Geographic variation in songs of the Bewick's wren: a search for correlations with avifaunal complexity

Summary Song characteristics of the Bewick's wren (Thryomanes bewickii) are compared from nine localities in the western United States. Character shifts, i.e., a difference in means, are evident for all song characters: Arizona and Colorado songs are especially short and long, respectively (Figs. 1, 2); songs of insular (Santa Cruz Is.) and nearby mainland populations in California are very dissimilar (Table 1); excluding the insular population, the frequency range of song phrases is positively correlated with latitude (Fig. 3). Variance shifts, i.e., a difference in repertoire size or Coefficients of Variation (CV's) of measured song characters, are also present; most notably, Arizona males have exceptionally stereotyped songs, with small song phrase repertoires (Table 2) and low CV's. Population densities and/or habitat structure undoubtedly influence signal design, but correlations reported here suggest that the avifaunal complexity and the corresponding vocal milieu should also be examined rigorously as possibly important influences.     

Bachelor and paired male rufous-and-white wrens use different singing strategies

To attract a breeding partner, males may behave differently when they are bachelors compared to when they are paired. Comparisons between groups of paired males versus groups of unpaired males in temperate-breeding animals have revealed such differences in signalling behaviour. Few studies, however, have explored how individual males alter their signalling behaviour with changes in pairing status, and very few investigations have explored paired versus unpaired male behaviour in tropical animals. During a 5-year study in Costa Rica, we analysed changes in the singing behaviour of male rufous-and-white wrens (Thryothorus rufalbus) when they were paired and when they were bachelors. We compared three aspects of male vocal behaviour: gross differences in song output, variation in repertoire use and differences in song structure. Males as bachelors had significantly higher song output and switched song types less frequently. Contrary to our expectation, bachelors sang significantly fewer song types from their repertoire compared to when those same males had a breeding partner. Songs sung by bachelor males were higher in syllabic diversity and had broader-bandwidth terminal syllables than the songs those males sang only when paired. Within song types, the fine structure of songs remained consistent across pairing status. Our results demonstrate that males change their singing behaviour with pairing status, delivering songs at a higher rate but with less variety when they are bachelors. Rufous-and-white wrens are renowned for their vocal duets, and we discuss the pattern of repertoire use in light of their duetting behaviour. These results enhance our understanding of how male behaviour varies with pairing status and the importance of vocal signalling behaviour in socially monogamous tropical animals.     

An experimental study of song type function in the bobolink (Dolichonyx oryzivorus)

Summary We tested the hypothesis that the alpha and beta songs of male bobolinks (Dolichonyx oryzivorus) have separate intrasexual and intersexual functions by presenting caged males and caged females on the territories of male bobolinks. Although all males in our study population share both alpha and beta songs, we found no evidence for a specialized intersexual function of alpha song or a specialized intrasexual function of beta song. Territorial males increased their singing rate of both alpha and beta songs during both the male presentations and the female presentations. The proportions of the two song types did not change between control and experimental periods for either male or female presentations. We found no evidence in support of specialized intrasexual and intersexual functions of note types. Only 10% of the vocalizations initiated by territorial males were complete alpha or beta songs, while 87% were song fragments, and the remaining 3 % were compound songs. We did find that territorial males responded to simulated male intruders by increasing the proportion of fragmented songs and decreasing the proportion of compound songs. Moreover, males included fewer notes per song initiated and sang a higher proportion of songs that included only introductory notes during male presentations. We conclude that the vocal response of territorial male bobolinks to conspecific male intruders is to sing short, simple sequences of either song type. We found no consistent changes in vocal behavior of territorial males in response to female presentations other than increases in song rates.     

