Vocal performance influences male receiver response in the banded wren

In a variety of songbirds the production of trilled song elements is constrained by a performance tradeoff between how fast a bird can repeat trill units (trill rate) and the range of frequencies each unit can span (frequency bandwidth). High-performance trills serve as an assessment signal for females, but little is known about the signal value of vocal performance for male receivers. We investigated the relationship between trill rate and frequency bandwidth in banded wren (Thryothorus pleurostictus) songs. Trilled song elements showed the same performance tradeoff found in other passerines and individuals differed in performance of some trill types. We tested the hypothesis that males of this species assess each other based on trill performance with a two-speaker experiment, in which territory owners were presented with alternating renditions of the same song type manipulated to differ in trill rate. Subjects were significantly more likely to approach the faster trill stimulus first. However, subjects that received trill types closer to the performance limit spent less time close to the fast speaker. Our results show that male banded wrens discriminate and respond differently to songs based on their vocal performance. Thus, performance of physically challenging songs may be important in intra- as well as inter-sexual assessment.     

A Tradeoff Between Performance and Accuracy in Bird Song Learning

Some of nature’s most complex behaviors, such as human speech and oscine bird song, are acquired through imitative learning. Accurate imitative learning tends to preserve patterns of behavior across generations, thus limiting the scope of cultural evolution. Less well studied are the routes by which cultural novelties arise during development, beyond simple copy error. In this study we assess, in a species of songbird, the relationship in song learning between two potentially conflicting learning goals: accuracy in copying and maximization of vocal performance. In our study species, the swamp sparrow (Melospiza georgiana), vocal performance can be defined for a given song type and frequency range by the rate of note repetition (‘trill rate’), with faster trills being more difficult to sing. We trained young swamp sparrows with song models with experimentally modified trill rates and characterized both the accuracy and performance levels of copies. Our main finding is that birds elevated the trill rates of low‐performance models, but at the expense of imitative accuracy. By contrast, birds reproduced normal and high‐performance models with typically high accuracy in structure and timing. Developmental mechanisms that enable songbirds to balance imitative accuracy and vocal performance are likely favored by sexual selection and may help explain some current patterns of variation in birdsong. Such mechanisms may also explain how behaviors that are learned by imitation can nevertheless respond to selection for high‐performance levels in their expression.     

Red-winged blackbirds Ageliaus phoeniceus respond differently to song types with different performance levels

Physical constraints on animal signals can have important consequences for communication. For bird songs that include a trill, performance is physically constrained by a trade-off between the rate at which notes are repeated in the trill and the sound frequency range covered, such that a trill cannot exceed a certain frequency range for a given note repetition rate. Producing trills that are closer to this performance limit is presumably more challenging for birds than producing trills that are farther from it. Male red-winged blackbirds Ageliaus phoeniceus have repertoires of two to eight song types containing trills that span a range of performance levels. We determined the approximate trill performance maximum for a population of red-winged blackbirds, then conducted playback experiments to measure the responses of territorial males to song types with either high or low performance levels relative to this limit. Males responded significantly more strongly to songs containing low performance trills. Our results show that male red-winged blackbirds can discriminate between different song performance levels, suggesting that vocal proficiency plays a role in male-male interactions.     

Morphological adaptation influences the evolution of a mating signal

Theory predicts that forces of natural selection can reduce the intensity of sexually selected traits. In this study, I investigate how morphological adaptation to feeding ecology influences a mating signal. In birds, changes in feeding ecology can cause rapid divergence in bill morphology. Because bills are also important for song production, feeding ecology may influence song divergence. During song, birds can rapidly change vocal tract resonance using bill movement, yet are constrained in rate and magnitude of bill movements resulting in a trade-off between trill rate and frequency bandwidth. Male swamp sparrows vary in their ability to produce rapid, broad-band trills and females prefer more physically demanding songs. Populations of swamp sparrows adapted to the feeding ecology of tidal marshes have larger bills than inland populations. Larger bills should increase the constraints of producing rapid, broad-band trills allowing for a test of how changes in feeding ecology affect a feature of song used in mate choice. I found significant differences in acoustic features of song consistent with the hypothesis that coastal males are less able to meet the physical demands of song production because of the constraints of having larger bills. As possible compensation for decreases in song performance, coastal populations exhibit an increase in song complexity. These changes support the current model of how motor constraints influence song production and suggest a mechanism by which feeding ecology can influence signal evolution.     

