Skewed performance distributions as evidence of motor constraint in sports and animal displays

Animal displays (i.e. movement-based signals) often involve extreme behaviours that seem to push signallers to the limits of their abilities. If motor constraints limit display performance, signal evolution will be constrained, and displays can function as honest signals of quality. Existing approaches for measuring constraint, however, require multiple kinds of behavioural data. A method that requires only one kind could open up new research directions. We propose a conceptual model of performance under constraint, which predicts that the distribution of constrained performance will skew away from the constraint. We tested this prediction with sports data, because we know a priori that athletic performance is constrained and that athletes attempt to maximize performance. Performance consistently skewed in the predicted direction in a variety of sports. We then used statistical models based on the skew normal distribution to estimate the constraints on athletes and displaying animals while controlling for potential confounds and clustered data. We concluded that motor constraints tend to generate skewed behaviour and that skew normal models are useful tools to estimate constraints from a single axis of behavioural data. This study expands the toolkit for identifying, characterizing, and comparing performance constraints for applications in animal behaviour, physiology and sports.

Learning to pause: Fidelity of and biases in the developmental acquisition of gaps in the communicative signals of a songbird

The temporal organization of sounds used in social contexts can provide information about signal function and evoke varying responses in listeners (receivers). For example, music is a universal and learned human behavior that is characterized by different rhythms and tempos that can evoke disparate responses in listeners. Similarly, birdsong is a social behavior in songbirds that is learned during critical periods in development and used to evoke physiological and behavioral responses in receivers. Recent investigations have begun to reveal the breadth of universal patterns in birdsong and their similarities to common patterns in speech and music, but relatively little is known about the degree to which biological predispositions and developmental experiences interact to shape the temporal patterning of birdsong. Here, we investigated how biological predispositions modulate the acquisition and production of an important temporal feature of birdsong, namely the duration of silent pauses ("gaps") between vocal elements ("syllables"). Through analyses of semi-naturally raised and experimentally tutored zebra finches, we observed that juvenile zebra finches imitate the durations of the silent gaps in their tutor's song. Further, when juveniles were experimentally tutored with stimuli containing a wide range of gap durations, we observed biases in the prevalence and stereotypy of gap durations. Together, these studies demonstrate how biological predispositions and developmental experiences differently affect distinct temporal features of birdsong and highlight similarities in developmental plasticity across birdsong, speech, and music. RESEARCH HIGHLIGHTS: The temporal organization of learned acoustic patterns can be similar across human cultures and across species, suggesting biological predispositions in acquisition. We studied how biological predispositions and developmental experiences affect an important temporal feature of birdsong, namely the duration of silent intervals between vocal elements ("gaps"). Semi-naturally and experimentally tutored zebra finches imitated the durations of gaps in their tutor's song and displayed some biases in the learning and production of gap durations and in gap variability. These findings in the zebra finch provide parallels with the acquisition of temporal features of speech and music in humans.

Tonality over a broad frequency range is linked to vocal learning in birds

Many birds emit tonal song syllables even though the sound sources generate sound with rich upper harmonic energy content. This tonality is thought to arise in part from dynamically adjusted filtering of harmonic content. Here, we compare tonality of song syllables between vocal learners and non-learners to assess whether this characteristic is linked to the increased neural substrate that evolved with vocal learning. We hypothesize that vocal learning ability is correlated with enhanced ability for generating tonal sounds, because vocal production learners might also have an enhanced ability to articulate their vocal tracts and sound source for producing tonality. To test this hypothesis, we compared vocal learners and non-learners from two groups (186 passerines and 42 hummingbirds) by assessing tonality of song syllables. The data suggest that vocal learners in both clades have evolved to sing songs with higher tonality than the related, non-vocal learning clades, which is consistent with stronger roles for broadband dynamic filtering and adjustments to the sound source. In addition, oscine songs display higher tonality than those of hummingbirds. A complex interplay of vocal tract biomechanics, anatomical differences of the sound source as well as increased motor control through vocal learning facilitates generation of broad tonality.

