters near Rincon, Puerto Rico, and to determine what acoustic cues provide consistent information about source distance. Narrow‐band tones and broadband sounds of similar frequency content (100–22 000 Hz) and duration (2.0 s) to sound units of humpback whale song were broadcast from an underwater speaker. Sounds were recorded at known distances ranging from 70–850 m from the sound source. The received intensity was negatively correlated with distance propagated; broadband sounds showed a stronger correlation than narrow‐band sounds. Analysis of the inten...

Analysis of Australian humpback whale song using information theory

Songs produced by migrating whales were recorded off the coast of Queensland, Australia over 6 consecutive weeks in 2003. Approximately 50 songs were analyzed using information theory techniques. The average length of the songs estimated by correlation analysis was approximately 100 units, with song sessions lasting from 300 to over 3100 units. Song entropy, a measure of structural constraints and complexity, was estimated using three different methodologies: (1) the independently identically distributed model; (2) first-order Markov model; and (3) the nonparametric sliding window match length (SWML) method, as described in Suzuki et al. [J. Acoust. Soc. Am. 119, 1849 (2006)]. The analysis finds the songs of migrating Australian whales are consistent with the hierarchical structure proposed by Payne and McVay (Science 173, 585−597 (1971)], and recently confirmed by Suzuki et al. for singers on the breeding grounds. Both the SWML entropy estimates and the song lengths for the Australian singers were lower than that reported by Suzuki et al. for Hawaiian whales in 1976−1978. These lower SWML entropy values indicate a higher level of predictability within songs. The average total information in the Australian sequence of song units was approximately 35 bits/song. Aberrant songs (10%) yielded entropies similar to the typical songs. [Sponsored by ONR and DSTO.]

Anthropogenic noise and baleen whales: A perspective from humpback whale acoustics

The effects of anthropogenic noise on baleen whales is a topic of intense interest not only to marine mammalogists but also to a variety of commercial interests, the military, oceanographers, and researchers that use sounds in the ocean. One of the major difficulties in setting standards for exposure levels and the effective use of anthropogenic sounds is a lack of knowledge of the auditory properties of whales. The auditory sensitivity and frequency range of hearing (audiogram) are not known, as well as how anthropogenic sounds physically affect the auditory system, the psychological well being, and the use of sounds by whales. One approach to gain some insight in this area is to study the acoustics of a baleen whale species, the humpback whale. The source levels and frequency range of humpback whale song units have been measured and can provide insights into what these whales expose each other to. The highest rms source level was 173 dB for a specific unit. The average sound pressure levels of chorusing have been measured and can provide insights into the long-term acoustic environment of humpback whales. The highest daily average of sound pressure level for chorusing sounds was 130 dB.

Vocalizations of blue whales in the Antarctic: Implications for the use of passive acoustics for population studies on baleen whales

Stereotypical calls of baleen whales can be a diagnostic field characteristic for determining species and stocks. Repeated series of these calls may be used for long‐term population studies using passive acoustics. Vocalizations were recorded from 15 groups of blue whales (consisting of 42 individuals) during the 2001–02 and 2002–03 International Whaling Commission‐Southern Ocean Whale and Ecosystem Research (IWC‐SOWER) surveys in Antarctic waters north and east of the Ross Sea. Long‐duration vocalizations attributed to Antarctic blue whales included 28‐Hz ‘‘tones,’’ 28 Hz ‘‘downsweeps,’’ and the more complex ‘‘3‐unit’’ calls (considered to be song units). The 28‐Hz component was stable for all three calls, regardless of signal strength. The 3‐unit calls varied in their 2nd and 3rd units and were heard in only 10 out of the 15 groups. There was no evidence of repeated patterns (song phrases) of 3‐unit calls. The results of this study suggest that the 28‐Hz peak frequency may be used as a diagnostic feature for acoustic identification of Antarctic blue whales in the field. Additional research on the gender, group size, and associated behavior of these animals is necessary to ground‐truth long‐term acoustic population studies of baleen whales.