Repertoire Sharing and Song Similarity between Great Tit Males Decline with Distance between Forest Fragments

Birdsong can play a critical role in establishing a territory and finding a mate among individuals from local and foreign populations. Variation in birdsong among populations can be influenced by habitat fragmentation and might affect successful dispersal among habitat fragments. We studied variation in great tit song in a long‐term study population distributed over nine forest fragments. All individual males recorded had a known dispersal history within the fragmented forest habitat. We found spatial structure of declining song‐type sharing with distance, with a marked drop from an individual’s own forest fragment to another across a habitat gap. We also found decreasing song similarity among increasingly distant fragments in terms of temporal and spectral characteristics of shared song types. The change in acoustic structure was more gradual and seemed less affected by habitat discontinuity but also showed a tight correlation with dispersal index among forest fragments. Immigrant birds shared fewer song types with neighbouring birds that were born within the same forest fragment, but not less compared to birds born in another forest fragment within the study area. Our data provide detailed insight into the relationship between song differentiation and male dispersal and contribute to our understanding of the potential role of song in reproductive exchange and avian speciation. The fact that birds in small forest fragments shared more songs than birds in larger forest fragments confirms that song analysis has potential as a tool for conservation in rare species.     

Advertisement Call Variation in the Golden Rocket Frog (Anomaloglossus beebei): Evidence for Individual Distinctiveness

Individual distinctiveness in acoustic signals can allow receivers to respond appropriately to different individuals. The aim of this research was to examine signal variation and to investigate the relative importance of different acoustic properties for coding individual distinctiveness in the advertisement calls of male golden rocket frogs (Anomaloglossus beebei). We examined patterns of within‐individual and among‐individual variability in 760 advertisement calls of 40 males as well as repeatability in 16 additional males that were recorded on two different occasions. We examined eight call properties, and all properties exhibited significant among‐individual variation and moderate to high repeatability across days. We employed discriminant function analysis to examine individual distinctiveness statistically. These analyses assigned 79% of calls to the correct individual. Fine temporal properties, including pulse duration, pulse rate, and pulse interval, as well as the spectral property of dominant frequency, contributed most toward discrimination among individuals. These results indicate that individual male golden rocket frogs can be distinguished statistically by their advertisement calls. We use these findings to evaluate results of previous playback studies in A. beebei and to generate testable predictions regarding the potential for specific call properties to function in mate selection and social recognition.     

Evolution of intraspecific variation in the advertisement call of a cricket frog (Acris crepitans, Hylidae)

Advertisement calls of the cricket frog, Acris crepitans , show statistically significant variation among populations in all call variables measured. Call variables show strong clinal variation resulting in calls of lower frequency, longer duration and slower call rates produced by A. c. blanchardi in open habitat in the west of the range, and calls of higher frequency, shorter calls and faster call rates produced by A. c. crepitans in the pine forests in the eastern part of the range. This clinal variation does not result from pleiotropic effects of body size or any other morphological characters we measured.
The two subspecies usually reside in different habitats, but some A. c. blanchardi reside in an isolated pine forest in central Texas. By comparing the calls of this subspecies in open and forest habitat, and by statistically removing the effects of clinal variation for all populations, we determined that habitat explains some of the variation in call structure; this is not true of subspecies.
Our data reject several hypotheses that purport to explain the evolution of mate recognition signals. (1) We reject the notion of Paterson and others that there is strong stabilizing selection on species-specific mate recognition signals. (2) There is no support for the hypothesis that call variation is primarily due to pleiotropic effects of body size or other morphological characters over the geographic range we examined. (3) There is no evidence for reproductive character displacement. (4) Our data, as well as experimental studies of habitat acoustics, support the hypothesis that some differences in calls among habitats result from environmental selection on call structure to enhance call transmission. We suggest that the latter hypothesis does not explain the strong clinal component of call variation. This might result from the passive effects of gene flow between populations at the extremes of the range under selection generated by habitat acoustics.     