No evidence of repeated song divergence across multiple urban and non-urban populations of dark-eyed juncos (Junco hyemalis) in Southern California

Urbanization can affect species communication by introducing new selection pressures, such as noise pollution and different environmental transmission properties. These selection pressures can trigger divergence between urban and non-urban populations. Songbirds rely on vocalizations to defend territories and attract mates. Urban songbirds have been shown in some species and some populations to increase the frequencies, reduce the length and change other temporal features of their songs. This study compares songs from four urban and three non-urban populations of dark-eyed juncos (Junco hyemalis) throughout Southern California. We examined song length, trill rate, minimum frequency, maximum frequency, peak frequency and frequency bandwidth. We also compared songs recorded from one urban junco population in San Diego nearly two decades ago with recently collected data in 2018-2020. Over all comparisons, we only found significant differences between UCLA and the 2006/2007 UCSD field seasons in minimum and maximum frequencies. These findings partially support and are partially in contrast to previous urban song studies. As urban areas expand, more opportunities will arise to understand urban song divergence in greater detail.

No evidence of repeated song divergence across multiple urban and non-urban populations of dark-eyed juncos (Junco hyemalis) in Southern California

Urbanization can affect species communication by introducing new selection pressures, such as noise pollution and different environmental transmission properties. These selection pressures can trigger divergence between urban and non-urban populations. Songbirds rely on vocalizations to defend territories and attract mates. Urban songbirds have been shown in some species and some populations to increase the frequencies, reduce the length and change other temporal features of their songs. This study compares songs from four urban and three non-urban populations of dark-eyed juncos (Junco hyemalis) throughout Southern California. We examined song length, trill rate, minimum frequency, maximum frequency, peak frequency and frequency bandwidth. We also compared songs recorded from one urban junco population in San Diego nearly two decades ago with recently collected data in 2018-2020. Over all comparisons, we only found significant differences between UCLA and the 2006/2007 UCSD field seasons in minimum and maximum frequencies. These findings partially support and are partially in contrast to previous urban song studies. As urban areas expand, more opportunities will arise to understand urban song divergence in greater detail.

Woodpecker drum evolution: An analysis of covariation in elements of a multicomponent acoustic display among and within species

Multicomponent signals are found throughout the animal kingdom, but how these elaborate displays evolve and diversify is still unclear. Here, we explore the evolution of the woodpecker drum display. Two components of this territorial sexually selected signal, drum speed and drum length, are used by territory holders to assess the threat level of an intruding drummer. We explore the coevolution of these display components both among and within species. Among species, we find evidence for strong coevolution of drum speed and length. Within species, we find that drum speed and length vary largely independent of each other. However, in some species, there is evidence of covariation in certain portions of the drum length distribution. The observed differences in component covariation at the macro- and microevolutionary scales highlight the importance of studying signal structure both among and within species. In all cases of covariation at both evolutionary scales, the relationship between drum speed and length is positive, indicating mutual elaboration of display components and not a performance trade-off.

Signature whistles exhibit a 'fade-in' and then 'fade-out' pattern of relative amplitude declination

Bottlenose dolphins have individually distinct signature whistles that are characterized by a stereotyped frequency-time contour. Signature whistles are commonly exchanged with short time delays between calls. Dolphin whistles are produced by pressurized nasal sacs that increase and then decrease in pressure over emission. This study found that the relative amplitude modulation pattern over time exhibited the same fade-in and then fade-out pattern in the signature whistles of eight bottlenose dolphins at the Navy in San Diego, CA. Both the initial and final five percent of the whistle's duration also had significantly lower mean relative amplitude than the center five percent. The current analyses of the amplitude-time relationship was then integrated to a previously reported model of the negative relationship between relative log amplitude and log peak frequency. This produced a more robust model for accounting for the predictable aspects of the more broadly non-stereotyped amplitude modulations of signature whistles. Whether dolphins can intentionally manipulate these amplitude features or they are simple by-products of the sound production system, and further whether they are perceived and utilized by receivers, is an exciting area for continued research.

