Seasonal changes in blue tit crown color: do they signal individual quality?

Plumage coloration is generally perceived as a static traitand therefore not a good indicator of current condition. However,changing of feather colors after molt does occur and may haveimportant implications for signal function and sexual selection.We studied longitudinal changes in blue tit (Parus caeruleus)crown ultraviolet (UV)/blue color, a sexually selected trait,by repeatedly measuring the same individuals between early winterand late spring. Whereas crown UV reflectance (UV chroma andhue) decreased dramatically over time, brightness and saturationdid not show consistent patterns of change. The magnitude ofthe decline in coloration exceeded sexual and age dichromatismin hue and UV chroma, respectively. Hence, seasonal color changescould have strong effects on blue tit sexual signaling. Between-individualvariation in the decline in UV coloration was large and relatedto attributes of male, but not female, quality, such as sizeand condition. Thus, conspecifics could potentially gain informationabout male phenotypic quality by assessing color change overthe year. However, the degree of decline in male UV color didnot affect breeding success because neither the number of within-pairnor the number of extrapair offspring produced correlated withchanges in crown color. Seasonal changes in the expression ofplumage coloration are probably widespread, and maintainingplumage coloration could thus constitute an additional honesty-enforcingmechanism after molt is completed.     

Manipulation of male quality in wild tits: effects on paternity loss

Extrapair paternity (EPP) has proved to be widespread and highlyvariable among birds and other taxa, including socially monogamousspecies. A multitude of hypotheses have been put forward toexplain this variation, but its occurrence is not fully understood.Male age, social dominance rank, song and breeding density orsynchrony have been among the suggested correlates of EPP, butresults so far are inconclusive. We interspecifically cross-fosteredblue tits (Parus caeruleus) and great tits (Parus major) inthe wild, thus manipulating males to exhibit reduced socialdominance rank, sing aberrant songs, and consequently be perceivedas low-quality males as compared to controls. This allowed usto test if male quality had an influence on loss of paternity.We found no statistically significant differences between cross-fosteredand control males of either species, neither with respect tolevels of cuckoldry nor proportions of extrapair young (EPY)in the broods. Paternity levels were comparable to other studieson the same species. No effect of density could be detectedon levels of EPP either, while an age effect seemed to be presentat least in the blue tit, EPY being almost absent in broodsof older blue tit males. We conclude that the effects of malequality on paternity loss are minor, if any, in these populations.     

Song Amplitude of Rival Males Modulates the Territorial Behaviour of Great Tits During the Fertile Period of Their Mates

Bird song is a widely used model in the study of sexual selection. Variation in the expression of sexually selected traits is thought to reflect variation in male genetic and/or phenotypic quality. Vocal amplitude is a song parameter that has received little attention in the context of sexual selection, but there is some evidence that the intensity of bird song affects female preferences. Here, we tested whether the amplitude of broadcast song plays a role in male–male competition. We used song playback with varying song amplitude (within the natural amplitude range of the species) and a dummy bird taxidermy to simulate territorial intrusions in the great tit, Parus major, during the fertile period of the female and measured the response of the local male. The results show that playback amplitude significantly affected the subjects’ behaviour: after approaching to within 25 m around the loudspeaker, territorial males stayed longer within that perimeter after the playback of high‐amplitude songs compared with low‐amplitude songs. Our findings add to the small but growing body of evidence suggesting that vocal amplitude may be a sexually selected song trait.     

Macrogeographic Variation in Song – a Test of Competition and Habitat Effects in Blue Tits

Investigations into the environmental causes of phenotypic variation may reveal information regarding the selective pressures leading to the evolution of these phenotypes. Blue tit ( Parus caeruleus ) song varies geographically in the proportion of song types that contain a trill (i.e. a series of identical notes repeated in sequence at a very rapid rate produced at the end of a song). In order to determine the environmental factors influencing geographic variation in the proportion of blue tit songs with a trill, we conducted a comparative study. At macrogeographic and regional scales we examined the effect of three factors that could influence geographic variation in blue tit song: dominant vegetation type, intraspecific competition and interspecific competition with great tits ( Parus major ). Interspecific competition may result in song convergence or song divergence (hereafter called character shift). The results we obtained are consistent with predictions of the character shift hypothesis: the proportion of songs presenting a different syntax to great tit songs (songs with a trill) increases significantly with relative great tit density. Results do not support predictions of hypotheses concerning song convergence or intraspecific competition. The results are consistent with some of the predictions of the acoustic adaptation hypothesis.     

