Relative threat and recognition ability in the responses of tropical mockingbirds to song playback

It has been suggested that individual recognition based on song may be constrained by repertoire size in songbirds with very large song repertoires. This hypothesis has been difficult to test because there are few studies on species with very large repertoires and because traditional experiments based on the dear enemy effect do not provide evidence against recognition. The tropical mockingbird, Mimus gilvus, is a cooperative breeder with very large song repertoires and stable territorial neighbourhoods. The social system of this species allowed us to test individual recognition based on song independently from the dear enemy effect by evaluating male response to playback of strangers, neighbours (from shared and unshared boundaries), co-males (i.e. other males in the same social group) and own songs. Although subjects did not show a dear enemy effect, they were less aggressive to co-males than to all other singers. Our results suggest that recognition in tropical mockingbirds (1) does not simply distinguish between familiar and unfamiliar singers, (2) requires a small sample of both songs and song types, (3) does not rely on individual-specific sequences of song types and (4) is not likely to rely on group-specific vocal signatures potentially available in cooperatively breeding groups. We conclude that this is a case of true recognition and suggest that the lack of a dear enemy effect in this and other species with large repertoires may relate to the role of song in mate attraction and the perception of neighbours as a threat to future paternity.     

Interspecific Differences in the Response of Female Birds to Song Repertoires

The functional significance of the wide variation between bird species in the sizes of individual song repertoires is not understood. We have studied the effects of song repertoires on females. Song triggers copulation solicitation display in female sparrows treated with estradiol. Song sparrow males (Melospiza melodia) have repertoires of about 10 song types, and female song sparrows display significantly more to presentations of large repertoires than to a single song type. By contrast, male field sparrows (Spizella pusilla) and white-throated sparrows (Zonotrichia albicollis) each have only one song type, and their females show no significant increment in responsiveness to repertoires of several song types over a single song type. Swamp sparrows (Melospiza georgiana) fall in between, with male repertoires of three song types. Female swamp sparrows behave in intermediate fashion, responding more to several song types than to one, but the response increment is less than in song sparrows. Thus species differences that males exhibit in song repertoires are paralleled by differences in female responsiveness to multiple song types, implicating variations in female reactivity in the evolution of song repertoires. Female song sparrows respond preferentially to repertoires programmed in eventual variety rather than immediate variety, while field sparrows and white-throated sparrows show no discrimination.     

Habituation,Recovery and the Similarity of Song Types within Repertoires in Red-winged Blackbirds (Agelaius phoeniceus) (Aves,Emberizidae)

Male red-winged blackbirds (Agelaius phoeniceus) respond to playback of conspecific song on their territories with the song spread, a graded aggressive display in which males extend their wings to expose their red epaulets while singing. We show that the intensity of song spread display declines with repeated presentation of one song type, and recovers when song types are switched. Recovery is greater for switches between song types that are acoustically dissimilar than for switches between song types that are acoustically similar. Recovery is no different for switches between two song types taken from the repertoires of different males than for switches between song types recorded from the same male. Analysis of acoustic features also indicates that song types recorded from different males are not more dissimilar than are song types from the same male. Our results do not support the idea that repertoires of red-winged blackbirds are composed of similar song types in order to facilitate individual recognition. Rather, repertoires may be constructed of dissimilar song types, so as to help maintain the response of listeners despite habituation.     

Are northern mockingbirds classic open-ended song learners?

The northern mockingbird (Mimus polyglottos, Linnaeus) is considered a classic example of a species in which individuals learn songs throughout adulthood, but this assumption has not been tested rigorously. To assess whether mockingbirds should be categorized as open-ended learners, I conducted a longitudinal study and a field-based song-tutoring experiment. I recorded songs from 15 free-living, banded, adult males in an earlier year and a later year, and I classified 400 mimetic songs per year per individual, based on the species and vocalization type mimicked. For two of these “mimetic types,” I further classified all the song types of all individuals in their early and later samples. The number of song types increased significantly across years for both mimetic types, and the average number of observed mimetic types per individual was 46.4 and 47.7 in the early and late samples (p = .055). I found no evidence adults learned any tutor stimuli after six months of tutoring, but examples from the scientific literature suggest the tutoring regime might not have been adequate to pass the motivational threshold required for learning. I conclude that mockingbirds probably are open-ended learners, but that future research is needed to verify experimentally that adults can indeed imitate novel song types.     