Non-stereotyped amplitude modulation across signature whistle contours

Bottlenose dolphin signature whistles are characterized by distinctive frequency modulation over time. The stable frequency contours of these whistles broadcast individual identity information. Little is known however, about whether or not the amplitude contour is also stereotyped. Here, we examined the relative amplitude-time contour of signature whistle emissions from eight bottlenose dolphins (Tursiops truncatus) in the U.S. Navy Marine Mammal Program (MMP) in San Diego, CA. The results suggested that unlike the stable frequency-time contour, the amplitude-time contour of signature whistles were largely non-stereotyped, characterized by large variability across multiple whistle emissions. Relative amplitude was negatively related to log peak frequency, with more energy focused in the lower frequency bands. This trend was consistent over all eight dolphins despite having quite different signature whistle contours. This relationship led to the amplitude contours being slightly more stereotyped within than between dolphins. We propose that amplitude across signature whistle emissions may serve as an avenue for encoding additional communicative information. We encourage future studies to incorporate analyses of amplitude contours in addition to frequency contours of signature whistles in order to begin to understand what role it may play in the dolphin communication system.

Independent evolution of song diversity and song motor performance in canaries, goldfinches and allies indicates clade-specific trade-offs in birdsong

The diversity and the motor performance of birdsongs can both be sexually selected. In wood warblers, most species with high motor performance sing a greater proportion of trills, presumably to advertise performance, and thus have lower syllable diversity. We tested if this trade-off between motor performance and syllable diversity extends to canaries, goldfinches and allies, a clade with much longer and more varied songs. We assembled a molecular phylogeny and inferred song motor performance based on the speed of frequency modulation either in trills or in within-song intervals. The two metrics of performance were positively, but only mildly, related across species. While performance evaluated in intervals had high phylogenetic signal, performance evaluated in trills changed independently of phylogeny and was constrained by body size. Species in densely vegetated habitats sang fewer trills, but did not differ in motor performance. Contrary to wood warblers, song motor performance did not predict the proportion of trilled syllables nor within-song syllable diversity, perhaps because large differences in the song duration of canaries, goldfinches and allies prevent trills from severely compromising syllable diversity. Opposed results in wood warblers and in these finches indicate the existence of clade-specific trade-offs in the evolution of birdsong.

Shared songs are of lower performance in the dark-eyed junco

Social learning enables the adjustment of behaviour to complex social and ecological tasks, and underlies cultural traditions. Understanding when animals use social learning versus other forms of behavioural development can help explain the dynamics of animal culture. The dark-eyed junco (Junco hyemalis) is a songbird with weak cultural song traditions because, in addition to learning songs socially, male juncos also invent or improvise novel songs. We compared songs shared by multiple males (i.e. socially learned) with songs recorded from only one male in the population (many of which should be novel) to gain insight into the advantages of social learning versus invention or improvisation. Song types shared by multiple males were on average of lower performance, on aspects of vocal performance that have been implicated in agonistic communication in several species. This was not explained by cultural selection among socially learned songs (e.g. selective learning) because, for shared song types, song performance did not predict how many males shared them. We discuss why social learning does not maximize song performance in juncos, and suggest that some songbirds may add novel songs to culturally inherited repertoires as a means to acquire higher-quality signals.

Studying the silent side of birdsong

The quality of complex communication signals, such as birdsong, is difficult to assess and compare across individuals or species. A new study on skylark song avoids the problem of signal complexity by assessing motor performance during the silent gaps of songs. This provides a metric of song quality applicable to species with very diverse songs, which facilitates novel types of analyses and comparisons in avian bioacoustics.

See research article: http://www.biomedcentral.com/1741-7007/12/58.