Vocal interactions in common nightingales (<Emphasis Type="Italic">Luscinia megarhynchos</Emphasis>): males take it easy after pairing

Seasonal patterns of bird song have been studied intensively with a focus on individual males. However, little is known about seasonal patterns of singing during vocal interactions between males. Vocal interactions have been shown to be important in sexual selection as males may signal aspects of motivation or quality. Here, we investigated in nightingales (Luscinia megarhynchos) whether a male’s behaviour in vocal interactions at different stages of the breeding season is influenced by its mating status. We examined how males that differ in their subsequent mating success respond to a non-interactive, nocturnal playback presented during the period of mate attraction and subsequently during the egg-laying period. We found that mated males overlapped fewer songs and had a lower song rate during the egg-laying period compared to their responses during the mate-attraction period, whereas unpaired males did not vary in their responses between the two periods. Our results suggest that mating status is a key factor affecting singing behaviour in vocal interactions and that a time-specific singing pattern like song overlapping is used flexibly during vocal interactions. Because song overlapping is thought to be a signal of aggression in male–male vocal interactions, it seems that males vary the level of aggression in vocal interactions according to their mating status and to the stage in the breeding season.     

Song sharing in two populations of song sparrows (Melospiza melodia)

Sharing song types with immediate neighbors is widespread in birds with song repertoires, and sharing songs may confer a selective advantage in some cases. Levels of song sharing vary between different geographical populations of several bird species, and ecological differences often correlate with differences in singing behavior; in particular, males in migratory subspecies often share fewer songs than males in resident subspecies. The song sparrow (Melospiza melodia) appears to fit this pattern: resident song sparrows in western North America generally share 20–40% of their repertoire (of about eight songs) with each neighbor, while migratory subspecies from eastern North America often share 10% or less. We compared song sharing in two populations within a single subspecies of song sparrow (M. m. morphna) in Washington State. These populations, separated by only 120 km, nonetheless differ in migratory tendencies and several other ecological and life history variables. We recorded complete song repertoires from 11 male song sparrows in a high-elevation, migrating population at Gold Creek in west-central Washington, and compared them to two samples (n = 15 and n = 36) from a coastal, resident population at Discovery Park, Seattle, Washington. Despite major differences in habitat, population density, and migratory tendencies, song sharing among Gold Creek males was as high as that among Discovery Park males. In both populations, sharing was highest between immediate neighbors, and declined with distance. We conclude that at the within-subspecies level, neither migration nor population density affect song sharing in song sparrows, a song repertoire species. Received: 26 November 1998 / Received in revised form: 1 May 1999 / Accepted: 29 May 1999     

How a songbird with a continuous singing style modulates its song when territorially challenged

In songbirds of the temperate zone, often only males sing and their songs serve to attract females and to deter territorial rivals. In many species, males vary certain aspects of their singing behavior when engaged in territorial interactions. Such variation may be an honest signal of the traits of the signaler, such as fighting strength, condition, or aggressive motivation, and may be used by receivers in decisions on whether to retreat or to escalate a fight. This has been studied intensively in species that sing discontinuously, in which songs are alternating with silent pauses. We studied contextual variation in the song of skylarks (Alauda arvensis), a songbird with a large vocal repertoire and a continuous and versatile singing style. We exposed subjects to simulated territorial intrusions by broadcasting conspecific song and recorded their vocal responses. We found that males sing differently if they are singing spontaneously with no other conspecific around than if they are territorially challenged. In this last case, males produced lower-frequency syllables. Furthermore, they increased the sound density of their song: they increased the proportion of sound within song. They seem to do so by singing different elements of their repertoire when singing reactively. Furthermore, they increased the consistency of mean peak frequency: they repeated syllable types with less variability when singing reactively. Such contextual variation suggests that skylarks might use low frequencies, sound density, and song consistency to indicate their competitive potential, and thus, those song features might be important for mutual assessment of competitive abilities.     

Seasonal variation in the duetting behaviour of rufous-and-white wrens (<Emphasis Type="Italic">Thryothorus rufalbus</Emphasis>)