Northern Mockingbirds produce syntactical patterns of vocal mimicry that reflect taxonomy of imitated species

ABSTRACT Detailed studies of the patterns and processes associated with avian vocal mimicry are rare. Some vocal mimics, such as Northern Mockingbirds (Mimus polyglottos), often produce song types in rapid succession, but no published data exist concerning the syntactical organization of this behavior. We follow up on an undocumented assertion from the literature that mockingbirds cluster temporally their imitations of the same species. We examined long bouts of singing by 18 male mockingbirds and classified all songs as either mockingbird‐specific song or one of 55 mimetic types defined by the species mimicked and the song or call type of that species. Temporal pairs consisting of two exemplars of a single mimetic type were found at 2.6 times the level predicted by ordering songs by chance. Temporal pairs consisting of two mimetic types from the same species were found at 4.2 times chance levels. Temporal pairs consisting of mimetic types from two different species in the same family were found at 1.7 times chance levels, though this pattern was just above the significance threshold. We examined two functional hypotheses to explain these patterns, one involving female preferences and one involving interspecies communication, but found no support for either hypothesis. Detailed field observations of the social contexts in which temporal pairs are used by mockingbirds will be needed to better understand their function.     

Adult Song Sparrows do not Alter their Song Repertoires

To understand fully the function of vocal learning, it is important to know when, during an individual's lifetime, learning occurs. Songbirds are generally categorized into two groups with respect to their adult song learning ability. 'Open-ended' song learners are able to learn to produce new songs in adulthood, whereas 'age-limited' song learners can only acquire songs during their first year of life. Researchers have long assumed that certain oscine species are open-ended or age-limited song learners, but the evidence to date has been inadequate to test these assumptions for most species. We tested the hypothesis that song sparrows ( Melospiza melodia ) are age-limited song learners who do not alter their song repertoires in adulthood by examining the song repertoires of 24 color-banded males who were fully recorded in two, three or four different years. We compared sonagrams of the song types produced by males in different years and looked for any changes in repertoire composition (i.e. added or dropped song types). With few exceptions, males produced song repertoires that were identical in every year they were recorded. The exceptions (four males who did not produce one of their song types during one recording session) were all cases in which we believe that we missed recording a song type that a male did indeed have, not that the males dropped a song type. The finding that adult males do not alter the composition of their song repertoires provides strong evidence that song sparrows are age-limited song learners. Although it is possible that song sparrows make subtle within-song type changes across years, such changes would not necessarily constitute new song learning.     

Vocal Plasticity – are Pied Flycatchers,Ficedula Hypoleuca,Open‐Ended Learners?

In some songbird species, large song repertoires are advantageous in female attraction, whereas song sharing with neighbours may give an advantage in male–male competition. Open‐ended learners, with the ability to memorize new song elements throughout their lives, may learn from territorial neighbours and thus benefit from increasing both repertoire size and song sharing. A distinction needs to be made between true adult song learning, i.e. memorization of novel song elements, and vocal plasticity resulting in changes in the use of previously memorized elements, such as the use of hidden repertoires or increased production of previously rare syllable types. We assessed the ability of adult pied flycatcher Ficedula hypoleuca males to learn previously unheard song elements and to change their song production in response to playback of unfamiliar, conspecific song, emulating a singing neighbour. After a 1‐week playback treatment, three out of 20 subjects had learned foreign song elements, providing evidence from the wild that pied flycatchers are true open‐ended learners. However, the syllable sharing with the playback stimulus repertoires had not changed, and the males’ repertoires had decreased rather than increased. Hence, we did not find support for increased syllable sharing with neighbours or increased repertoire size as functions of adult song learning in pied flycatchers. Because pied flycatcher song seems to serve mainly for mate attraction, copying of attractive syllable types is a possible alternative function of adult song learning in this species.     