Being angry, singing fast? Signalling of aggressive motivation by syllable rate in a songbird with slow song

Syllable rate has been shown to play a role in male-male aggressive interactions and has been proposed to serve as a male quality indicator in several bird species. In those with fast syllable rates, males often increase rates when singing in aggressive context, and respond differently to test stimuli of varying rates. We asked whether the syllable rate fulfils a similar signalling function in the chiffchaff (Phylloscopus collybita), a songbird species with a slow syllable rate. We confronted 36 chiffchaff males with one of three playback types differing in syllable rate: control (non-manipulated rate), fast, or slow (artificially increased and decreased syllable rate, respectively). We recorded tested males' songs and behaviour before and during the experiment. Our results indicate that syllable rate might be an aggressive signal in chiffchaff. Males that physically attacked the loudspeaker during experiments sang faster songs spontaneously, and those that continued singing during the playback responded to fast and non-manipulated stimuli with substantial increase of syllable rate. Indirect evidence further suggests that syllable rate in chiffchaff is unlikely constrained by respiratory demands; thus, we propose that syllable rate in this species functions as a conventional signal of male aggressiveness rather than an index of quality.

Song performance and elaboration as potential indicators of male quality in Java sparrows

Bird songs have evolved under sexual selection pressure. Songs include multiple features that are subject to female preference, but recent comparative research has indicated evolutionary tradeoffs between song performance and complexity in some species. Trill, a repetition of the same sound, is a performance-related song trait; higher trill performance can be achieved at the cost of song complexity at the among-species or population level. The aim of this study was to examine whether such tradeoffs also account for within-species variation in Java sparrow songs, which include both multiple trill types and non-trill parts. We found a great individual variation in trill proportion, trill performance, and song complexity. A positive association between trill performance and body size suggested that trills can serve as an indicator of male quality. However, contrary to the tradeoffs predicted by previous studies based on other passerine species, trill performance and song complexity, i.e., note repertoire, were positively correlated: males in better condition can sing songs with larger note repertoires and higher trill performance, which may explain how trills and non-trill notes are both maintained and have co-evolved by sexual selection in Java sparrow songs.

Physically challenging song traits, male quality, and reproductive success in house wrens

Physically challenging signals are likely to honestly indicate signaler quality. In trilled bird song two physically challenging parameters are vocal deviation (the speed of sound frequency modulation) and trill consistency (how precisely syllables are repeated). As predicted, in several species, they correlate with male quality, are preferred by females, and/or function in male-male signaling. Species may experience different selective pressures on their songs, however; for instance, there may be opposing selection between song complexity and song performance difficulty, such that in species where song complexity is strongly selected, there may not be strong selection on performance-based traits. I tested whether vocal deviation and trill consistency are signals of male quality in house wrens (Troglodytes aedon), a species with complex song structure. Males' singing ability did not correlate with male quality, except that older males sang with higher trill consistency, and males with more consistent trills responded more aggressively to playback (although a previous study found no effect of stimulus trill consistency on males' responses to playback). Males singing more challenging songs did not gain in polygyny, extra-pair paternity, or annual reproductive success. Moreover, none of the standard male quality measures I investigated correlated with mating or reproductive success. I conclude that vocal deviation and trill consistency do not signal male quality in this species.

No Correlation Between Three Selected Trade-Offs in Birdsong Performance and Male Quality for a Species With Song Repertoires

Demanding performance of vocal signals, such as birdsong, may be evaluated by trade-offs among acoustic traits. If individuals differ in their ability to sustain physiologically demanding singing, then aspects of song performance resulting from such trade-offs could signal individual quality. Song performance can also differ among song types, and it is not known whether this influences the assessment of individual quality. We asked whether three trade-off-based measures of song performance indicate male age or aspects of condition (body condition, hematocrit and ectoparasite load) in the dark-eyed junco (Junco hyemalis), a species with small repertoires. Across a sample of over 100 males, no measure of song performance was related to male age or condition, nor did song performance improve with age for those males recorded in consecutive years. In all cases, the variation in song performance explained by these predictors was small (<4%). Instead, the more song types we recorded from a male, the more likely we were to record high-performance songs, and this sampling effect was stronger than putative correlations with male quality. These results complement a previous study on this population showing that most variation in performance is found among song types rather than among males. Collectively, the lack of association between trade-off-based aspects of song performance and male age or condition, plus variation among song types that interferes with rapid assessment of a male's best performance, indicate that these aspects of song performance do not allow a good assessment of male quality in juncos, and perhaps more generally in species with song repertoires.