Inferring performance in the songs of dark-eyed juncos (Junco hyemalis)

Within bird species, songs differ in their attractiveness tofemales or effectiveness in male–male interactions. Somesongs are more difficult to sing than others, and receiversmay use a singer's performance of difficult songs as a meansfor evaluating the quality of the singer. The concept of songperformance aims at quantifying how physiologically demandingare different songs. Using variation between song types of dark-eyedjuncos, Junco hyemalis, we show that some song traits tradeoff with costly aspects of song output—short intervalsbetween syllables or loud sound amplitude—suggesting thatthose traits are difficult to sing. First, after controllingfor other traits, long syllables require longer intervals forrecovery. This supports the idea that a measure of "respiratoryperformance" could be based on the relative lengths of syllablesand intervals. Second, some syllable traits trade off stronglywith sound amplitude, suggesting that these traits may be difficultto sing at high amplitudes. The ratio of frequency bandwidthand trill rate has been used to infer performance in other birdspecies, but we found no evidence that frequency bandwidth tradesoff with any aspect of song output in the junco. The negativeassociation of bandwidth with trill rate may instead be a passiveconsequence of syllable length, with longer syllables randomlyaccumulating frequency modulation. We conclude that bird receiversmay best evaluate how well a song is performed if they integratemultiple cues and discuss how researchers may similarly devisemeasures of song performance.     

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.     

Different sexual traits show covariation among genotypes: implications for sexual selection

An unresolved question in sexual selection research is whetherdifferent secondary sexual traits are developmentally independentor instead whether their degree of expression is a manifestationof a general resource pool (i.e., condition) within the organism.If degree of expression of different sexual traits reflectsability to accumulate condition, then covariation should existacross genotypes in the expression of these traits, even ifthey are very different in kind. Here we present evidence forpredicted covariation between morphological (sex comb size)and behavioral (courtship song) sexual traits among geneticlines of Drosophila bipectinata Duda extracted from a naturalpopulation. There is evidence that both these traits in Drosophilaare condition dependent and subject to sexual selection. Wedetected significant body size–independent differencesin comb size among 32 lines. Replicate lines exhibiting relativelyhigh and low values of comb size were then subjected to analysesof courtship song. High sex comb lines exhibited shorter meanburst period and shorter mean burst duration than low sex comblines. These song differences occurred only during the distantpursuit phase of male courtship and existed despite factoringout individual variations in sex comb size, the trait on thebasis of which test lines were originally chosen. The resultsverify the prediction of an association between condition-dependentsecondary sexual traits across genotypes and, therefore, supportthe existence of an overall genetic quality related to conditionacquisition.     

Song discrimination suggests premating isolation among sympatric indigobird species and host races

Indigobirds (Vidua spp.) are host-specific brood parasites thathave diversified in a recent radiation apparently driven byhost colonization. Behavioral imprinting of both male and femaleindigobirds on host song is thought to promote rapid speciationbecause it results in assortative mating between indigobirdsassociated with a particular host. We conducted a song playbackexperiment to test whether male indigobirds discriminate amongpotential competitors based on song. Of particular interestwas the behavior of two sympatric host races of the Cameroonindigobird Vidua camerunensis that differ only in host songmimicry and other components of their vocal repertoires. Territorialmales of the two V. camerunensis host races and Vidua raricola,a morphologically distinct indigobird species, were tested withplaybacks of each other's songs. Males of all three groups respondedmost aggressively to songs of their own species and/or hostrace, as evidenced by strong and highly significant differencesin a variety of response variables. This differential territorialdefense suggests that an intruding male with different songsdoes not represent a competitive threat and is expected if femalesmate assortatively with respect to song. Thus, our results provideevidence of premating reproductive isolation among recentlyevolved indigobird species and host races.     