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.     

Vocal performance influences female response to male bird song: an experimental test

Female songbirds are thought to assess males based on aspectsof song, such as repertoire size or amount of singing, thatcould potentially provide information about male quality. Arelatively unexplored aspect of song that also might serve asan assessment signal is a male's ability to perform physicallychallenging songs. Trilled songs, such as those produced byswamp sparrows (Melospiza georgiana), present males with a performancechallenge because trills require rapid and precise coordinationof vocal tract movements, resulting in a trade-off between trillrate and frequency bandwidth. This trade-off defines a constrainton song production observed as a triangular distribution inacoustic space of trill rate by frequency bandwidth, with anupper boundary that represents a performance limit. Given thisbackground on song production constraints, we are able to identifya priori which songs are performed with a higher degree of proficiencyand, thus, which songs should be more attractive to females.We determined the performance limit for a population of swampsparrows and measured how well individual males performed songsrelative to this limit ("vocal performance"). We then comparedfemale solicitation responses to high-performance versus low-performanceversions of the same song type produced by different males.Females displayed significantly more to high-performance songsthan to low-performance songs, supporting the hypothesis thatfemales use vocal performance to assess males.     

Variation in Trill Characteristics in Tree Pipit Songs: Different Trills for Different Use?

Transmitting information about singer's quality is an important function of song in many bird species, and this information should be useful in territorial interactions. Fast trills, being physically demanding song structures, are particularly suitable candidates for signalling of quality or aggressive motivation. We have evaluated trill characteristics in songs within a population of the Tree Pipit, a common European songbird with no sexual dimorphism, in which song apparently plays a key role in territory defence as well as mate choice. Two types of relatively fast trills (each of them in multiple variants differing in complexity) were commonly observed in repertoires of Tree Pipit males. Trill rates significantly differed among individuals, suggesting that these song structures may carry information about male quality in this species. We tested by playback experiments whether both trill types are used in territorial encounters. Only one of the trill types was sung by males in response to playback, regardless on the trill type played to them. In an immediate response to playback, they increased the frequency of use of this trill, and also significantly increased the trill rate in comparison with spontaneous songs. This confirmed field observations, suggesting that this trill is important in male–male interactions. On the contrary, the use of the fastest, apparently more demanding, trill type actually decreased after the simulated territorial intrusion. We hypothesize that the latter one is more directed towards females, and that while performance of both trill types may reflect male quality, they are primarily used in different contexts.     

Directional female preference for an exaggerated male trait in canary (Serinus canaria) song

Motor constraints on vocal production impose a trade-off between trill rate and frequency bandwidth within birdsong. We tested whether domesticated canary (Serinus canaria) females, reared either in acoustic isolation or in aviary conditions, had a preference for broad bandwidth songs with artificially increased syllable rates. The copulation solicitation display (CSD) was used as an index of female preference. As predicted, both naive and experienced females were especially responsive to syllables with a broad bandwidth emitted at an artificially increased rate. Female preference for supernormal stimuli provide support for the honest-signalling hypothesis and our results are consistent with recent findings indicating that production of song phrases maximizing both bandwidth and syllable rate may be a reliable indicator of male physical or behavioural qualities. We suggest that female preference for vocal emissions, which simultaneously maximize these two parameters, could be a widespread pattern within songbirds.     

Conservation of multivariate female preference functions and preference mechanisms in three species of trilling field crickets

Divergence in mate recognition systems among closely related species is an important contributor to assortative mating and reproductive isolation. Here, we examine divergence in male song traits and female preference functions in three cricket species with songs consisting of long trills. The shape of female preference functions appears to be mostly conserved across species and follows the predictions from a recent model for song recognition. Multivariate preference profiles, combining the pulse and trill parameters, demonstrate selectivity for conspecific pulse rates and high trill duty cycles. The rules for integration across pulse and trill timescales were identical for all three species. Generally, we find greater divergence in male song traits than in associated female preferences. For pulse rate, we find a strong match between divergent male traits and female peak preferences. Preference functions for trill parameters and carrier frequency are similar between species and show less congruence between signal and preference. Differences among traits in the degree of trait–preference (mis)match may reflect the strength of preferences and the potential for linkage disequilibrium, selective constraints and alternative selective pressures, but appear unrelated to selection for mate recognition per se.     