On the relation between loudness and the increased song frequency of urban birds

Songbirds often sing at higher frequency (pitch) in urban, noise-polluted areas, which reduces acoustic masking by low-frequency anthropogenic noise. Such frequency shifts, however, are less efficient at overcoming background noise than simply singing louder. Therefore, it was suggested that high-frequency singing might not be a functional adjustment to noise, but a physiological consequence of singing louder (also known as the Lombard effect). We tested for the first time the main tenet of this hypothesis, for birdsong whether increasing sound amplitude has a concomitant effect on song frequency, using a representative species with higher urban minimum frequency, the dark-eyed junco, Junco hyemalis. The frequency bandwidth of songs and syllables increased with amplitude, involving lower minimum frequency in louder songs and syllables. Therefore, louder singing does not explain the higher minimum frequency of urban dark-eyed juncos. Amplitude and peak frequency were weakly positively related across but not within songs, suggesting that increased frequency is not an obligatory outcome of singing louder. Instead, birds may adjust both amplitude and frequency in response to changing noise or motivation across songs. Our results suggest that adjustments in song frequency and amplitude are largely independent and, thus, can be complementary rather than alternative vocal adjustments to noise. We discuss oscine vocal physiology and details of the behaviour of urban birds, both of which we argue are consistent with the increased frequency of urban birdsong generally being a functional adjustment to noise, rather than a consequence of singing louder.

Directional cultural change by modification and replacement of memes

Evolutionary approaches to culture remain contentious. A source of contention is that cultural mutation may be substantial and, if it drives cultural change, then current evolutionary models are not adequate. But we lack studies quantifying the contribution of mutations to directional cultural change. We estimated the contribution of one type of cultural mutations--modification of memes--to directional cultural change using an amenable study system: learned birdsongs in a species that recently entered an urban habitat. Many songbirds have higher minimum song frequency in cities, to alleviate masking by low-frequency noise. We estimated that the input of meme modifications in an urban songbird population explains about half the extent of the population divergence in song frequency. This contribution of cultural mutations is large, but insufficient to explain the entire population divergence. The remaining divergence is due to selection of memes or creation of new memes. We conclude that the input of cultural mutations can be quantitatively important, unlike in genetic evolution, and that it operates together with other mechanisms of cultural evolution. For this and other traits, in which the input of cultural mutations might be important, quantitative studies of cultural mutation are necessary to calibrate realistic models of cultural evolution.

Song environment affects singing effort and vasotocin immunoreactivity in the forebrain of male Lincoln's sparrows

Male songbirds often establish territories and attract mates by singing, and some song features can reflect the singer's condition or quality. The quality of the song environment can change, so male songbirds should benefit from assessing the competitiveness of the song environment and appropriately adjusting their own singing behavior and the neural substrates by which song is controlled. In a wide range of taxa, social modulation of behavior is partly mediated by the arginine vasopressin or vasotocin (AVP/AVT) systems. To examine the modulation of singing behavior in response to the quality of the song environment, we compared the song output of laboratory-housed male Lincoln's sparrows (Melospiza lincolnii) exposed to 1 week of chronic playback of songs categorized as either high or low quality, based on song length, complexity, and trill performance. To explore the neural basis of any facultative shifts in behavior, we also quantified the subjects' AVT immunoreactivity (AVT-IR) in three forebrain regions that regulate sociosexual behavior: the medial bed nucleus of the stria terminalis (BSTm), the lateral septum (LS), and the preoptic area. We found that high-quality songs increased singing effort and reduced AVT-IR in the BSTm and LS, relative to low-quality songs. The effect of the quality of the song environment on both singing effort and forebrain AVT-IR raises the hypothesis that AVT within these brain regions plays a role in the modulation of behavior in response to competition that individual males may assess from the prevailing song environment.