Seasonal patterns of vocal mimicry in northern mockingbirds Mimus polyglottos

Many aspects of birdsong vary seasonally, but we know almost nothing about seasonal variation in vocal mimicry, a conspicuous feature of the songs of 15–20% of bird species. I sampled spontaneous song from nine, free‐living, male northern mockingbirds Mimus polyglottos four times during the calendar year – three times during the breeding season, and once during the non‐breeding season. Results showed that mockingbirds did not mimic summer migrants significantly more during any sampling period. Results also showed significant differences across sampling periods in the frequency, but not diversity, of mimicry. Frequency of mimicry was highest in late spring (61.3%) and lowest during the non‐breeding season (48.3%), suggesting mimicry might play a role in reproductive stimulation of multiple‐brooded females. Because mockingbirds mimicked summer migrants throughout the year, regardless of whether migrants were present, mimicry is unlikely to facilitate interspecies communication. Seasonal patterns also suggest that females might be attracted to a high frequency of mimicry, but not to a high diversity of mimicry. I argue that observational studies of seasonal variation in additional mimics could provide key insights into the functional role of vocal mimicry.     

Tuning towards tomorrow? Common nightingales Luscinia megarhynchos change and increase their song repertoires from the first to the second breeding season

In many oscines, song repertoire size correlates with male quality and female mate choice, and can vary with age. In a cross-sectional field study in common nightingales Luscinia megarhynchos , one y old birds had substantially smaller repertoires than did older ones. In laboratory experiments males can acquire new song types during this period. This longitudinal field study therefore investigates whether individual nightingales increase their repertoires from the first to the second breeding season. We report a striking repertoire turnover, with an average overall increase of 24% of the first season's repertoire, resulting from added and dropped song types (54% and 30%, respectively). The number of added song types correlated positively with the size of the first season's repertoire. These results are consistent with the notion that repertoire size in nightingales correlates with male quality, although the overlap between repertoire sizes of first and second season birds makes it impossible to discriminate age based solely on repertoire size. Comparing the number of song types an individual sang in both seasons ('permanent song types') revealed a lower overlap than reported for subsequent seasons. The frequencies with which these were sung in the first season were less predictive of how often they were sung in the second season than was the case between later years. This drastic repertoire turnover from the first to the second season may be a selective process in response to the local song types, constrained by genetic makeup and shaped by early experience.     

Song Sharing in Neighboring and Non-Neighboring Thrush Nightingales (Luscinia luscinia) and its Implications for Communication

In many passerines, males have repertoires of different songs of which some songs are often shared with other males. Sharing of song repertoires among males can provide insights into the context in which songs were acquired and on the role of song repertoires in inter- and intrasexual communication. Here we studied repertoire sharing in male territorial thrush nightingales ( Luscinia luscinia ). We compared male vocal repertoires of the basic song components, full songs, and the sequencing of songs in a bout. The results show that males differed significantly in the size of their song repertoires but not in the size of the repertoire of basic song components. Moreover, they shared almost all (80%) the repertoire of song components but only 30% of their song types. Neighboring males shared significantly more song types than did non-neighboring males but interestingly they did not share more basic song components than non-neighboring males. These results show that the repertoire of basic song components is under much less sexual selection than the size of song repertoires. Sharing of song repertoires among neighbors presumably results from repertoire conversion over time and from males returning to their territories in the following season. Repertoire sharing then can be an indicator of territory tenure and thus it can be important in repelling rivals and in female choice.     

Seasonal Variation in Female Response to Male Calling Song in the Field Cricket, Gryllus rubens

Conspicuous traits that make males attractive to females may make them vulnerable to predators. Females that approach conspicuous males may increase their risk of predation. This means that selection for reduced male conspicuousness in the presence of predators may be due to sexual selection resulting from altered female behavior in the face of increased predator risk. We examine this hypothesis in the field cricket, Gryllus rubens, in which male calling song attracts both conspecific females for mating and parasitoid flies (Ormia ochracea) which kill their hosts within a week. Female crickets are also parasitized by these flies as a result of associating with calling males. In northern Florida crickets that emerge in the spring are not subject to fly parasitism whereas autumn crickets encounter large numbers of flies. We predicted that autumn females should be less attracted to male song than spring females. We tested female response to male calls in a rectangular arena in which male calling song was broadcast from a speaker. Spring females readily approached the speaker but autumn females were less likely to approach and remain in the vicinity of the speaker. These results emphasize the importance of considering how risk affects the evolution of conspicuous male behavior both directly through its effect on the male and indirectly through its effect on female responses to males.     