Seasonal variation in animal signalling behaviour has been well documented and has contributed much to our understanding of male signals. In contrast, we know little about seasonal variation in female signals or signals produced jointly by males and females, such as the vocal duets of birds. Here, we examine how singing behaviour changes in relation to time of year and breeding stage in rufous-and-white wrens (Thryothorus rufalbus), neotropical songbirds where both males and females sing and where breeding partners coordinate songs to produce vocal duets. We recorded a colour-marked population of birds over an extended time period encompassing multiple breeding stages. Across all time frames and breeding stages, males sang at higher rates than females and male solos were more common than duets or female solos. Males and females showed divergent seasonal patterns of singing. Females sang more often early in the year, during the pre-breeding season, and female song tapered off as the breeding season progressed. Duetting followed a parallel pattern, which resulted from females showing less duet responsiveness to their partner’s songs later in the year. Male independent song rate peaked at the onset of the rainy season – a time when females become fertile – and males showed the highest level of duet responsiveness during this period. Our results suggest that early in the year, duets appear to be cooperative displays, functioning in joint territory defence and/or the coordination of breeding activities. When females are fertile, however, increased duet responsiveness by males is consistent with mate or paternity guarding.     

Population structure of humpback whales in the western and central South Pacific Ocean as determined by vocal exchange among populations

For cetaceans, population structure is traditionally determined by molecular genetics or photographically identified individuals. Acoustic data, however, has provided information on movement and population structure with less effort and cost than traditional methods in an array of taxa. Male humpback whales (Megaptera novaeangliae) produce a continually evolving vocal sexual display, or song, that is similar among all males in a population. The rapid cultural transmission (the transfer of information or behavior between conspecifics through social learning) of different versions of this display between distinct but interconnected populations in the western and central South Pacific region presents a unique way to investigate population structure based on the movement dynamics of a song (acoustic) display. Using 11 years of data, we investigated an acoustically based population structure for the region by comparing stereotyped song sequences among populations and years. We used the Levenshtein distance technique to group previously defined populations into (vocally based) clusters based on the overall similarity of their song display in space and time. We identified the following distinct vocal clusters: western cluster, 1 population off eastern Australia; central cluster, populations around New Caledonia, Tonga, and American Samoa; and eastern region, either a single cluster or 2 clusters, one around the Cook Islands and the other off French Polynesia. These results are consistent with the hypothesis that each breeding aggregation represents a distinct population (each occupied a single, terminal node) in a metapopulation, similar to the current understanding of population structure based on genetic and photo‐identification studies. However, the central vocal cluster had higher levels of song‐sharing among populations than the other clusters, indicating that levels of vocal connectivity varied within the region. Our results demonstrate the utility and value of using culturally transmitted vocal patterns as a way of defining connectivity to infer population structure. We suggest vocal patterns be incorporated by the International Whaling Commission in conjunction with traditional methods in the assessment of structure.     

Temporal performance roles during vocal interactions in nightingales (Luscinia megarhynchos B.)

Summary The temporal patterning of vocal interactions between territorial nightingales (Luscinia megarhynchos B). was investigated during nocturnal dyadic singing. We distinguished three forms of temporal performance roles (Table 1; Fig. 3): inserter (preferred song start 0.5–1.0 s after offset of a neighbour's song), overlapper (preferred song start 0.5–1.0 s after onset of a neighbour's song), and Autonomous bird (no evidence for temporal responses adjusted to the timing of a neighbour's singing; Fig. 4). With the beginning of the daily dawn chorus, mutual temporal adjustment of song performances could no longer be ascertained (Fig. 2). To test the flexibility of song onset timing, we presented subjects with three playback programs (I, II, III) simulating a non-flexible conspecific bird. Sequences of songs recorded from a stranger that shared no song types with the subjects were used; intersong pauses were unaltered in I, lengtened in II, and shortened in III. Results confirmed that the timing of song onset can be influenced by the temporal patterning of auditory stimuli (Table 1; Fig. 5): Inserters and overlappers sang at a slower rate during program II than during program I. During program III, which by accelerated stimulus succession caused frequent overlap of songs, inserters and overlappers interrupted their singing behaviour (Fig. 6). This suggests that overlapping, which results in signal interference, is of some functional significance in vocal interactions.     