Release vocalizations in neotropical toads (Bufo): ecological constraints and phylogenetic implications

The release vocalizations of four nominal neotropical toad taxa ( Bufo achalensis , Bufo limensis , Bufo spinulosus , Bufo arenarum ) which compose three sympatric species pairs, were quantitatively analysed and homologous call types statistically compared. The first three taxa are closely related members of the mainly Andean Bufo spinulosus species group in which advertisement calls are absent. The specific vocal repertoire consisted of a uniform, unpulsed release call and one (in B. arenarum, two) pulsed release trill(s) which were given either singly or in series of up to three single calls. The uniform call was similar in structure and dominant (= basic) frequency in all taxa and probably represents an unspecific acoustic signal which is used to avoid heterospecific amplexus between sympatric toads. The release trills significantly differed in most call features among the taxa and, using discriminant analyses, even single calls were correctly assigned to sympatric pairs of species. In contrast, calls of allopatric pairs of species were confounded at rates of up to 36% indicating that selection towards species-specific signals increases when different species live in sympatry. The release trills of two allopatric populations (Perú, Argentina) assigned to B. spinulosus were similar in structure, but nevertheless features of pulse train permitted an unequivocal distinction, suggesting a long-lasting independent evolution. The taxonomic significance of this finding remains to be evaluated in further investigations. The complex vocal repertoire also offered the opportunity to assess phylogenetic relationships among the taxa. The extra-Andean B. achalensis seems to be closer related to the Andean B. spinulosus than this species is to the widely sympatric B. limensis – a species with several primitive character states indicating an early separation from the ancestral stock.     

The predictive value of trill performance in a large repertoire songbird,the nightingale Luscinia megarhynchos

In animal communication, elaborate signals have been shown to be under sexual selection and often to reliably indicate a signaler's quality, condition, or motivation. For instance, the performance of physically challenging signals such as trills – i.e. rapidly repeated elements of broad frequency bandwidth – is considered to reflect signaler quality. Nightingales Luscinia megarhynchos are renowned for their outstanding song repertoire sizes, and most songs include a variety of complex trills. In the present study, we examined whether performance of trills can reliably reflect male quality. We show that vocal performance of trills predicts the age of a male. Older males sang trills that were closer to the performance limit than did younger males. Moreover, males with narrower beaks sang more consistent trills than did males with wider beaks. Vocal performance of trills, however, did not significantly predict other measures of biometric quality such as body size or body condition of the males. The findings suggest that receivers could benefit from the predictive value of physically demanding song traits in assessing age as an important quality component of potential mates or rivals. Particularly in species with high singing versatility, signaler assessment based on readily assessable structures may be adaptive, as this will allow receivers to quickly gather relevant information about the singer without attending to the full song repertoire.     

Vocalizations and morphology: interpreting the divergence among populations of Chough Pyrrhocorax pyrrhocorax and Alpine Chough P. graculus

Capsule Differences in vocalizations among populations are mostly explained on morphological bases, but historical factors may have played a significant role in differentiation processes.

Aims To investigate the relationships among vocal and morphological variation in two corvids: Chough and Alpine Chough.

Methods We used data from 11 populations of Chough and seven populations of Alpine Chough spanning the Palearctic distribution of the two species. Three data sets (morphometry, spectrotemporal parameters of trill calls and acoustic repertoire) were analysed and their variation compared with uni- and multivariate techniques.

Results In both species, morphological differences among populations were correlated to spectro-temporal variation of trills; in particular, frequencies of calls were negatively correlated to wing length (an indicator of body size). By considering only co-existing populations of the two species, the magnitude of morphological and spectrotemporal divergence was similar.

Conclusions In both species, birds from populations with similar morphology uttered similar call types and trills with close spectrotemporal features. In particular, larger-sized populations, as expressed by wing length, emitted lower pitched calls. However, the fact that dissimilarities in repertoire and morphology were correlated cannot be explained only by appealing to functional explanations, as the observed intraspecific morphological variability does not seem to be high enough to promote consistent changes in the pool of calls. In this case, historical factors might have contributed to the present pattern of differentiation.     

