Song as an aggressive signal in songbirds

Neotropical wrens learn new duet rules as adults

Although song development in songbirds has been much studied as an analogue of language development in humans, the development of vocal interaction rules has been relatively neglected in both groups. Duetting avian species provide an ideal model to address the acquisition of interaction rules as duet structure involves time and pattern-specific relationships among the vocalizations from different individuals. In this study, we address the development of the most striking properties of duets: the specific answering rules that individuals use to link their own phrase types to those of their partners (duet codes) and precise temporal coordination. By performing two removal experiments in canebrake wrens (Cantorchilus zeledoni), we show that individuals use a fixed phrase repertoire to create new phrase pairings when they acquire a new partner. Furthermore, immediately after pairing, individuals perform duets with poor coordination and poor duet code adherence, but both aspects improve with time. These results indicate that individuals need a learning period to be able to perform well-coordinated duets that follow a consistent duet code. We conclude that both duet coordination and duet code adherence are honest indicators of pair-bond duration.

Vocal matching: the what, the why and the how

Over the years, vocal matching has progressed beyond being an interesting behavioural phenomenon to one that now has relevance to a wide range of fields. In this review, we use birds and cetaceans to explain what vocal matching is, why animals vocally match and how vocal matching can be identified. We show that while the functional aspects of vocal matching are similar, the contexts in which matching is used can differ between taxa. Whereas vocal matching in songbirds facilitates mate attraction and the immediate defence of resources, in parrots and cetaceans it plays a role in the maintenance of social bonds and the promotion of behavioural synchrony. We propose criteria for defining vocal matching with the aim of stimulating more matching studies across a wider range of taxa, including those using other, non-vocal, communication modalities. Finally, we encourage future studies to explore the importance of vocal learning in the development of vocal matching, and the information it may provide to third parties in the communication network.

Effect of acute stressor on reproductive behavior differs between urban and rural birds

The life‐history trade‐off between self‐maintenance and reproduction posits that investment in one function decreases investment in the other. Manipulating the costs and benefits of functions involved in a trade‐off may alter this interaction. Here we ask whether investment in self‐maintenance during a stress response alters territorial behavior in wild Dark‐eyed Juncos and whether rural and urban birds, which are known to differ in the magnitude of the stress response (greater in rural), also differ in the degree to which stress reduces territorial behavior. In rural and urban habitats, we measured territorial behavior using song playback, followed by either an acute stressor (capture and collection of a blood sample) or a nonstressful control situation. The following day, we again measured territorial behavior, predicting greater reduction in territorial behavior in individuals exposed to the stressor but a lesser reduction in territorial behavior in the urban as compared to the rural environment. We further assessed individual and population differences in response to stressors by measuring flight initiation distance, breath rate, and corticosterone levels in the blood. The rural population had a higher physiological and behavioral stress response than the urban population, and acute capture stress had a lasting (24 h) negative effect on territorial behavior, but only in the rural habitat. However, individual‐level differences in measures of the stress response did not explain variation in the impact of stress on territorial behavior. Our findings show that stressors can have a negative effect on territorial behavior, but that this effect may differ between populations that vary in their stress ecology.

The vocal repertoire of the domesticated zebra finch: a data-driven approach to decipher the information-bearing acoustic features of communication signals

Although a universal code for the acoustic features of animal vocal communication calls may not exist, the thorough analysis of the distinctive acoustical features of vocalization categories is important not only to decipher the acoustical code for a specific species but also to understand the evolution of communication signals and the mechanisms used to produce and understand them. Here, we recorded more than 8000 examples of almost all the vocalizations of the domesticated zebra finch, Taeniopygia guttata: vocalizations produced to establish contact, to form and maintain pair bonds, to sound an alarm, to communicate distress or to advertise hunger or aggressive intents. We characterized each vocalization type using complete representations that avoided any a priori assumptions on the acoustic code, as well as classical bioacoustics measures that could provide more intuitive interpretations. We then used these acoustical features to rigorously determine the potential information-bearing acoustical features for each vocalization type using both a novel regularized classifier and an unsupervised clustering algorithm. Vocalization categories are discriminated by the shape of their frequency spectrum and by their pitch saliency (noisy to tonal vocalizations) but not particularly by their fundamental frequency. Notably, the spectral shape of zebra finch vocalizations contains peaks or formants that vary systematically across categories and that would be generated by active control of both the vocal organ (source) and the upper vocal tract (filter).