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.     

Element repertoire: change and development with age in Whitethroat Sylvia communis song

Song repertoires are often important determining factors in sexual selection. In several species, older males have larger repertoires than 1-year-old males. The development of large song repertoires by an individual is, however, poorly understood. We studied song element repertoire changes in five individual male Whitethroats Sylvia communis sampled as 1- and 2-year olds. These males increased the size of their element repertoire between their first and second year, but song length and number of different elements per song did not change. On average, 44.3% of the song elements in the first-year repertoire were also found in the second-year repertoire. Elements shared between years were found earlier in the songs and tended to occur in sequences. Sequences of shared elements also seemed to be conserved between years. The study suggests that the song element repertoire of the second year is partly based on the first-year repertoire, which may explain why large song repertoires are mainly expressed by males at least 2 years of age. It would appear, therefore, that song element repertoire size could be a reliable signal of male age.     

Conventional and network analyses of song organization and complexity in northern House Wrens (Troglodytes aedon parkmanii)

Among songbirds with large song-type repertoires, there may be functional variation in how individuals organize and display song-type diversity over time. Past studies focusing on conventional measures of song production have been extremely productive. However, network analysis offers a novel set of tools to quantify additional, previously unstudied elements of song organization and display. We studied protracted bouts of singing by 10 male House Wrens (Troglodytes aedon) to (1) test functional hypotheses of variation in song diversity in this species, and (2) evaluate the utility of network metrics in such research. Our analysis included a variety of conventional measures of song production and several standard metrics from network theory to quantify how variably the many song types in a male’s repertoire could be connected to one another and the limitations or diversity of their song sequences. Analysis of conventional variables showed that males produced more and longer songs, at shorter intervals, containing more syllables and more different syllable types, and also more different song types, prior to than after pairing and early in the morning regardless of breeding stage. These results are consistent with the hypothesis that song diversity functions in mate attraction and possibly in territory signaling. In contrast, analyses of network metrics revealed variety in song sequencing by males, but comparatively few and weak effects associated with either breeding stage or time of day. Overall, most song types connected to only a few others and a relatively small proportion of all possible song-type transitions actually occurred. Hence, much of the variety in song sequencing that was possible with the large song repertoires of males was not realized. The latter outcomes, brought to light via network analyses, highlight an important paradox for future research on this and related species with large song repertoires.     

Rufous‐capped Warblers Basileuterus rufifrons show seasonal,temporal and annual variation in song use

In the majority of songbird species, males have repertoires of multiple song types used for mate attraction and territory defence. The wood‐warblers (family Parulidae) are a diverse family of songbirds in which males of many migratory species use different song types or patterns of song delivery (known as ‘singing modes’) depending on context. The vocal behaviour of most tropical resident warblers remains undescribed, although these species differ ecologically and behaviourally from migratory species, and may therefore differ in their vocal behaviour. We test whether male Rufous‐capped Warblers Basileuterus rufifrons use distinct singing modes by examining song structure and context‐dependent variation in their songs. We recorded multiple song bouts from 50 male Warblers in a Costa Rican population over 3 years to describe seasonal, diel and annual variation in song structure and vocal behaviour. We found that Rufous‐capped Warbler songs are complex, with many syllable types shared both within and between males’ repertoires. Males varied their song output depending on context: they sang long songs at a high rate at dawn and during the breeding season, and shortened songs in the presence of a vocalizing female mate. Unlike many migratory species, Rufous‐capped Warblers do not appear to have different singing modes; they did not change the song variants used or the pattern of song delivery according to time of day, season or female vocal activity. Our research provides the first detailed vocal analysis of any Basileuterus warbler species, and enhances our understanding of the evolution of repertoire specialization in tropical resident songbirds.     