Contexts and behavioral correlates of trill vocalizations in wild white-faced capuchin monkeys (Cebus capucinus)

Primate vocalizations that appear to occur independently of specific contexts typically are considered to be contact calls. However, results from several recent studies indicate that these calls function to facilitate social interactions. White-faced capuchin monkeys (Cebus capucinus) emit a high-frequency vocalization, termed a "trill," in social interactions and during travel. In this study, immatures emitted most trills, but adult females also trilled; by contrast, adult males rarely trilled. Infants emitted the majority of trills, and they trilled at significantly higher rates than adult females. Infants trilled most when approaching other individuals. Furthermore, infants emitted proportionately more trills than other age classes when approaching other individuals. I therefore focused on the detailed context and immediate behavioral correlates of trilling by infants. Infants that trilled when approaching others tended to interact affiliatively with them subsequently (i.e., climbing on, touching, receiving grooming, and performing food inspection) more than infants that did not trill when approaching. Therefore, infant trilling may have had an immediate effect on the recipient's behavior.     

Song is a reliable signal of bill morphology in Darwin's small tree finch Camarhynchus parvulus, and vocal performance predicts male pairing success

The shape and movement of the vocal tract are known to influence bird song. Current theory predicts that large bill and body size are correlated with low frequency song and slow trill rate. It is also widely accepted that song characteristics are important for mate choice by females. We investigated the relationship between bill morphology, song characteristics, and pairing success in Darwin's small tree finch Camarhynchus parvulus , on the Galapagos Islands. Contrary to predictions from a previous cross-species study on Darwin's finches, we found that individuals with larger bill size produced songs with slow trill rate, high dominant frequency, and broad frequency bandwidth, indicating that song is a reliable signal of bill morphology. Vocal performance as indicated by the deviation from an upper performance limit was higher in paired than unpaired males. Pairing was not skewed in favour of a particular bill size, and both small and large billed males that sang high performance song had high pairing success. The reliable signalling function of song has implications for female choice and territorial defence, given that both females and conspecific competitors can assess the relative size of males' bills through song, while females may use vocal performance as a signal of male quality.     

The evolution of birdsong on islands

Islands are simplified, isolated ecosystems, providing an ideal set‐up to study evolution. Among several traits that are expected to change on islands, an interesting but poorly understood example concerns signals used in animal communication. Islands are typified by reduced species diversity, increased population density, and reduced mate competition, all of which could affect communication signals. We used birdsong to investigate whether there are systematic changes in communication signals on islands, by undertaking a broad comparison based on pairs of closely related island‐mainland species across the globe. We studied song traits related to complexity (number of different syllables, frequency bandwidth), to vocal performance (syllable delivery rate, song duration), and also three particular song elements (rattles, buzzes, and trills) generally implicated in aggressive communication. We also investigated whether song complexity was related to the number of similar sympatric species. We found that island species were less likely to produce broadband and likely aggressive song elements (rattles and buzzes). By contrast, various aspects of song complexity and performance did not differ between island and mainland species. Species with fewer same‐family sympatric species used wider frequency bandwidths, as predicted by the character release hypothesis, both on continents and on islands. Our study supports the hypothesis of a reduction in aggressive behavior on islands and suggests that discrimination against closely related species is an important factor influencing birdsong evolution.     

Pygmy Marmosets Modify Call Structure When Paired

Pygmy marmosets ( Cebuella pygmaea ) modified the structure of their trill vocalizations in response to pairing with a new mate. Prior to pairing, individual marmosets displayed different acoustic parameters of trill structure. Within the first 6 wk after pairing, three of four pairs produced trills that were more homogeneous between mates. Pairs that had the greatest individual difference in trill structure prior to pairing displayed convergence on a greater number of parameters than pairs having fewer differences in prepairing trill structure. The vocal structure following pairing remained highly stable in the two pairs that could be recorded 3 yr after initial pairing. Changes in trill structure that occurred within the first 6 wk after pairing were greater than spontaneous changes that occurred during baseline control recording sessions on the same animals. These results suggest that nonhuman primates show subtle modification of vocal structure, especially in response to changes in social environment. Similar findings have been reported in birds and other mammals.     

Birdsong performance and the evolution of simple (rather than elaborate) sexual signals

Sexual signals are often elaborate as a result of sexual selection for signals of individual quality. Contrary to expectation, however, the elaboration of signals such as birdsong is not related to the strength of sexual selection across species. With a comparative study across wood warblers (family Parulidae), we show a compromise between advertising the performance of trills (syllable repetitions) and song complexity, which can result in the evolution of simple, rather than elaborate, song. Species with higher trill performance evolved simple songs with more extensive trilled syntax. This advertises trill performance but reduces syllable diversity in songs. These two traits are commonly sexually selected in songbirds, but indexes of sexual selection were not related to either in wood warblers. This is consistent with sexual selection targeting different traits in different species, sometimes resulting in simple signals. We conclude that the evolution of sexual signals can be unpredictable when their physiology affords multiple or, as here, opposing ways of advertising individual quality.     