Song repertoire size and female preferences in song sparrows

Summary Captive female song sparrows (Melospiza melodia), after treatment with estradiol, perform copulation solicitation displays when presented with songs of conspecific males. Females respond more strongly to eight song types than to four, and to sixteen song types than to eight. These results are consistent with the hypothesis that female song sparrows prefer large repertoires, rather than preferring normal or natural repertoire sizes (5 to 13 song types).The results with captive females might be taken to imply that females in the field prefer as mates those males with the largest repertoire sizes. This hypothesis was tested by observing pairing in a field population. In each of 2 years, there was no significant correlation between male repertoire size and date of initial pair formation. Furthermore, there was no correlation between repertoire size and the speed with which a second mate was acquired after removal of the first. It is concluded that repertoire size does not have an important influence on female choice of mates in song sparrows.     

Estimating the complexity of bird song by using capture-recapture approaches from community ecology

Repertoire size, the number of unique song or syllable types in the repertoire, is a widely used measure of song complexity in birds, but it is difficult to calculate this exactly in species with large repertoires. A new method of repertoire size estimation applies species richness estimation procedures from community ecology, but such capture-recapture approaches have not been much tested. Here, we establish standardized sampling schemes and estimation procedures using capture-recapture models for syllable repertoires from 18 bird species, and suggest how these may be used to tackle problems of repertoire estimation. Different models, with different assumptions regarding the heterogeneity of the use of syllable types, performed best for different species with different song organizations. For most species, models assuming heterogeneous probability of occurrence of syllables (so-called detection probability) were selected due to the presence of both rare and frequent syllables. Capture-recapture estimates of syllable repertoire size from our small sample did not differ significantly from previous estimates using larger samples of count data. However, the enumeration of syllables in 15 songs yielded significantly lower estimates than previous reports. Hence, heterogeneity in detection probability of syllables should be addressed when estimating repertoire size. This is neglected using simple enumeration procedures, but is taken into account when repertoire size is estimated by appropriate capture-recapture models adjusted for species-specific song organization characteristics. We suggest that such approaches, in combination with standardized sampling, should be applied in species with potentially large repertoire size. On the other hand, in species with small repertoire size and homogenous syllable usage, enumerations may be satisfactory. Although researchers often use repertoire size as a measure of song complexity, listeners to songs are unlikely to count entire repertoires and they may rely on other cues, such as syllable detection probability.Communicated by A. Cockburn     

Spatial and age-related variation in use of locally common song elements in dawn singing of song sparrows <Emphasis Type="Italic">Melospiza melodia</Emphasis>: old males sing the hits

In many songbirds, individuals have repertoires of multiple song types, some of which may be shared with others in the local area. Hypotheses about the evolution of song repertoires differ as to whether selection acts primarily on repertoire size itself or the ability to match songs of neighbours. We used a 16-channel acoustic location system to record neighbourhoods of song sparrows (Melospiza melodia melodia) during the dawn chorus. We asked whether males sing all songs with similar frequency as predicted by the Repertoire Size Hypothesis, whether males preferentially sing highly shared songs as predicted by the General Sharing Hypothesis, or whether use of highly shared songs is associated with phenotype as predicted by the Conditional Sharing Hypothesis. Contrary to the Repertoire Size Hypothesis, most males did not sing all songs equally often. Contrary to the General Sharing Hypothesis, we found no general tendency to overproduce highly shared songs. The degree to which males overproduced highly shared songs was repeatable across days, indicating consistent individual differences, and varied across neighbourhoods. Moreover, and consistent with the Conditional Sharing Hypothesis, older males were more likely to overproduce highly shared songs. If highly shared song is a conventional signal of aggression, with the threat of receiver retaliation maintaining honesty, older males may be more willing or able to risk conflict. Alternatively, males may learn which songs are effective signals for an area. Finally, age-related variation in vocal performance may shape the adaptive value of highly shared song.     

Individual recognition of song repertoires in two wood warblers

We examined recognition of individual neighbors in hooded warblers (Wilsonia citrina) and Kentucky warblers (Oporornis formosus) by comparing responses to playbacks of neighbors' songs from appropriate and inappropriate boundaries of the subjects' territories. Kentucky warblers, which sing one song type each, responded more strongly to the songs of neighbors broadcast from incorrect than from correct boundaries. Hooded warblers use their repertoires of three to nine song types in two distinct modes of singing: repeat mode involves repeated presentation of one song type; mixed mode involves presentation of two or more song types in irregular sequence. Playbacks of neighbors' mixed-mode songs from appropriate and inappropriate boundaries indicated capabilities of individual recognition similar to those reported previously for repeat-mode songs (Godard 1991) and to those of Kentucky warblers. Repertoires of moderate size, therefore, have no pronounced influence on eventual recognition of individual neighbors in hooded warblers.     

