Songs of isolation-reared sparrows function in communication,but are significantly less effective than learned songs

Summary Songs of male song sparrows (Melospiza melodia) and swamp sparrows (M. georgiana) isolated before hatching from conspecific song were played to males and females of both species. Isolate songs of these sparrows resemble natural conspecific song in several aspects of gross structure, but differ from natural song in note structure. Male territory owners of both species responded more to conspecific isolate song than to heterospecific normal song. Captive females of both species, previously treated with estradiol, courted in response to isolate song but not to heterospecific song. We conclude that there is sufficient speciesspecific information in isolate song of both species to allow a degree of normal function. To assess the importance of the structural refinements added through learning, we compared response to isolate and natural conspecific songs. Male territory owners and captive females responded more to natural than to isolate songs in both species. Learning thus significantly increases the potency of song in both intersexual and intrasexual communication.     

Effects of early auditory experience on the development of local song preference in female swamp sparrows

A growing number of studies show that learning about male mating signals can shape the way females discriminate among males and may influence the evolution of both female preferences and the male traits under selection. Female songbirds commonly prefer local songs over foreign songs from a different population. In some species, however, the extent of variation among songs sung by different males within a population is as great as the variation observed between populations, raising the question of how females are able to discriminate local from foreign songs. Here, we report that laboratory-reared female swamp sparrows (Melospiza georgiana) not only show a preference for the particular song types with which they were tutored as compared with both foreign songs and unfamiliar local songs but also show preference for unfamiliar local songs over unfamiliar foreign songs. An acoustic analysis comparing tutor songs and those presented as unfamiliar local and foreign songs suggests that female swamp sparrows might be attuned to the specifics of local note phonology when assessing the attractiveness of unfamiliar songs. Our results demonstrate that early auditory experience influences response to geographic song variation in female swamp sparrows, and suggest the possibility that female songbirds may generalize what they learn from songs early in life to novel songs heard in adulthood. Additional work is needed to evaluate the contribution of unlearned predispositions for local song.     

Female response to song reflects male developmental history in swamp sparrows

One explanation for why female songbirds attend to male song is that the quality of a male's song is associated with the quality of his developmental history. We tested this hypothesis by playing back to female swamp sparrows (Melospiza georgiana) songs recorded from males of either inferior or superior developmental histories, as assessed by their rates of mass gain during the first 18 days post-hatching. Females showed significantly higher levels of courtship display in response to songs of males with superior growth than to songs of males with inferior growth. Out of nine song traits measured, only song duration correlated with variation in female response; duration was also the only trait that differed significantly in univariate comparisons between the superior growth songs and the inferior growth songs. In a multivariate analysis, however, inferior growth songs were best discriminated from superior growth songs by combining three song traits: trill rate, stereotypy, and the number of notes per syllable. We suggest that early developmental stress degrades song in many small ways, and that it is the cumulative effect of the resulting deficits that explains lower female response.     

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.     

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.     

Recognition of individual neighbors by song in the song sparrow,a species with song repertoires

Summary Previous theory and research have suggested that bird species with song repertoires in general, and song sparrows (Melospiza melodia) in particular, cannot readily discriminate between the songs of neighbors and strangers. In a recent study (Stoddard et al. 1991) we showed that song sparrows can in fact discriminate neighbors from strangers on the basis of song. In this study we sought to demonstrate that song sparrows can make the finer discrimination between individual neighbors and that they can do so on the basis of a single song type. We compared the response of territorial males to song playback of neighbors and strangers at three locations: the neighbor's regular boundary, the opposite boundary, and the center of the territory. The birds showed strong neighbor-stranger discrimination at the regular boundary but not at the opposite boundary, nor in the center of the territory. The differences in song discrimination between different boundary locations indicate that song sparrows associate particular songs with particular territories, effectively discriminating between individual neighbors on the basis of song. Song repertoires themselves do not interfere with neighbor recognition to the extent originally postulated. As speakers are moved inside the territory from the border, however, the degree of discrimination diminishes. We believe that differences in speaker placement may have contributed to the variability in neighbor-stranger discrimination observed in previous studies of the song sparrow and perhaps other repertoire species as well. This interpretation is consistent with data from another song sparrow population showing that half the territory takeovers are by immediate neighbors.     

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.     

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.     