Dawn song of male blue tits as a predictor of competitiveness in midmorning singing interactions

During the dawn chorus, territorial male songbirds vocalise intensively within signalling range of several conspecific males and can therefore be considered members of a busy communication network. The more or less continuous singing over a long period of time under standardised stimulus conditions makes the dawn song a potentially important information source both for simple receivers and for eavesdroppers. Male blue tits (Parus caeruleus) vary in features of their dawn song, e.g. older males sing longer strophes, and females choose males that sing longer strophes as extra-pair partners. However, so far, dawn song in the blue tit has been investigated separately from other singing behaviour of the same males. In this study, we investigate aspects of blue tit male quality, reflected in dawn song characteristics, and their predictive value for how males behave during singing interactions later in the morning. We acted as simple receivers by recording the singing activity of one male at a time at dawn and compared features of its dawn song, such as onset before sunrise, repertoire size, mean bout length, strophe length and percentage performance time to responses of the same male to a territory intrusion simulated by playback of synthesised songs later during the same morning. We assume that an aggressive response towards an intruder will involve a fast approach to the loudspeaker broadcasting strophes of blue tit song, searching for the intruder (flying around), and a high amount of counter singing and overlapping of the intruders songs. Aspects of vigour of response to the simulated intrusion could be predicted from all five investigated dawn song parameters as well as male age. This is, to our knowledge, the first indication that a simple receiver could extract reliable information from a males dawn singing behaviour about its competitiveness later in the day.Communicated by P.K. McGregor     

Species divergence in sexually selected traits: increase in song elaboration is related to decrease in plumage ornamentation in finches

Elaborate plumage and complex songs of male birds are two of the best-known examples of sexually selected traits, yet the interaction between these traits is poorly understood. Theory suggests that among a suite of potential displays, animals will emphasize traits that are most conspicuous, least costly, or best signal condition and reduce display of other traits. Here we examined the relationship between song and plumage elaborations in cardueline finches, songbirds that are highly variable in plumage displays and songs, but that share a similar mating system. We statistically controlled for body mass, habitat characteristics, and phylogenetic relationships and found that across species song complexity was strongly negatively related to elaboration of plumage ornamentation. When plumage coloration was partitioned into carotenoid-based and melanin-based components, song complexity was negatively related to elaboration of male carotenoid-based coloration but unrelated to elaboration of melanin-based coloration. These observations support the idea that, for condition-dependent traits, animal species trade off trait expression in response to changes in the costs or the information content of these traits. We discuss several alternative explanations for the observed pattern.     

Limited condition dependence of male acoustic signals in the grasshopper Chorthippus biguttulus

In many animal species, male acoustic signals serve to attract a mate and therefore often play a major role for male mating success. Male body condition is likely to be correlated with male acoustic signal traits, which signal male quality and provide choosy females indirect benefits. Environmental factors such as food quantity or quality can influence male body condition and therefore possibly lead to condition-dependent changes in the attractiveness of acoustic signals. Here, we test whether stressing food plants influences acoustic signal traits of males via condition-dependent expression of these traits. We examined four male song characteristics, which are vital for mate choice in females of the grasshopper Chorthippus biguttulus. Only one of the examined acoustic traits, loudness, was significantly altered by changing body condition because of drought- and moisture-related stress of food plants. No condition dependence could be observed for syllable to pause ratio, gap duration within syllables, and onset accentuation. We suggest that food plant stress and therefore food plant quality led to shifts in loudness of male grasshopper songs via body condition changes. The other three examined acoustic traits of males do not reflect male body condition induced by food plant quality.     

Acoustic preference functions and sexual selection on the male calling song in the grasshopper Chorthippus biguttulus

Male courtship songs have two functions in species recognition and intraspecific mate choice. Female preference might thus exert different types of selection pressure on male song traits. We used a combination of acoustic mate choice experiments and statistical analyses to examine how traits of the calling songs of male nightingale grasshoppers,Chorthippus biguttulus , are influenced by different sexual selection pressures. We recorded calling songs of males and tested their attractiveness to females in acoustic mate choice experiments. The attractiveness values were a good estimate of the potential male mating success. In experiments with a pair of males, females copulated significantly more often with the male that had the higher attractiveness value. To detect directional, stabilizing, disruptive or correlative selection acting on male song properties we used linear and nonlinear regressions between male song traits and female response behaviour. Three signal traits were revealed to be under directional selection: song loudness, pause to syllable ratio and the mean duration of gaps within syllables. A nonlinear regression testing for correlative selection showed that a fourth song trait, rhythm, in combination with mean gap duration was also important for female mate choice. With these traits and trait combinations we were able to explain 42% of the variance in attractiveness between males. Since we found no evidence for stabilizing selection, but ample evidence for directional selection, we conclude that selection on the traits examined is related to mate choice mainly in the context of intraspecific sexual selection and probably less so in species recognition. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Mate choice and imprinting in birds studied by cross-fostering in the wild