Geographical variation in the vocalizations of the suboscine Thorn‐tailed Rayadito Aphrastura spinicauda

Structural variation in acoustic signals may be related either to the factors affecting sound production such as bird morphology, or to vocal adaptations to improve sound transmission in different environments. Thus, variation in acoustic signals can influence intraspecific communication processes. This will ultimately influence divergence in allopatric populations. The study of geographical variation in vocalizations of suboscines provides an opportunity to compare acoustic signals from different populations, without additional biases caused by song learning and cultural evolution typical of oscines. The aim of this study was to compare vocalizations of distinct populations of a suboscine species, the Thorn‐tailed Rayadito. Four types of vocalizations were recorded in five populations, including all three currently accepted subspecies. Comparisons of each type of vocalization among the five populations showed that some variation existed in the repetitive trill, whereas no differences were found among alarm calls and loud trills. Variation in repetitive trills among populations and forest types suggests that sound transmission is involved in vocal differences in suboscines. Acoustic differences are also consistent with distinguishing subspecies bullocki from spinicauda and fulva, but not the two latter subspecies from each other. Our results suggest that the geographical differentiation in vocalizations observed among Thorn‐tailed Rayadito populations is likely to be a consequence of different ecological pressures. Therefore, incipient genetic isolation of these populations is suggested, based on the innate origin of suboscine vocalizations.     

Individuality in the vocalizations of infant and adult coppery titi monkeys (Plecturocebus cupreus)

As social animals, many primates use acoustic communication to maintain relationships. Vocal individuality has been documented in a diverse range of primate species and call types, many of which have presumably different functions. Auditory recognition of one's neighbors may confer a selective advantage if identifying conspecifics decreases the need to participate in costly territorial behaviors. Alternatively, vocal individuality may be nonadaptive and the result of a unique combination of genetics and environment. Pair-bonded primates, in particular, often participate in coordinated vocal duets that can be heard over long distances by neighboring conspecifics. In contrast to adult calls, infant vocalizations are short-range and used for intragroup communication. Here, we provide two separate but complementary analyses of vocal individuality in distinct call types of coppery titi monkeys (Plecturocebus cupreus) to test whether individuality occurs in call types from animals of different age classes with presumably different functions. We analyzed 600 trill vocalizations from 30 infants and 169 pulse-chirp duet vocalizations from 30 adult titi monkeys. We predicted that duet contributions would exhibit a higher degree of individuality than infant trills, given their assumed function for long-distance, intergroup communication. We estimated 7 features from infant trills and 16 features from spectrograms of adult pulse-chirps, then used discriminant function analysis with leave-one-out cross-validation to classify individuals. We correctly classified infants with 48% accuracy and adults with 83% accuracy. To further investigate variance in call features, we used a multivariate variance components model to estimate variance partitioning in features across two levels: within- and between-individuals. Between-individual variance was the most important source of variance for all features in adults, and three of four features in infants. We show that pulse-chirps of adult titi monkey duets are individually distinct, and infant trills are less individually distinct, which may be due to the different functions of the vocalizations.     

Temporal stability and change in a microgeographical pattern of song variation in the rufous-collared sparrow

We studied the pattern of song variation in the rufous-collared sparrow, Zonotrichia capensis, in the 1987 and 2000 breeding seasons in a 7-km² area covering open and closed habitats. We measured 14 quantitative song variables over a total of 390 individuals and found consistent differences between habitats in both years. In particular, songs of individuals from closed habitat sites had trills with longer trill intervals and lower frequencies than those of individuals from open habitats. This pattern of variation is interpreted as a song cline that correlates with the environmental gradient. Although this cline was stable in location and shape, it also showed several differences between years. The songs recorded during 2000 had trills with longer trill intervals and lower minimum frequencies than those recorded during 1987. Thus, this study is the first to directly document temporal changes in song variables in a Z. capensis population. This change would be expected if all the habitats studied were more closed in 2000 than in 1987, but we did not find obvious differences in habitat features between seasons. However, this modification of song structure is compatible with alternative explanations, such as subtle differences in habitat characteristics, changes in climatic variables, a delayed effect of a past modification of the environment and neutrality of the change.