Nutritional correlates and mate acquisition role of multiple sexual traits in male collared flycatchers

The information content of a sexual signal may predict its importance in a multiple signal system. Many studies have correlated sexual signal expression with the absolute levels of nutrient reserves. In contrast, the changes of nutrient reserves associated with signal expression are largely unknown in the wild due to technical limitations although they are important determinants of signal information content. We compared two visual and eight acoustic sexual traits in male collared flycatchers to see whether the nutritional correlates of expression predict the role of the signal in sexual selection. We used single point assays of plasma lipid metabolites to estimate short-term changes in nutritional state in relation to sexual trait expression during courtship. As a measure of sexual selection, we estimated the relationship with pairing latency after arrival in a 4-year dataset. Males which found a mate rapidly were characterized by large wing and forehead patches, but small song strophe complexity and small figure repertoire size. Traits more strongly related to pairing latency were also more closely related to changes in nutrient reserves. This indicates a link between signal role and information content. Small wing patches and, surprisingly, complex songs seemed to indicate poor phenotypic quality and were apparently disfavoured at mate acquisition in our population. Future studies of the information content of sexual traits, especially dynamic traits such as song, may benefit from the use of plasma metabolite profiles as non-invasive indicators of short-term changes in body condition.

Annual variation in vocal performance and its relationship with bill morphology in Lincoln's sparrows

Morphology may affect behavioural performance through a direct, physical link or through indirect, secondary mechanisms. Although some evidence suggests that the bill morphology of songbirds directly constrains vocal performance, bill morphology may influence vocal performance through indirect mechanisms also, such as one in which morphology influences foraging and thus the ability to perform some types of vocal behaviour. This raises the possibility for ecologically induced variation in the relationship between morphology and behaviour. To investigate this, I used an information theoretic approach to examine the relationship between bill morphology and several measures of vocal performance in Lincoln's sparrows (Melospiza lincolnii). I compared this relationship between two breeding seasons that differed markedly in ambient temperatures, phenology of habitat maturation, and food abundance. I found a strong curvilinear relationship between bill shape (height/width) and vocal performance in the seemingly less hospitable season but not in the other, leading to a difference between seasons in the population's mean vocal performance. Currently, I do not know the cause of this annual variation. However, it could be due to the effects of bill shape on foraging and therefore on time budget, energy balance, or some other behavioural or physiological response that manifests mostly under difficult environmental conditions or, alternatively, to associations between male quality and both vocal performance and bill shape. Regardless of the cause, these results suggest the presence of an indirect, ecologically mediated link between morphology and behavioural performance, leading to annual variation in the prevailing environment of acoustic signals.

The deterrent effect of bird song in territory defense

Using the responses of territory owners to playback to infer the territorial function of acoustic signals is common practice. However, difficulties with interpreting the results of such experiments have obscured our understanding of territorial signalling. For instance, a stronger response to playback is often interpreted as more aggressive, but there is no consensus as to whether this should be in response to the least or most threatening simulated intruder. Rather than following a gradual increase or decrease, the relationship between signal intensity and response strength may instead describe a peaked curve. We manipulated banded wren (Thryophilus pleurostictus) songs to simulate low-, median-, and high-performance singers and used these songs as stimuli in playback experiments. Banded wrens were less likely to approach the high-performance stimulus compared with the low- and median-performance stimuli. However, the birds that did approach the high-performance stimulus sang more than those that approached the low-performance stimulus. In addition, birds were more likely to match the songs when exposed to the median- and high-performance stimuli compared with the low-performance stimuli, and song matching predicted approach behavior. These results are in accordance with theoretical models of aggressive encounters in which low-performance opponents are challenged without further assessment. Median- and high-performance opponents, however, may require further assessment, and the latter may be perceived as too intimidating for approach.