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     

Song-type sharing in song sparrows: implications for repertoire function and song learning

One hypothesis for the function of song repertoires is that males learn multiple song types so that they may share songs with neighbors, allowing them to match during territorial interactions. In at least one song sparrow population, in Washington, territorial males share a high proportion of song types with their neighbors and use these shared songs in matching. We recorded song sparrows in Pennsylvania and quantified sharing of whole songs and song segments. We found that song sharing is an order of magnitude less common in the Pennsylvania population. We found sharing of song segments to be significantly more common than the sharing of whole songs in three of the five fields we examined, while we found no significant differences between whole and partial song sharing in the remaining two fields. Finally, we found no evidence that sharing is greater between birds in the same field compared to birds in different fields. Taken with the data from Washington song sparrows, these results provide evidence for intraspecific geographic variation in the organization of song repertoires, and suggest that song sharing has not been a strong selective force in the evolution of song repertoires in song sparrows as a species. Furthermore, Washington and Pennsylvania song sparrows differ in how they learn song, in that Washington birds copy whole songs, while Pennsylvania birds appear to copy and recombine song segments, as has been found in laboratory studies of song learning. Thus both song learning and the function of song repertoires differ between populations of song sparrows. Such intraspecific geographic variation offers a unique opportunity to explore the ecological and historical factors which have influenced the evolution of song. Received: 30 June 1997 / Accepted after revision: 8 March 1998     

The behavioral regulation of monogamy in gibbons (Hylobates muelleri)

Summary Responses of mated individuals to playbacks of the songs of solitary females and males permit evaluating the behavioral mechanisms maintaining monogamy in gibbons. When female songs are played back from the centers of their ranges, mated female gibbons typically initiate duets and group approaches toward playback sites. Female songs played back from range boundary locations elicit duetting responses. Responses to solitary female songs do not differ from responses to song duets used by established mated pairs to mediate patterns of intergroup spacing. Mated males lead silent group approaches toward the sites of male song playbacks. These results suggest that range defense by female gibbons forces males into accepting monogamous mating relationships and that monogamy in gibbons is regulated by intersexually-supported, intrasexual aggression.     

Female and male serins (Serinus serinus) respond differently to derived song traits

We tested if male or female behavior towards manipulated song indicates intra- or inter-sexual selection of two characteristics of serin song that are extreme and evolutionarily derived in this species: high frequency and fast syllable rate. In a first experiment, we monitored vocal responses and attendance to song playbacks. Female behavior indicated a preference for high-frequency song and suggested an aggressive function for fast syllable rates, as fast songs inhibited vocal response. Males did not show discrimination of frequency or syllable rate with this experimental design. The second experiment used a simple approach/no approach design, and in this experiment, males showed stronger discrimination between stimuli than did females. Therefore, sex differences in discrimination appear not to result from differences in perceptual abilities but from differences in the context of stimulus presentation. The second experiment also supported a role of song frequency in female choice, as the effect of frequency was limited to females: males did not respond differently to song frequency and approached high-frequency songs less than females did. Results of this experiment also supported an aggressive function for fast syllable rates, as the effect of fast songs did extend to male behavior. Taken together, our results indicate that the high frequency and fast syllable rate of serin song cannot result from a single selection process: while high frequency may have evolved by inter-sexual selection, syllable rate provokes a pattern of response that is more consistent with intra-sexual selection.     

Sexual dimorphism in house sparrow eggs

Recent evidence has revealed an apparently high degree of control by female birds over the physiological aspects of their reproduction and offspring sex allocation, consistent with adaptive hypotheses of sex allocation and differential investment in their offspring. In the house sparrow, we investigated possible mechanisms that may be used by females to enhance the fitness returns from a reproductive effort. Using molecular techniques, we demonstrate that house sparrow eggs containing male embryos are significantly larger than those containing female embryos. We also found that male embryos were laid randomly with respect to laying order. We speculate that this sexual dimorphism of eggs is adaptive, because male house sparrows show greater variance in condition-dependent reproductive success than females. More important, the result provides further evidence of the ability of females to detect or control ovulation of either male or female ova and to differentially invest in one sex over the other. Received: 19 January 2000 / Revised: 29 June 2000 / Accepted 20 July 2000     

Response of territorial male song sparrows to song types and variations

Summary The song repertoires of male song sparrows (Melospiza melodia) are comprised of distinct song types and many overlapping variations within each type. This study examines the sensitivity of territorial male song sparrows to both multiple song types and variation within types. A playback experiment was conducted using stimulus tapes that had either single songs (but not variations), song variations (but not types) or a single song. Response during playback was similar for all three stimulus groups, but diminished sooner after playback for the single-song tape. Thus our results indicate that male song sparrows are sensitive to song variability both within and among song types.     

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.     

Do male plumage and song characteristics influence female off-territory forays and paternity in the hooded warbler?