Singing the Popular Songs? Nightingales Share More Song Types with Their Breeding Population in Their Second Season than in Their First

Signals used in communication often change throughout an individual’s life course. For example, in many song bird species, males modify their song especially between their first and second breeding season. To address one possible reason of such modification, we investigated whether common nightingales Luscinia megarhynchos adjust their song type repertoires to sing the song types commonly occurring in their breeding population. We analysed nocturnal singing of six nightingales in their first and second breeding season and compared their repertoire composition and usage to the ‘typical’ repertoire and usage on the breeding ground (represented by seven reference birds). Songs that were maintained between the first and second season by the six focal birds occurred in most of the repertoires of the seven reference birds and were sung often. In contrast, song types that were dropped from the repertoires occurred less often in the reference birds’ repertoires and were sung less often. Furthermore, in the first year, each focal nightingale’s repertoire was less similar to the reference birds’ repertoires than in the second year. Thus, nightingales adjusted their singing towards the songs popular in the breeding grounds by keeping song types that were common and frequently sung by other individuals in their breeding area and by disposing of infrequently performed ones. This resulted in increased similarity with the population’s repertoire from the first to the second year. We discuss possible ontogenetic processes that may lead to such an adjustment and suggest an improved ability to match song types as possible adaptive value.     

Social Guidance of Vocal Learning by Female Cowbirds: Validating its Functional Significance

Juvenile male M. a. ater cowbirds, who have never heard other male cowbirds sing, develop distinctively different repertoires when housed with M. a. ater females from their own area versus M. a. obscurus females from a distant population. Because female cowbirds do not sing, the differences in the males' songs do not arise through vocal imitation. Here we provide data demonstrating that the songs of female-housed males are functionally, as well as acoustically, distinctive. The songs of 8 groups of males were tested where the groups differed by age of singer, acoustic experience, and identity of social companion. The playback results demonstrate that non-singing female cowbirds not only stimulate the male to modify song content, but song potency. As such, they demonstrate the critical role female cowbirds may assume in the proximate and ultimate regulation of vocal development.     

Female marsh wrens do not provide evidence of anatomical specializations of song nuclei for perception of male song

In songbirds the forebrain nuclei HVC (high vocal center) and RA (robust nucleus of the archistriatum) are larger in individuals or species that produce larger song repertoires, but the extent to which the size of these nuclei reflects a need for either producing or perceiving large repertoires is unknown. We, therefore, tested the hypothesis that species differences in the size of song nuclei reflect a commitment of “brain space” to the perceptual processing of conspecific song. The two species of marsh wren (Cistothorus palustris western and eastern) provide a good test case. Western males produce larger song repertoires, and have larger HVC and RA than do eastern males. Female marsh wrens do not sing, and if they use their song nuclei to assess conspecific male song repertoires, then we predicted that measurable cellular and nuclear parameters of HVC and RA would be greater in western than eastern female wrens. For males we confirmed that the volumes of HVC and RA, and cellular parameters of HVC, are greater in western than in eastern birds. These nuclei were also considerably larger in males than in conspecific females. Western and eastern female wrens, however, did not differ in any measured parameters of HVC or RA. Females of these wren species thus do not provide any direct evidence of anatomical specializations of song nuclei for the perceptual processing of conspecific male song. 1994 John Wiley & Sons, Inc.     

Sexual Differences in the Responses of Captive Great Reed Warblers (Acrocephalus arundinaceus) to Variation in Song Structure and Repertoire Size

The function and evolution of repertoires in songbirds is still not clearly understood. This study involves the great reed warbler Acrocephalus arundinaceus and playback of recorded song to both captive males and females under controlled laboratory conditions. Females were implanted with oestradiol and the copulation solicitation display used as an index of sexual response. Males were not implanted and crest-raising used as an index of aggressive response. Two types of songs were used (short and long) and two sizes of repertoire (high and low). Females only responded to long songs, and responded significantly more to high than low repertoires. It was concluded that long songs have a sexual function and that high repertoires are the result of intersexual selection through female choice. Males responded to short and long songs, but did not discriminate between high and low repertoires. Adding a live male to the experiment made no difference to female responses but males responded significantly more. When field studies are also taken into consideration the overall conclusion is that song quality and repertoire size is particularly important in female choice, and that short songs and a male presence are more important in aggressive behaviour. Integration of both field and laboratory studies on males and females should lead to more rigorous testing of hypotheses concerning the function and evolution of bird songs.