Evolution of nocturnal flight calls in migrating wood-warblers: apparent lack of morphological constraints

Many migratory songbirds produce flight calls that vary, sometimes strikingly, among species in duration, frequency, modulation, and pattern, and little is known about what factors are responsible for this variation. Negative correlations between body mass or bill length and song frequencies are well documented in birds, but no studies have examined these associations for flight calls. We explored relationships between flight call and song frequencies and body mass or bill length in 33 species of wood-warblers. Phylogenetically controlled and uncontrolled analyses showed no significant relationship between body mass or bill length and flight call frequency; however, we found significant differences between flight call and song frequencies and a significant relationship between maximum frequencies of flight calls and songs. Our findings suggest that factors other than body mass and bill length are responsible for variation in flight call frequencies in wood-warblers. We suggest that different ecological and atmospheric properties might play important roles in selection for flight call frequencies.     

Who is Calling? Intraspecific Call Variation in the Aromobatid Frog Allobates femoralis

Many territorial species respond less aggressively to familiar neighbours than to unfamiliar floating strangers based on individual differences in acoustic signals. This form of social recognition, termed neighbour–stranger discrimination (NSD) or dear-enemy phenomenon has been reported so far from three anuran species. To investigate the potential of auditory signal features to convey information on sender's identity, we determined patterns of within-male and between-male variability in the advertisement call of the aromobatid frog Allobates femoralis . We examined 285 calls from 19 males to assess those call properties showing sufficient and reliable inter-individual differences to function as possible recognition cues. Beside calls per call bout and call rate, all other examined call properties were more variable among males than within males. Generally, temporal call features showed higher between- and within-male variability ratios than spectral properties and contributed mostly to distinguish individual males in the discriminant-function analysis. Mean classification success of 64.9% correctly assigned calls to individual males is mainly attributable to three temporal call properties (duration of note 1 and 4, note repetition rate). Altogether, our results suggest that there is sufficient variation in the advertisement call to discriminate statistically among individual males. However, assessed call differences between A. femoralis males were rather small, suggesting that potential NSD might be based either on a combination of call features or even on the whole pattern of individual call variation instead on single call properties. Habituation–discrimination experiments in the field using modified playback signals to test for differential behavioural responses are required to confirm this hypothesis.     

Geographical differentiation, acoustic adaptation and species boundaries in mainland citril finches and insular Corsican finches, superspecies Carduelis [citrinella]

Aim In birds, differentiation of acoustic characters is an important mechanism of reproductive isolation that may lead to an ethological–acoustic barrier, resulting in the formation of new species. We examined acoustic variation in mainland citril and insular Corsican finch populations, with the aim of assessing the degree of acoustic differentiation between both members of the superspecies Carduelis [citrinella] and documenting possible variation between local subpopulations that are geographically isolated. Location We chose study sites throughout the geographical ranges of citril and Corsican finches. For the citril finch, we obtained samples from the Black Forest (Germany), the Cevennes (France) and the Pyrenees (Spain); for the Corsican finch, we obtained samples from the islands Capraia and Sardinia (Italy) and Corsica (France). Methods We analysed frequent contact calls and elements of the perch song. Vocalization patterns of the study populations were compared by means of discriminant and hierarchical cluster analyses. Results There were significant differences in vocalization characteristics of perch songs and contact calls, which permitted unambiguous discrimination of citril and Corsican finch populations. However, we also detected significant differences in contact calls between mainland citril finch subpopulations. There was a pattern of clinal variation in vocalization: short, steeply modulated signals in the northern part of the geographical range (Black Forest) and long, shallowly modulated signals in the southern part (Pyrenees). Main conclusions Acoustically, mainland citril and insular Corsican finches separate well in their contact calls and perch songs. However, variation in the two vocalization patterns between subpopulations of mainland citril finches indicates that acoustic characteristics can evolve very quickly, not only on islands but also on the mainland. Local habitat differences may play a crucial role in the rapid evolution of these signals under full or partial isolation of small subpopulations. To judge the importance of signal variation as a pre‐mating isolating barrier, future studies will have to determine whether members of the distinct subpopulations are able to match their signals to each other if they re‐meet, and whether intraspecific species recognition is still possible.     