Extra-pair fertilizations are common in many socially monogamous species, and paternity studies have indicated that females may use male vocal performance and plumage ornaments as cues to assess male quality. Female off-territory forays may represent a key component of female choice and male extra-pair mating success, and female foray behaviour is expected to be strongly influenced by indictors of male quality. In this study, we examined how male song and ornamentation affect how often females left their territories, which males they visited and extra-pair paternity in a socially monogamous passerine, the hooded warbler (Wilsonia citrina). We radiotracked 17 females during the fertile period and quantified male vocal performance (song output and rate) and plumage characteristics (size of the black melanin hood and colour of the black hood, yellow cheeks and breast areas). We obtained blood samples and determined paternity at 35 nests including those of 14 females that we radiotracked. Eleven (65%) of the 17 females forayed off-territory, whilst fertile and female foray rate was positively correlated with the number of extra-pair young in the nest. Females that left their territories more frequently were paired with males that sang at a low rate. In addition, extra-pair mates had higher song rates than the social mates they cuckolded (5.3 songs/min vs. 4.4 songs/min). Female off-territory forays or extra-pair paternity were not significantly related to male plumage characteristics. Our results indicate that a high song rate influences both the foray behaviour of a male’s social mate and the likelihood that he will sire extra-pair offspring with neighbouring females.     

Soft song is a reliable signal of aggressive intent in song sparrows

Animals frequently use signals to modulate aggressive interactions. Establishing that a signal is aggressive or threatening requires demonstrating that it is more commonly used in agonistic contexts, that it predicts subsequent aggressive behaviors by the sender, and that receivers respond differently to this signal. Like many birds, song sparrows (Melospiza melodia) produce a low-amplitude “soft song” vocalization that has been hypothesized to be an aggressive signal. Soft song meets the first two criteria, but previous research has failed to demonstrate that soft song provokes aggression or that receivers even perceive soft song differently from normal loud song. We used a playback experiment with taxidermic mount presentation to test whether territorial male song sparrows respond differently to loud and soft song playbacks. Subjects reacted more strongly to the soft song playback by approaching the mount more closely, increasing wing wave displays, and increasing the proportion of their own songs that were soft songs, with further trends toward increasing the number of flights and attacks. These results confirm that soft song is a conventional signal of aggression in song sparrows and that increased receiver retaliation maintains its reliability.     

Soft song in song sparrows: response of males and females to an enigmatic signal

Low-amplitude “soft song” is used by a variety of songbirds; in some species during aggressive encounters, in others during courtship, and yet others in both these contexts. In song sparrows (Melospiza melodia), soft song has thus far been observed only in aggressive encounters, where its production is a more reliable predictor of attack than any other signaling behavior. We used song playback to test the response of both male and female song sparrows to soft song. The design of the playback experiments took into account the existence of two classes of soft song: crystallized soft song, which consists of song types also found in the broadcast repertoire, and warbled soft song, which consists of less-structured song types not found in the broadcast repertoire. Female song sparrows responded with significantly less courtship display to the playback of crystallized soft song than to that of normal broadcast song, and response to warbled soft song was if anything lower than to that of crystallized soft song. Male song sparrows responded equally aggressively to normal broadcast song as to crystallized soft song, and equally aggressively to warbled soft song as to crystallized soft song. The female results support the conclusion that neither form of soft song functions in courtship. The male results suggest that the reliability of soft song as a signal of aggressive intent is not maintained by a receiver retaliation rule.     

Discrimination of vocal performance by male swamp sparrows

In aggressive communication, the interests of signalers and receivers are directly opposed, presenting a challenge to the maintenance of reliable signaling. Index signals, whose production is constrained by physical ability, offer one solution to the reliable signaling problem. Vocal performance, the ability to produce physically challenging songs, is likely such a signal in swamp sparrows. Maximum vocal performance varies between males and is correlated with aspects of quality. However, vocal performance can be modulated in aggressive contexts by increasing the frequency bandwidth and trill rate of songs. This study examines receiver response to (1) differences in performance of the same song types by different signalers and (2) individual modulation of performance between contexts. Results demonstrate that male receivers show differential response to between-male differences in song type performance, but do not show differential response to the smaller scale modulations of performance produced by individuals singing the same song type at different times. This pattern suggests that vocal performance cannot be effectively cheated and may therefore serve as a good example of an index signal.     

Female choice for complex song in the European starling: a field experiment

Male European starlings Sturnus vulgaris sing long complex songs that appear to be important in the courtship of females but which also influence competitive interactions between males. We tested the hypothesis that females choose mates on the basis of the complexity of their songs, rather than on the quality of the territories the males defended. In order to determine whether certain territories were preferred over others, the first set of birds to settle in the experimental nest-boxes was removed and a second set allowed to settle. Consistent preferences for certain nest-boxes were indicated by correlations between the settlement patterns of the first and second sets of birds. However, males with the most complex song did not necessarily occupy the most preferred nest sites. Males with more complex song acquired mates faster. This relationship remained significant when nest-site preference was statistically controlled, indicating that female starlings chose males with complex song rather than those that defended preferred nest sites. A number of morphological variables were also found to be uncorrelated with female choice. Song complexity in European starlings increases with age, and the evolution of song complexity in this species is consistent with an age-indicator model of sexual selection. Males with larger repertoires were also in better condition, indicating that females obtain high-quality mates by choosing on the basis of male song. Received: 29 April 1995/Accepted after revision: 9 September 1995