Temporal patterning of within-song type and between-song type variation in song repertoires

In many songbird species, males sing a repertoire of distinct song types. Song sparrows typically are described as having repertoires of about a dozen song types, but these song types are themselves quite variable and some songs are produced that appear intermediate between two types. In this study we quantify the similarity between successive songs produced by song sparrows in order to determine if differences between song types are emphasized or deemphasized in bouts of continuous singing. In spite of the high degree of variation within song types and similarity between song types observed in this species, we show that transitions from one song type to the next are distinct as compared to transitions within sequences of the same type (Figs. 4 and 5). Variation does not accumulate across sequences of the same song type, and the average variation observed within a continuous sequence of the same song type is significantly less than is predicted from the total variation recorded for that type across many different bouts (Fig. 6). These results support the view that song sparrows include two levels of variation in their singing: differences between song types as is commonly observed in species with song repertoires, and an independent level of variation observed for songs of the same type.     

Responses to playback of whistle songs and normal songs in male nightingales: effects of song category, whistle pitch, and distance

In most species of song birds, males develop song repertoires of several different songs. Among this variety, different songs may be used differently in communication and, thus, may have different functions. Here we studied vocal responses to playback of structurally different songs in male territorial nightingales (Luscinia megarhynchos). Nightingales have enormous vocal repertoires of about 200 song types. Among these songs, one category, whistle songs, sticks out syntactically and acoustically. Here we tested whether or not male nightingales match whistle songs with whistle songs and, if so, whether they also match the pitch of the broadcast whistles. Furthermore, we also tested if nightingales treat whistle songs as a separate category of songs. We conducted interactive playback experiments on nocturnal song in which each male received three playback treatments that differed in the number of whistle songs broadcast. Males responded differently during playbacks by singing significantly more whistle songs when the playback tapes contained many whistle songs than when they contained no whistle songs. Males also frequently matched the pitch of the broadcast whistle songs. In contrast to responses during playback, after the playback terminated males sang more whistle songs when no whistle songs were broadcast than when many whistle songs were broadcast. These findings suggest that whistle songs have a specific signal value and that nightingales treat them as a special song category. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00265-002-0511-1.     

Developing Metapopulation Connectivity Criteria from Genetic and Habitat Data to Recover the Endangered Mexican Wolf

Restoring connectivity between fragmented populations is an important tool for alleviating genetic threats to endangered species. Yet recovery plans typically lack quantitative criteria for ensuring such population connectivity. We demonstrate how models that integrate habitat, genetic, and demographic data can be used to develop connectivity criteria for the endangered Mexican wolf (Canis lupus baileyi), which is currently being restored to the wild from a captive population descended from 7 founders. We used population viability analysis that incorporated pedigree data to evaluate the relation between connectivity and persistence for a restored Mexican wolf metapopulation of 3 populations of equal size. Decreasing dispersal rates greatly increased extinction risk for small populations (<150–200), especially as dispersal rates dropped below 0.5 genetically effective migrants per generation. We compared observed migration rates in the Northern Rocky Mountains (NRM) wolf metapopulation to 2 habitat‐based effective distance metrics, least‐cost and resistance distance. We then used effective distance between potential primary core populations in a restored Mexican wolf metapopulation to evaluate potential dispersal rates. Although potential connectivity was lower in the Mexican wolf versus the NRM wolf metapopulation, a connectivity rate of >0.5 genetically effective migrants per generation may be achievable via natural dispersal under current landscape conditions. When sufficient data are available, these methods allow planners to move beyond general aspirational connectivity goals or rules of thumb to develop objective and measurable connectivity criteria that more effectively support species recovery. The shift from simple connectivity rules of thumb to species‐specific analyses parallels the previous shift from general minimum‐viable‐population thresholds to detailed viability modeling in endangered species recovery planning. Desarrollo de Criterios de Conectividad Metapoblacional a Partir de Datos Genéticos y de Hábitat para Recuperar al Lobo Mexicano en Peligro de Extinción     