Geographic and Temporal Variation of the Male Zebra Finch Distance Call

Temporal and geographic variation of acoustic signals can provide insights into dispersal patterns, population history and speciation. Vocalizations that are transmitted from one generation to the next are of particular value in this respect because they can reveal patterns of gene flow, effectively behaving as population markers. The male zebra finch Taeniopygia guttata distance call is one such vocalization; sons learn their father's distance call in the first 40 d of life and it is individually stereotyped thereafter. We investigated geographic variation in the zebra finch by comparing the structure of distance calls recorded from 61 males from six populations across the continent‐wide range of the Australian subspecies T. g. castanotis. Intra‐population variation was high, in many cases greater than the variation among all males recorded, possibly because of population interchange. However, three of six call variables measured, including the newly discovered modulated element, varied geographically although the pattern of distance call variation did not agree with that of geographic proximity of populations. The proportion of calls with a modulated element increased dramatically over 7 yr in central Australia but there was no change over a similar period of time in south‐eastern Australia where no calls contained the element. The findings suggest that interchange among widely separated populations may be commonplace in Australian zebra finches, with the possible exception of those from south‐eastern Australia.     

Acoustic communication in the Kittiwake <Emphasis Type="Italic">Rissa tridactyla</Emphasis>: potential cues for sexual and individual signatures in long calls

Sex and individual recognition systems vary among species and can have various functions in different contexts. In order to determine the basis of identification by voice in the Kittiwake (Rissa tridactyla), the greeting calls of 32 individuals (18 males and 14 females) were recorded in May–June 2004 on the Kittiwake colony of Hornøya island (Barents sea) and analysed. On the basis of coefficient of variation calculations and discriminant analyses, we show (1) that calls are sexually dimorphic and that the dimorphism is mainly based on the value of the fundamental frequency, and (2) that calls are individually distinct, individuality being due to a complex of temporal and frequency parameters located in different parts of the signal. This coding strategy is discussed in the context of the colonial breeding habitat of the species.     

Seasonal variability in habitat structure may have shaped acoustic signals and repertoires in the black-capped and boreal chickadees

Many songbird species have evolved multiple vocalizations, or repertoires, that function to communicate various biological signals. More diverse repertoires may have evolved in response to the effects of seasonal variation in habitat structure on signal transmission. Such changes in habitat necessarily occur for migrating species, but they also occur for resident species that occupy deciduous forests. The black-capped chickadee (Poecile atricapillus) possesses a chick-a-dee call and a fee-bee song, but the closely related boreal chickadee (P. borealis) lacks a song. Consistent with the habitat variability hypothesis, the black-capped chickadee possesses a larger repertoire and primarily occupies deciduous forests, whereas the songless boreal chickadee occurs more often in coniferous forests. We explored the ecological basis of this hypothesis by recording audio playbacks of two species in two habitat types during two seasons. Specifically, we played both songs and calls of the black-capped chickadee and calls of the boreal chickadee in deciduous and coniferous habitats, prior to and after leaf-out. We measured attenuation and degradation in re-recorded vocalizations. For black-capped chickadees, the song was less degraded than the call in post-leaf, deciduous forests. The boreal chickadee call attenuated more quickly in all treatments, but maintained its acoustic structure better than both black-capped chickadee vocalizations in coniferous forests. Our results support the hypothesis that variable habitats provided a seasonal transmission benefit for both song and call in the black-capped chickadee, but that the transmission benefit of song is lost in the less variant coniferous forests, which may underlie the absence of a song in the boreal chickadee.     

Geographic Variation in the Long Calls of Male Orangutans (Pongo spp.)

Relatively few data exist on population differences in the vocal behavior of mammals. Geographic variation in calls is of special interest because of the implications for resolving evolutionary and behavioral questions. For example, information on geographic variation in vocalizations complements morphological and molecular data used to infer phylogenetic relationships and provides evidence for the mechanisms underlying call development. A quantitative acoustic analysis of orangutan long calls was undertaken, comparing flanged adult males from four geographically distinct sites across Borneo and Sumatra, revealing consistent differences among the calls of individuals. Long calls produced by orangutans from the four sites in Borneo and Sumatra differ in quantitative acoustic measures. Discriminant function analysis reveals that acoustic variables can be used in combination to assign calls to the correct individual, site and island at rates higher than that expected by chance. Specifically, four acoustic parameters proved reliable for distinguishing among the individuals, between sites, and across the two islands that arguably represent populations from separate species or subspecies. Although Bornean and Sumatran long calls share a repetitive structure and show similar call rates (0.100–0.500 LCs/h) and maximum frequency bands (0.400–1.500 kHz), they differ significantly in the number of pulses per call, call speed, call duration, bandwidth, pulse duration, and dominant frequency. Strong consistency in these acoustic parameters is also seen among males within sites and the observed variation may allow for individual recognition. Individual identification by call structure presumably benefits dispersed orangutans, where individuals characteristically forage independently and both encounters and interactions with signaling males are highly variable and largely dependent on context. Acoustic recognition of callers facilitates the choice of which males to join or avoid, thus allowing receivers to manipulate potential costs and benefits of association.     