The fitness consequences of honesty: Under-signalers have a survival advantage in song sparrows

How honest or reliable signaling can evolve and be maintained has been a major question in evolutionary biology. The question is especially puzzling for a particular class of signals used in aggressive interactions: threat signals. Here, we report a study on song sparrows (Melospiza melodia) in which we assayed males with playbacks on their territories to quantify their aggressiveness (flights and close proximity) and aggressive signaling levels (rates of soft song, a close-range signal reliably predicting attack) and asked whether these traits affect individuals' survival on territory. We found that the effect of aggressive signaling via soft song interacted with aggressive behaviors such that there was a negative correlational selection: among males with low aggression, those males that signaled at higher levels (over-signalers) had higher survival whereas among males with high aggression those that signaled at low levels (under-signalers) survived longer. In other words, males that deviate from reliable signaling have a survival advantage. These results, along with previous research that suggested most of the deviation from reliable signaling in this system is in the form of under-signaling (high-aggression males signaling at low levels) pose a puzzle for future research on how this reliable signaling system is maintained.

Aggressive Bimodal Communication in Domestic Dogs, Canis familiaris

Evidence of animal multimodal signalling is widespread and compelling. Dogs’ aggressive vocalisations (growls and barks) have been extensively studied, but without any consideration of the simultaneously produced visual displays. In this study we aimed to categorize dogs’ bimodal aggressive signals according to the redundant/non-redundant classification framework. We presented dogs with unimodal (audio or visual) or bimodal (audio-visual) stimuli and measured their gazing and motor behaviours. Responses did not qualitatively differ between the bimodal and two unimodal contexts, indicating that acoustic and visual signals provide redundant information. We could not further classify the signal as ‘equivalent’ or ‘enhancing’ as we found evidence for both subcategories. We discuss our findings in relation to the complex signal framework, and propose several hypotheses for this signal’s function.

Vocal production and playback of altered song do not affect ZENK expression in black-capped chickadees (Poecile atricapillus)

The two-note fee bee song of the black-capped chickadee (Poecile atricapillus) is sung at many different absolute frequencies, but the relative frequencies between the start and end of the fee note (the glissando) and between the fee and the bee notes (the inter-note ratio) are preserved regardless of absolute frequency. If these relative frequencies are experimentally manipulated, birds exhibit reduced behavioural responses to playback of altered songs both in field studies and laboratory studies. Interestingly, males appear to be sensitive to alterations in the glissando, while females appear to be sensitive to alterations in both the glissando and the inter-note ratio. In this study, we sought to determine whether the behaviour of male and female chickadees corresponds to differences in zenk protein immunoreactivity (ZENK-ir) in auditory perceptual regions following playback of fee bee songs with typical and altered pitch ratios. Overall, there was a small but significant sex difference in ZENK-ir (females>males), but altering relative frequencies did not reduce ZENK-ir compared to typical song. Birds did vocalize less in response to playback of songs that lacked an inter-note interval, but amount of singing fee bee song, chick-a-dee calls, or gargles was not correlated with ZENK-ir in perceptual regions (caudomedial nidopallium, NCM and caudomedial mesopallium, CMM) or in HVC, which is part of the song system. Our results confirm that ZENK-ir in NCM and CMM is not involved in fine-grain perceptual discrimination, however it did not support the idea that increased vocalizing increases ZENK-ir in HVC.