Vocal and visual stimuli enabling copulation behavior in female buntings

Summary The loud and elaborate songs of male songbirds are throught to serve in territorial defense and to stimulate reproductive behavior in the female. We report here that in contrast to several other species, song alone is inadequate to induce sexual (lordosis) behavior in female indigo and lazuli buntings (Passerina cyanea and P. amoena); they require a more elaborate stimulus configuration to develop the full expression of sexual receptivity as indicated by a stereotyped copulation solicitation display. A live male must be present near the female, singing must ensue, and in addition a second and unique vocal utterance must be heard by the female; this is a soft buzzy-sounding vocalization, audible for no more than a few meters, not recognized previously as a critical arousal signal of the male songbird repertoire. Females also utter similar soft sounds in an intimate vocal exchange with the male leading to solicitation of copulation.     

Developmental stress affects the attractiveness of male song and female choice in the zebra finch (Taeniopygia guttata)

Developmental stress has recently been shown to have adverse effects upon adult male song structure in birds, which may well act as an honest signal of male quality to discriminating females. However, it still remains to be shown if females can discriminate between the songs of stressed and non-stressed males. Here we use a novel experimental design using an active choice paradigm to investigate preferences in captive female zebra finches (Taeniopygia guttata). Nine females were exposed to ten pairs of songs by previously stressed and non-stressed birds that had learned their song from the same tutor. Song pairs differed significantly in terms of song complexity, with songs of stressed males exhibiting lower numbers of syllables and fewer different syllables in a phrase. Song rate and peak frequency did not differ between stressed and non-stressed males. Females showed a significant preference for non-stressed songs in terms of directed perching activity and time spent on perches. Our results therefore indicate that developmental stress affects not only the structure of male song, but that such structural differences are biologically relevant to female mate choice decisions.     

Mate choice by female white-crowned sparrows in a mixed-dialect population

Summary It has been argued that the song dialects of white-crowned sparrows, Zonotrichia leucophrys, may function to restrict gene flow between dialect populations, thus promoting adaptation to local environmental conditions. One proposed mechanism whereby this may occur is if a female mates preferentially with males singing her natal dialect, typically the song type of her father. Previous field and laboratory studies of the role played by song differences in mate choice by female white-crowned sparrows have produced equivocal results. These investigations were conducted in populations in which a single song dialect was predominant. We examined mate choice in a population in which two song dialects are equally common. We found that songs of mates of individual females in successive years were no more likely to be of the same dialect than expected by chance. In addition, individual females did not mate preferentially with males whose songs matched those of their fathers. We conclude that, in this population, females choose mates on the basis of characteristics other than song type.     

Tutor choice and imitation accuracy during song learning in a wild population of the Puget Sound white-crowned sparrow

Songbirds learn to sing by modeling their songs on the songs of other males through a process of social learning. Models of social learning predict that animals should be selective in what and when they learn. In this study, we asked whether young males in a wild population of the Puget Sound white-crowned sparrow, Zonotrichia leucophrys pugetensis, were selective in their choice of tutor models and what factors influenced how accurately they imitated tutors’ songs. We first examined two strategies for tutor choice: whether pupils have a conformity bias and/or a preference for high-quality tutors. In keeping with a conformity bias, tutors that sang song types that were relatively common within a radius of about 500 m of their territory were more likely to be imitated than were tutors that sang rarer song types. Most potential tutors were not imitated by pupils. Aspects of tutor quality, such as age, pairing status, and survival to the next year had no effect on whether a tutor’s song was imitated. Secondly, we tested whether pupil repertoire size, pupil quality, and local abundance of tutor models affected the accuracy of song imitations. We found a trade-off between repertoire size and tutor imitation accuracy with males that sang two or more song types developing significantly poorer imitations than males that sang one type. We discuss possible functions of a conformity learning strategy and factors that could produce a trade-off between imitation accuracy and repertoire size.