Sexual-selection theories generally assume that mating preferences are heritable traits. However, there is substantial evidence that the rearing environment may be important for the development of mating preferences, indicating that they may be learnt, or modified by experience. The relative importance of such sexual imprinting across species remains largely unexplored. Here, we report results of a large-scale cross-fostering experiment in the wild in which nestling birds were raised by parents of a different species. We show that resulting sexual imprinting may have a negative effect on pairing success in one species (the great tit, Parus major), but not in two other species (the blue tit, P. caeruleus and the pied flycatcher, Ficedula hypoleuca). A remarkable variation thus seems to exist, even between species that are congeneric and have similar breeding ecologies. The cross-fostering resulted in heterospecific pairings between the two tit species (female blue tit breeding with male great tit), which has never, to our knowledge, been previously documented. However, the chicks fledging from these nests were all blue tit.     

Zebra finch song reflects current food availability

Food availability is a major evolutionary force that has direct effects on an individual’s body condition. Since sexually selected traits are often condition-dependent, they are likely to reflect food availability or other ecological constraints. Here we test whether bird song, which is thought to be under intense female choice, is sensitive to food availability and might be used by females to assess male body condition and/or territory quality. We manipulated food availability of male zebra finches (Taeniopygia guttata) and assessed the within-individual effects of the treatment on song parameters that are thought to be important in mate choice. We found no effect of food availability on syllable repertoire, proportion of sound versus silence within a song, and mean song frequency. In contrast, treatment birds showed a reduced song rate, an increased latency to sing, and a lowered song amplitude and fundamental frequency. Our study demonstrates that zebra finch song reflects food availability and that songs of well-fed males contain traits that have previously been reported to be more attractive to females. This adds strong support to the general assumption that female song preferences evolved because song reflects male quality and/or territory quality. Moreover, our study provides corroborative evidence for the notion that variation in environmental factors plays an important role in the evolution of mating signals.     

Courtship signals and mate choice of the flies of inbred Drosophila montana strains

We studied genetic variation in fly mating signals and mate choice in crosses within and between inbred strains of Drosophila montana. Male songs and the cuticular hydrocarbons of both sexes as well as some of the flies’ behavioural traits differed significantly between strains. This did not, however, cause sexual isolation between strains. In fact, courtship was shorter if the female was courted by a male of a foreign strain than when courted by their own male. Heterosis was found for courtship duration and the carrier frequency of male song. Diallel analysis of male song revealed additive genetic variation in four out of the five traits studied. Two traits showed dominance variation and one of these, carrier frequency, expressed unidirectional dominance with alleles for higher carrier frequency being dominant. Direction of dominance in carrier frequency was the same as the direction of sexual selection exercised by D. montana females on this trait, which suggests that sexual selection could be a driving force in the evolution of song towards a higher carrier frequency.     

Differential rates of phenotypic introgression are associated with male behavioral responses to multiple signals

Sexual selection on multiple signals may lead to differential rates of signal introgression across hybrid zones if some signals contribute to reproductive isolation but others facilitate gene flow. Competition among males is one powerful form of sexual selection, but male behavioral responses to multiple traits have not been considered in a system where traits have introgressed differentially. Using playbacks, mounts, and a reciprocal experimental design, we tested the hypothesis that male responses to song and plumage in two subspecies of red‐backed fairy‐wren (Malurus melanocephalus) explain patterns of differential signal introgression (song has not introgressed, whereas plumage color has introgressed asymmetrically). We found that males of both subspecies discriminated symmetrically between subspecies’ songs at a long range, but at a close range, we found that aggression was equal for both subspecies’ plumage and songs. Taken together, our results suggest that male behavioral responses hinder the introgression of song, but allow for the observed asymmetrical introgression of plumage. Our results highlight how behavioral responses are a key component of signal evolution when recently divergent taxa come together, and how differential responses to multiple signals may lead to differential signal introgression and novel trait combinations.     