Social coordination in animal vocal interactions. Is there any evidence of turn-taking? The starling as an animal model

Turn-taking in conversation appears to be a common feature in various human cultures and this universality raises questions about its biological basis and evolutionary trajectory. Functional convergence is a widespread phenomenon in evolution, revealing sometimes striking functional similarities between very distant species even though the mechanisms involved may be different. Studies on mammals (including non-human primates) and bird species with different levels of social coordination reveal that temporal and structural regularities in vocal interactions may depend on the species' social structure. Here we test the hypothesis that turn-taking and associated rules of conversations may be an adaptive response to the requirements of social life, by testing the applicability of turn-taking rules to an animal model, the European starling. Birdsong has for many decades been considered as one of the best models of human language and starling songs have been well described in terms of vocal production and perception. Starlings do have vocal interactions where alternating patterns predominate. Observational and experimental data on vocal interactions reveal that (1) there are indeed clear temporal and structural regularities, (2) the temporal and structural patterning is influenced by the immediate social context, the general social situation, the individual history, and the internal state of the emitter. Comparison of phylogenetically close species of Sturnids reveals that the alternating pattern of vocal interactions varies greatly according to the species' social structure, suggesting that interactional regularities may have evolved together with social systems. These findings lead to solid bases of discussion on the evolution of communication rules in relation to social evolution. They will be discussed also in terms of processes, at the light of recent neurobiological findings.

Song plasticity over time and vocal learning in clay-colored thrushes

Songbirds have been traditionally classified into close-ended or open-ended learning species according to the length of the sensitive period during which birds are able to memorize new vocalizations. Closed-ended learners are generally not capable of changing their song after the first year of life, while open-ended learners show song plasticity as adults. A few Turdus species have been suggested to be open-ended learners, but no long-term study has been conducted to investigate their song plasticity over time. We analyzed the songs of clay-colored thrushes, T. grayi, over four successive breeding seasons to assess song plasticity in their syllable repertoires within and between breeding seasons. A total of 16,262 syllables were classified through visual inspection of spectrograms and multidimensional scaling analysis based on spectrogram correlations. On average, 563 ± 153 (SD) syllables per male per breeding season were analyzed. Male repertoire size was 9-20 syllable types. Males were capable of modifying their syllable repertoire between the initial and final periods of the breeding season. Song plasticity within breeding seasons may be associated with imitation between neighboring males, suggesting song learning in males that were ≥2 years old. This short-term plasticity is not enough, however, to explain the high proportion of change (mean = 65 % syllable types) in repertoire composition between breeding seasons in adult males. Song plasticity resulting from annual changes in repertoire composition could be explained by open-ended learning, but another mechanism, extended memory and re-expression, could also explain long-term plasticity. Experimental studies controlling the acoustic environment are needed to determine which mechanism is responsible for such a high level of song plasticity.

Differential coexpression of FoxP1, FoxP2, and FoxP4 in the Zebra Finch (Taeniopygia guttata) song system

Heterozygous disruptions of the Forkhead transcription factor FoxP2 impair acquisition of speech and language. Experimental downregulation in brain region Area X of the avian ortholog FoxP2 disrupts song learning in juvenile male zebra finches. In vitro, transcriptional activity of FoxP2 requires dimerization with itself or with paralogs FoxP1 and FoxP4. Whether this is the case in vivo is unknown. To provide the means for future functional studies we cloned FoxP4 from zebra finches and compared regional and cellular coexpression of FoxP1, FoxP2, and FoxP4 mRNA and protein in brains of juvenile and adult male zebra finches. In the telencephalic song nuclei HVC, RA, and Area X, the three investigated FoxPs were either expressed alone or occurred in specific combinations with each other, as shown by double in situ hybridization and triple immunohistochemistry. FoxP1 and FoxP4 but not FoxP2 were expressed in RA and in the HVCRA and HVCX projection neurons. In Area X and the surrounding striatum the density of neurons expressing all three FoxPs together or FoxP1 and FoxP4 together was significantly higher than the density of neurons expressing other combinations. Interestingly, the proportions of Area X neurons expressing particular combinations of FoxPs remained constant at all ages. In addition, FoxP-expressing neurons in adult Area X express dopamine receptors 1A, 1B, and 2. Together, these data provide the first evidence that Area X neurons can coexpress all avian FoxP subfamily members, thus allowing for a variety of regulatory possibilities via heterodimerization that could impact song behavior in zebra finches.