Whip it good! Geographic consistency in male songs and variability in female songs of the duetting eastern whipbird Psophodes olivaceus

Geographic variation in male bird songs has been studied extensively, but there have been few investigations of geographic variation in female songs or sex differences in patterns of geographic variation. We compared patterns of variation in male and female songs of eastern whipbirds Psophodes olivaceus by analyzing recordings from 16 populations across the species’ geographic range in eastern Australia. We found remarkably different patterns of geographic variation between the sexes. Female eastern whipbird songs are easily categorized into discrete song types. Song types are shared between nearby females, but female songs show pronounced differences at a continental scale. In contrast, male eastern whipbird songs show high consistency throughout the species’ geographic range. All recorded males share the ability to transpose the frequency of the introductory whistle and most recorded males share the ability to vary the direction of the slope of the terminal whip crack. For eight of nine measured variables, female songs show significantly higher levels of variation than male songs. We discuss whether sex differences in dispersal, song learning strategies, and song function may explain these sex differences in patterns of song variation. We suggest that eastern whipbirds have experienced a decoupling of male and female song learning strategies and that the songs of each sex have responded to different selective pressures in the context of countersinging interactions. We speculate that consistency in male songs throughout the geographic range of eastern whipbirds may arise through female preference for males that perform large bandwidth whip cracks.     

Larger body size on islands affects silvereye Zosterops lateralis song and call frequency

Differences in song repertoires and characteristics of island and mainland populations of the same avian species are usually explained by dispersal, cultural evolution and/or habitat differences. The influence of morphology is often overlooked, even though island populations are frequently morphologically distinct from mainland populations, and morphology could affect vocalizations. I compared morphological features, songs, contact calls and alarm calls of six isolated island populations of silvereye Zosterops lateralis with those of two mainland populations to examine whether differences between mainland and island vocalizations were consistent across vocalization types, and whether these differences could be linked to morphological differences. Vocalizations were lower in frequency on islands. Island individuals were larger (both in mass and body structure), and body mass was an important predictor of frequency in contact and alarm calls. I argue that this strong association results from the island rule (islands promote larger body sizes) and cascading effects of morphology on vocalization frequency in this species.     

Whip it good! Geographic consistency in male songs and variability in female songs of the duetting eastern whipbird Psophodes olivaceus

Geographic variation in male bird songs has been studied extensively, but there have been few investigations of geographic variation in female songs or sex differences in patterns of geographic variation. We compared patterns of variation in male and female songs of eastern whipbirds Psophodes olivaceus by analyzing recordings from 16 populations across the species' geographic range in eastern Australia. We found remarkably different patterns of geographic variation between the sexes. Female eastern whipbird songs are easily categorized into discrete song types. Song types are shared between nearby females, but female songs show pronounced differences at a continental scale. In contrast, male eastern whipbird songs show high consistency throughout the species' geographic range. All recorded males share the ability to transpose the frequency of the introductory whistle and most recorded males share the ability to vary the direction of the slope of the terminal whip crack. For eight of nine measured variables, female songs show significantly higher levels of variation than male songs. We discuss whether sex differences in dispersal, song learning strategies, and song function may explain these sex differences in patterns of song variation. We suggest that eastern whipbirds have experienced a decoupling of male and female song learning strategies and that the songs of each sex have responded to different selective pressures in the context of countersinging interactions. We speculate that consistency in male songs throughout the geographic range of eastern whipbirds may arise through female preference for males that perform large bandwidth whip cracks.     