Ecological gradient of sexual selection: elevation and song elaboration in finches

Ecological gradients in natural and sexual selection often result in evolutionary diversification of morphological, life history, and behavioral traits. In particular, elevational changes in habitat structure and climate not only covary with intensity of sexual selection in many taxa, but may also influence evolution of mating signals. Here we examined variation in courtship song in relation to elevation of breeding across cardueline finches-a subfamily of birds that occupies the widest elevational range of extant birds and shows extensive variation in life histories and sexual selection along this range. We predicted that decrease in sexual selection intensity with elevation of breeding documented in this clade would result in a corresponding evolutionary reduction in elaboration of courtship songs. We controlled for the effects of phylogeny, morphology, and habitat structure to uncover a predicted elevational decline in courtship song elaboration; species breeding at lower elevations sang more elaborated and louder songs compared to their sister species breeding at higher elevations. In addition, lower elevation species had longer songs with more notes, whereas frequency components of song did not vary with elevation. We suggest that changes in sexual selection account for the observed patterns of song variation and discuss how elevational gradient in sexual selection may facilitate divergence in mating signals potentially reinforcing or promoting speciation.     

FEMALE CHOICE FOR AN INDICATOR OF MALE SIZE IN THE SONG OF THE BLACK-HORNED TREE CRICKET,OECANTHUS NIGRICORNIS (ORTHOPTERA: GRYLLIDAE: OECANTHINAE)

The calling song of male crickets, including Oecanthus nigricornis (Walker), attracts females for mating and provides a model system of sexual communication. We give the first conclusive identification of a feature of cricket song that is both attractive to females and indicates a phenotypic feature (body size) that determines male mating success and female reproductive benefits. We do this by first testing for correlations between song characteristics and aspects of male phenotype that are hypothesized to indicate male quality. We show that song is a reliable indicator of male size and male age, and that large male size is associated with increased female fecundity. We then use playbacks of synthetic songs that mimic natural variation in song parameters to study song preferences and we compare preferences under different presentation regimes to determine whether choices are based on relative song quality or some fixed criterion. Females show a preference for the lower frequency songs produced by large males, but only during simultaneous playbacks. Thus female choice is based on the relative quality of calls that can be sampled simultaneously. These results provide strong support for the hypothesis that females use variation in calling song to assess male mate quality.     

The design of complex sexual traits in male barn swallows: associations between signal attributes

Variation in the expression of sexually selected traits among individuals is widely investigated on the premise that these traits evolved to signal male quality. Significant repeatabilities of sexual signals and their associations with condition, mating success, survivorship and age may be the signatures of sexual selection. However, little is known about the relationship between these sexual attributes. Here we studied 28 acoustic and visual traits in the barn swallow, Hirundo rustica, that may potentially function in sexual selection. Based on effect sizes calculated at the between-individual level, we assessed the relationship between repeatability, condition-dependence, attractiveness, age-dependence and viability indicator value of sexual traits using sexual signals as the units of analyses. Those traits that showed high within-year repeatability also showed high between-year repeatability, indicating that between-individual variation is consistent within and among seasons. In addition, age-dependence of traits, probably causing between-year variation, was negatively related to between-year repeatability. Condition-dependence was negatively correlated with effect sizes for the extent to which traits predicted viability. Therefore, traits that are positively related to immediate condition are those that are negatively related to survival, which may be the signature of a trade-off between current and future reproductive success ultimately reflecting signal reliability. No other significant relationship was found between trait attributes. We conclude that multiple sexual signals reflect different aspects of male quality in the barn swallow.     

Geographic variation in bird songs: examination of the effects of sympatric related species on the acoustic structure of songs

Birdsong evolution has influenced by various ecological and social factors. When related species that sing similar songs coexist, the acoustic properties of the songs of one or both species may shift, and the songs may diverge. We investigated geographic variation in the songs of the Japanese tit (Parus minor) and the varied tit (Poecile varius) in the Ryukyu Archipelago, Japan, whose islands harbor either one or both species. The songs of the two species exhibited similar structure, but acoustic measurements differed between them. For example, varied tits sang songs at higher frequency than Japanese tits did. The songs of both species varied geographically. At sites with higher relative densities of varied tits, Japanese tits sang lower frequency songs, indicating that in areas of coexistence, Japanese tits sang songs that had acoustically diverged from those of varied tits. Song variation in varied tits was not related to sympatry with Japanese tits. These asymmetric results suggest that the subordinate Japanese tit modified the acoustic characteristics of its song to avoid harassment by the dominant varied tit. We observed no effects of genetic divergence or local intraspecific density on Japanese tit or varied tit songs. This study used geographic variation to examine hypotheses of song evolution, and the results highlight the importance of character displacement.