Two novel vocalizations are used by veeries (Catharus fuscescens) during agonistic interactions

Avian vocalizations are common examples of the complex signals used by animals to negotiate during agonistic interactions. In this study, we used two playback experiments to identify agonistic signals in a songbird species with several acoustically complex songs and calls, the veery. In the first experiment, we compared veery singing behavior in response to simulated territorial intrusions including playback of three variations of veery song: 1) song alone as a control, 2) songs with added whisper calls, and 3) songs with introductory notes removed. In the second experiment, we used multimodal stimuli including songs, whisper calls and songs with introductory notes removed, along with a robotic veery mount. Focal males readily responded to all of the playback stimuli, approached the speaker and/or robotic mount, and vocalized. Male veeries gave more whisper calls, and sang more songs without the introductory note in response to all types of playback. However, veeries responded similarly to all types of stimuli presented, and they failed to physically attack the robotic mount. These results indicate that rival veeries use two different types of novel vocalizations: whisper calls and songs lacking the introductory note as agonistic signals, but do not allow us to discern the specific functions of these two vocalizations.

Six playback experimental designs fail to demonstrate acoustic preferences in common waxbills (Estrildidae: Estrilda astrild)

Playback experiments in controlled laboratory settings have been instrumental in studies of animal communication and signal evolution. Nonetheless, applying common methods to wild or non-model species can be challenging. We tested male and female common waxbills (Estrilda astrild) for behavioural preferences of conspecific vs. heterospecific songs, and of conspecific song vs. calls. Although common waxbills are related to model species used in this type of experiments, we could not demonstrate the predicted preference for conspecific over heterospecific song, or differential behavioural responses for conspecific song vs. calls. We confirm these results with experimental designs that either monitor movements, vocal responses, or active eliciting of playbacks, and that test birds in isolation or not, with or without visual stimuli accompanying playbacks. We discuss difficulties that may commonly apply to playback experiments with wild and social animals, and stress the importance of using simple discrimination tasks to validate common methods in novel species.

Song trait similarity in great tits varies with social structure

For many animals, long-range signalling is essential to maintain contact with conspecifics. In territorial species, individuals often have to balance signalling towards unfamiliar potential competitors (to solely broadcast territory ownership) with signalling towards familiar immediate neighbours (to also maintain so-called “dear enemy” relations). Hence, to understand how signals evolve due to these multilevel relationships, it is important to understand how general signal traits vary in relation to the overall social environment. For many territorial songbirds dawn is a key signalling period, with several neighbouring individuals singing simultaneously without immediate conflict. In this study we tested whether sharing a territory boundary, rather than spatial proximity, is related to similarity in dawn song traits between territorial great tits (Parus major) in a wild personality-typed population. We collected a large dataset of automatized dawn song recordings from 72 unique male great tits, during the fertile period of their mate, and compared specific song traits between neighbours and non-neighbours. We show here that both song rate and start time of dawn song were repeatable song traits. Moreover, neighbours were significantly more dissimilar in song rate compared to non-neighbours, while there was no effect of proximity on song rate similarity. Additionally, similarity in start time of dawn song was unrelated to sharing a territory boundary, but birds were significantly more similar in start time of dawn song when they were breeding in close proximity of each other. We suggest that the dissimilarity in dawn song rate between neighbours is either the result of neighbouring great tits actively avoiding similar song rates to possibly prevent interference, or a passive consequence of territory settlement preferences relative to the types of neighbours. Neighbourhood structuring is therefore likely to be a relevant selection pressure shaping variation in territorial birdsong.

