Spectrogram cross-correlation can be used to measure the complexity of bird vocalizations

Birdsong is an important signal in mate attraction and territorial defense. Quantifying the complexity of these songs can shed light on individual fitness, sexual selection, and behavior. Several techniques have been used to quantify song complexity and be broadly categorized into diversity indices, measures of stationary probabilities, and measures of sequential variations. However, these methods are unable to account for important acoustic features like the frequency bandwidth and the variety in the shape of syllables which are an integral part of these vocal signals. This study proposes a new complexity measure that considers intra-song note variability and calculates a weighted index for birdsongs using spectral cross-correlation. We compared the previously described methods to understand the advantages and limitations based on the factors that would be affecting the complexity of songs. We developed a new method- Note Variability Index (NVI), which incorporates the spectral features of notes while quantifying complexity. This measure alleviates the need for manual annotations of notes that can be error-prone. We used Spectrogram Cross-Correlation (SPCC) to compare notes within a song and used the output values to quantify song complexity. To check for the efficacy of the new method, we generated synthetic songs to caricature extremes in song complexity and compared selected conventional complexity measures along with the NVI. We provide case-specific limitations of these methods. Additionally, to examine the efficacy of this new method in real-world scenarios, we used natural birdsongs from multiple species across the globe with varying song structures to compare conventional methods with NVI. To our knowledge, NVI is the only song complexity method that captures the variation of spectral features of notes in songs where the conventional methods fail to distinguish between similar song structures with different note types. As NVI does not need a manual classification of notes, it can be easily implemented for any type of birdsong with existing sound analysis softwares; it is very quick, avoids the possible biases in note classification, and can possibly be automated for large datasets in the future.

Song complexity in relation to repertoire size and phonological syntax in the breeding song of Purple Sunbird

There are multiple measures for bird song complexity such as repertoire size, phonological or compositional syntax and complex vocal mechanism (CVM). We examined these in an old-world passerine, Purple Sunbird. First, we identified and acoustically characterised the repertoire size (of notes and phrases). We then assessed positional fidelity and ordering of notes within phrases. We found 23 distinct notes by aural-visual inspection of spectrograms which was validated using a Classification and Regression Tree based on 5 acoustic parameters. These notes combined in different iterations to form 30 different phrases. Phrases comprised of an overall structure with an introductory note (prefix) at the onset, followed by notes occurring at specific positions within the phrase body, and terminated with a trill (suffix syllable(s)). Prefix was present in 93% of phrases whereas suffix syllable(s) occurred in 27% of phrases only. We found that notes exhibited positional fidelity and combined in specific order to form a phrase. This is indicative of underlying phonological syntax that limits the ways in which notes combine to form phrase and offers insights to song complexity. Finally, we found that suffix syllables exhibit the presence of mini-breath (very short inter-note interval) which are known to be produced by CVM.

Loss of cultural song diversity and the convergence of songs in a declining Hawaiian forest bird community

The effects of population decline on culturally transmitted behaviours in animals have rarely been described, but may have major implications to population viability. Learned vocal signals in birds are of critical importance to behaviours associated with reproduction, intrasexual interactions and group cohesion, and the complexity of vocal signals such as song can serve as an honest signal of an individual's quality as well as the viability of a population. In this study, we examined how rapid population declines recently experienced by Hawaiian honeycreepers on the island of Kaua‘i (USA) may have influenced the diversity, complexity and similarity of learned honeycreeper songs. We analysed the acoustic characteristics of songs recorded during three time periods over a 40-year time frame for three species of declining Kaua‘i honeycreepers. We detected a loss of song complexity and diversity over the 40-year time period that paralleled dramatic population declines. Concurrent with the loss of complexity, we also found that the acoustic characteristics of the three honeycreepers' songs became more similar to one another. To our knowledge, this is the first documentation of convergence of acoustic characteristics among rapidly declining species. The reduction in song complexity and diversity and convergence of songs not only signals a loss of culturally transmitted behaviours in these endemic Hawaiian honeycreepers, but also potential challenges to the recovery of these rapidly declining species. Moreover, the present study highlights that there is a ‘hidden’ cost to declining populations beyond just the loss of individuals that is not often considered, the loss of culturally transmitted social behaviours.

Signal complexity communicates aggressive intent during contests, but the process is disrupted by noise

Contestants use displays to signal their aggressive intent and settle disputes before they escalate. For birds, this is often in the form of song, which can vary in structural complexity. The role of song complexity in signalling aggressive intent has not been fully established, and its efficacy could be influenced by background noise levels. Using playback experiments, we found that in European robins, Erithacus rubecula, song complexity signalled sender aggression and affected receiver response. However, increased noise impacted the ability of contestants to adjust response based on opponent song complexity. These findings provide new evidence regarding the use of acoustic signal complexity for assessing opponent aggression and that noise can influence contest behaviour by interrupting this process, which could impose fitness consequences.

Are commonly used metrics of bird song complexity concordant?

Researchers studying the evolution of animal communication often ask what generates and maintains signal complexity, but they define and measure complexity in different ways. If different metrics are not concordant, then studies using them are probably not comparable. In this study, we asked whether 7 metrics of bird song complexity vary in tandem among individuals of a single species, the Rock Wren (Salpinctes obsoletus). The included metrics were chosen because they are regularly used by researchers in the field to test hypotheses within the literature. Results indicated that none of the metrics positively correlated with any others, suggesting that bird songs are not under general selective pressures favoring increased complexity across multiple features. If metrics of signal complexity are not correlated, then care should be taken when designing analyses and comparing results. Researchers should always clearly define the “complexity” under investigation and verify that it has significance to the study species. Contradictory outcomes among existing research into signal “complexity” may result from the fact that this single word is used to represent multiple independent traits.