Variation in pika (Ochotona collaris,O. princeps) vocalizations within and between populations

Understanding geographic call variation can resolve evolutionary and behavioural questions, yet the factors influencing divergent acoustic signals remain poorly understood in mammals. We explored call variation between collared pikas in Yukon and Alaska and American pikas in Alberta, and between individuals within a population of collared pikas. Classification trees were used to determine the extent of call divergence between populations and the elements of calls driving these differences. Pika populations had significant differences in call structure, and individual pikas were classified to their correct populations with up to 94% accuracy. To investigate possible mechanisms responsible for interspecific variability, we tested the acoustic adaptation hypothesis by using a playback experiment to explore whether American and collared pika calls transmit with less degradation across their own species' habitat than the habitat of their congener. We found no support for the acoustic adaptation hypothesis. Geographic call variation in these two species of pikas likely reflects genetic divergence, and may be a result of separate evolutionary histories. We calculated the potential for individual coding for both time and frequency measurements of calls. High frequency harmonics showed greater between‐ than within‐individual variation, and may act as sources of information regarding individual identity.     

Voice variance may signify ongoing divergence among black-legged kittiwake populations

Acoustic features are important for individual and species recognition. However, while dialectal variations in song characteristics have been described in many songbirds, geographical divergence in vocal features across populations has seldom been studied in birds that are not thought to have song-learning abilities. Here, we document marked differences in the vocal structure of calls of two populations of black-legged kittiwakes ( Rissa tridactyla ), a seabird whose call is considered as not being learned from other individuals. We found that calls vary both within and between populations. Within-population variation may convey individual identity, whereas the marked differences in frequency and temporal parameters observed between the two populations may reveal ongoing divergence among kittiwake populations. Moreover, we were unable to detect any sex signature in adult calls in a Pacific population (Middleton, Alaska), while these were detected in an Atlantic population (Hornøya, Norway), potentially affecting sexual behaviours. Despite the fact that these calls seemed to change over the reproductive season and across years, the individual signature remained fairly stable. Such vocal differences suggest that Pacific and Atlantic populations may be undergoing behavioural divergences that may reveal early stages of speciation, as is suggested by molecular data.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 289–297.     

Persistence of song types in Darwin's finches,Geospiza fortis,over four decades

Learned bird songs evolve via cultural evolution, with song patterns transmitted across generations by imitative learning. In Darwin''s finches of the Galápagos Islands, males learn songs from their fathers, and song types can be maintained across multiple generations. However, little is known about the time frame over which specific song types are preserved, in the face of copy errors and corresponding modifications to song structure. Here we investigate cultural evolution in songs of male Geospiza fortis, at Academy Bay, Santa Cruz Island, comparing songs recorded in 1961 by R. Bowman (20 individuals) to those recorded in 1999 by J. Podos (16 individuals). For each individual, we characterized four timing and six frequency parameters, and assessed inter-individual variation in song structure using multivariate analysis. Several 1961 song types persisted into 1999, some with remarkable fidelity. Variation among song types was extensive during both years, and we detected no changes in 10 vocal parameters across the sampling period. These results illustrate temporal continuity in a culturally acquired trait, and raise questions about mechanisms that promote stability in song structure.     

High levels of gene flow among song dialect populations of the Puget Sound white‐crowned sparrow

Populations within a species can show geographic variation in behavioral traits that affect mating decisions or limit dispersal. This may lead to restricted gene flow, resulting in a correlation between behavioral variation and genetic differentiation. Populations of a songbird that differ in a learned behavioral trait, their song dialects, may also differ genetically. If song dialects function as mating barriers, evolutionary processes such as genetic drift should lead to divergence in allele frequencies among dialect populations. The Puget Sound white‐crowned sparrow (Zonotrichia leucophrys pugetensis) is an excellent study system with a well‐defined series of song dialects along the Pacific Northwest coast. A previous study found low genetic differentiation based on four microsatellite loci; however, available loci and analyses techniques have since dramatically improved and allow us to reassess gene flow in this species. We also add extra samples to fill in gaps and add a new level of analysis of geographic variation. Based on acoustic similarities, we group six song dialects into two geographically larger “northern” and “southern” song themes. One southern dialect is acoustically more similar to dialects in the north, which makes the genetic profile of birds singing this dialect particularly interesting. Traditional F‐statistics, analysis of molecular variance as well as Bayesian techniques confirmed the earlier result that geographic variation in song does not correlate with the neutral genetic structure of the sampled dialect populations. The song themes also did not differ genetically, and the origin of the extralimital northern‐theme dialect cannot be determined. We compare this result to findings in several other species and discuss how the timing of learning and dispersal allow vocalizations to vary independently of patterns of genetic divergence.     

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.