Songbirds never forget: long-lasting behavioural change triggered by a single playback event

Many behavioural studies rely on playback experiments. While it is known that songbirds decrease behavioural responses after short-term repeated stimulation, long-term behavioural changes due to playbacks are unknown. We studied the response to playbacks in a free-living songbird in the long-term, while also studying the repeatability of the behaviour. Locomotor behaviour (a proxy of aggressiveness) decreased one year after first exposure to a single playback. Song response, however, remained consistent, suggesting that these two behaviours may provide different information. Locomotor behaviour was less repeatable than the song response to playback, the latter showing significant between-years repeatability. To the best of our knowledge, our study is the first to report long-term decrease in response to playbacks in a songbird, and that some aspects of the response to playback are repeatable. Similar studies in other species or populations of the great tit are important, to examine the generality of our findings.

Negotiation of territorial boundaries in a songbird

How do territorial neighbors resolve the location of their boundaries? We addressed this question by testing the predictions of 2 nonexclusive game theoretical models for competitive signaling: the sequential assessment game and the territorial bargaining game. Our study species, the banded wren, is a neotropical nonmigratory songbird living in densely packed territorial neighborhoods. The males possess repertoires of approximately 25 song types that are largely shared between neighbors and sequentially delivered with variable switching rates. Over 3 days, boundary disputes among pairs of neighboring males were synchronously recorded, their perch positions were marked, and their behavioral interactions were noted. For each countersinging interaction between 2 focal males, we quantified approach and retreat order, a variety of song and call patterns, closest approach distance, distance from the territorial center, and female presence. Aggressors produced more rattle-buzz songs during the approaching phase of interactions, whereas defenders overlapped their opponent's songs. During the close phase of the interaction, both males matched frequently, but the key determinant of which one retreated first was song-type diversity-first retreaters sang with a higher diversity. Retreaters also produced more unshared song types during the interaction, and in the retreating phase of the interaction, they overlapped more. A negative correlation between song-type diversity asymmetry and contest duration suggested sequential assessment of motivational asymmetry. The use of this graded signal, which varied with distance from the center and indicated a male's motivation to defend a particular position, supported the bargaining model. The bargaining game could be viewed as a series of sequential assessment contests.

Assessing vocal performance in complex birdsong: a novel approach

BACKGROUND

Vocal performance refers to the ability to produce vocal signals close to physical limits. Such motor skills can be used by conspecifics to assess a signaller's competitive potential. For example it is difficult for birds to produce repeated syllables both rapidly and with a broad frequency bandwidth. Deviation from an upper-bound regression of frequency bandwidth on trill rate has been widely used to assess vocal performance. This approach is, however, only applicable to simple trilled songs, and even then may be affected by differences in syllable complexity.

RESULTS

Using skylarks (Alauda arvensis) as a birdsong model with a very complex song structure, we detected another performance trade-off: minimum gap duration between syllables was longer when the frequency ratio between the end of one syllable and the start of the next syllable (inter-syllable frequency shift) was large. This allowed us to apply a novel measure of vocal performance ¿ vocal gap deviation: the deviation from a lower-bound regression of gap duration on inter-syllable frequency shift. We show that skylarks increase vocal performance in an aggressive context suggesting that this trait might serve as a signal for competitive potential.

CONCLUSIONS

We suggest using vocal gap deviation in future studies to assess vocal performance in songbird species with complex structure.

The evolution of honest communication: integrating social and physiological costs of ornamentation

Much research on animal communication has addressed how costs such as social costs or physiological costs favor the accuracy of signals. Previous work has largely considered these costs separately, but we may be missing essential connections by studying costs in isolation. After all, social interactions produce rapid changes in hormone titers which can then affect individual behavior and physiology. As a result, social costs are likely to have widespread physiological consequences. Here, I present a new perspective on the factors that maintain honest signals by describing how the interplay between social costs and physiological costs may maintain an accurate link between an animal's abilities and ornament elaboration. I outline three specific mechanisms by which the interaction between social behavior and hormones could favor honest signals and present specific predictions for each of the three models. Then, I review how ornaments alter agonistic behavior, agonistic behavior influences hormones, and how these hormonal effects influence fitness. I also describe the few previous studies that have directly tested how ornaments influence hormones. Finally, opportunities for future work are discussed. Considering the interaction between social behavior and physiology may address some challenges associated with both social and physiological models of costs. Understanding the dynamic feedbacks between physiology and social costs has potential to transform our understanding of the stability of animals' communication systems.

Individual differences affect honest signalling in a songbird

Research in the past decade has established the existence of consistent individual differences or 'personality' in animals and their important role in many aspects of animal behaviour. At the same time, research on honest signalling of aggression has revealed that while some of the putative aggression signals are reliable, they are only imperfectly so. This study asks whether a significant portion of the variance in the aggression-signal regression may be explained by individual differences in signalling strategies. Using the well-studied aggressive signalling system of song sparrows (Melospiza melodia), we carried out repeated assays to measure both aggressive behaviours and aggressive signalling of territorial males. Through these assays, we found that aggressive behaviours and aggressive signalling were both highly repeatable, and moreover that aggressive behaviours in 2009-2010 predicted whether the birds would attack a taxidermic mount over a year later. Most significantly, we found that residual variation in signalling behaviours, after controlling for aggressive behaviour, was individually consistent, suggesting there may be a second personality trait determining the level of aggressive signalling. We term this potential personality trait 'communicativeness' and discuss these results in the context of honest signalling theories and recent findings reporting prevalence of 'under-signalling'.

Female competition and aggression: interdisciplinary perspectives

This paper introduces a Theme Issue combining interdisciplinary perspectives in the study of female competition and aggression. Despite a history of being largely overlooked, evidence is now accumulating for the widespread evolutionary significance of female competition. Here, we provide a synthesis of contributions to this Theme Issue on humans and other vertebrates, and highlight directions for future research. Females compete for resources needed to survive and reproduce, and for preferred mates. Although female aggression takes diverse forms, under most circumstances relatively low-risk competitive strategies are favoured, most probably due to constraints of offspring production and care. In social species, dominance relationships and threats of punishment can resolve social conflict without resort to direct aggression, and coalitions or alliances may reduce risk of retaliation. Consistent with these trends, indirect aggression is a low cost but effective form of competition among young women. Costs are also minimized by flexibility in expression of competitive traits, with aggressive behaviour and competitive signalling tailored to social and ecological conditions. Future research on female competition and the proximate mediators of female aggression will be greatly enhanced by opportunities for interdisciplinary exchange, as evidenced by contributions to this Theme Issue.

A test of multiple hypotheses for the function of call sharing in female budgerigars, Melopsittacus undulatus

In many social species group, members share acoustically similar calls. Functional hypotheses have been proposed for call sharing, but previous studies have been limited by an inability to distinguish among these hypotheses. We examined the function of vocal sharing in female budgerigars with a two-part experimental design that allowed us to distinguish between two functional hypotheses. The social association hypothesis proposes that shared calls help animals mediate affiliative and aggressive interactions, while the password hypothesis proposes that shared calls allow animals to distinguish group identity and exclude nonmembers. We also tested the labeling hypothesis, a mechanistic explanation which proposes that shared calls are used to address specific individuals within the sender-receiver relationship. We tested the social association hypothesis by creating four-member flocks of unfamiliar female budgerigars (Melopsittacus undulatus) and then monitoring the birds' calls, social behaviors, and stress levels via fecal glucocorticoid metabolites. We tested the password hypothesis by moving immigrants into established social groups. To test the labeling hypothesis, we conducted additional recording sessions in which individuals were paired with different group members. The social association hypothesis was supported by the development of multiple shared call types in each cage and a correlation between the number of shared call types and the number of aggressive interactions between pairs of birds. We also found support for calls serving as a labeling mechanism using discriminant function analysis with a permutation procedure. Our results did not support the password hypothesis, as there was no difference in stress or directed behaviors between immigrant and control birds.