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.     

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.     

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.     

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.     

Singing onstage: female and male common nightingales eavesdrop on song type matching

In bird communication, listening individuals may obtain information on the quality and motivation of a male not only from solo-singing, but also from song interactions and listeners base their future decisions in territorial and mating contexts on such public information. Eavesdropping on male interactions may thus have a strong influence on sexual selection. In singing interactions, temporal coordination (e.g. overlapping vs. alternating) of two singers as well as structural interaction patterns (e.g. song type matching or repertoire matching) have been described, but the latter is far less studied. By conducting dual-speaker playback experiments with common nightingales Luscinia megarhynchos, we simulated an interaction where one singer was repeatedly song–type matching his counterpart. Playbacks were broadcast to male and female nightingales, and their approach behaviour and singing responses (in the case of male focals) were analysed. We found that both, males and females, spent more time with the matched bird, whereas males additionally sang more songs towards the matching bird. This can be taken as strong hint that eavesdropping occurs in nightingale communication and that listening to male vocal contests might be an important strategy for both sexes to adjust their behavioural output. With regard to the function of song matching, we assume that song-matching is not an aggressive signal per se in nightingales. We rather conclude that vocal leaders within an interaction, here the matched bird, may elicit stronger responses in conspecifics than vocal followers, here the matching bird.     

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.     

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.     

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.     

Pitch shifts and song structure indicate male quality in the dawn chorus of black-capped chickadees

The fee-bee song of male black-capped chickadees (Poecile atricapillus) is considered a single-type song that singers transpose up and down a continuous frequency range. While the ability to shift song pitch in this species provides a mechanism for song matching as an aversive signal in male-male territorial song contests, the functional significance of this behaviour during the solo performances of males during the dawn chorus is unclear. We analysed the dawn chorus songs and singing behaviour of males whose winter-flock dominance status we determined. We used correlation analysis to show that pitch shifts were accompanied by changes to other fine structural characteristics in song, including temporal and relative amplitude parameters. We also found that songs of socially dominant males and songs of their most subordinate flockmates could be distinguished using these methods by the way they performed a between-note frequency measure accompanying pitch shifts. That is, a ratio measure of the internote frequency interval remained constant for songs of high-ranking birds despite changes in absolute pitch, while low-ranking males sang a smaller ratio as they shifted to higher absolute pitches. These findings identify previously unrecognised variation in the songs of black-capped chickadees. More importantly, they indicate a mechanism by which pitch shifting during the dawn chorus of black-capped chickadees could provide a reliable indicator of relative male quality.Communicated by I. Hartley     

Song length variation serves multiple functions in the white-crowned sparrow

A wide variety of hypotheses has been proposed to explain the structural diversity in bird song repertoires. Song diversity is frequently described in terms of song “types” or within-type “variants.” Male Puget Sound white-crowned sparrows, Zonotrichia leucophrys pugetensis, produce variants of their single adult song type by altering the number of repetitions of syllables in the terminal trill. We tested whether variation in trill length correlated with distance to the receiver and with signaling context as predicted by the eavesdropping avoidance hypothesis and the strategic signaling hypothesis. In accordance with the eavesdropping avoidance hypothesis, males sang variants with shorter trills, and sang quieter and less frequently when near their mate during the incubation phase than during spontaneous singing while unpaired. Males also sang variants with short trills, but at a high rate and variable amplitude when within 10 m of an opponent during close male–male territorial interactions. In agreement with the strategic signaling hypothesis, males decreased trill length immediately before chasing an opponent, but did not change length consistently prior to flight. We conclude that the occurrence of short quiet songs sung near the mate agrees with predictions of the eavesdropping avoidance hypothesis, while short songs sung near other males are best explained by the strategic signaling hypothesis. Trill length variation may be a conventional signal of aggressive intentions in male–male contests stabilized by receiver-imposed retaliation costs.     

Changes in singing behavior of male black-capped chickadees (Parus atricapillus) following mate removal

We removed the mates of ten male black-capped chickadees (Pares atricapillus) during the nest-building period to determine the effect of female presence on dawn singing. During the first dawn chorus following mate removal, males sang significantly longer, increased movement within their territory, and increased the percentage of their territory covered while singing. After the female was returned, these parameters returned to the pre-removal values. Males did not alter the frequency range or modal frequency of their songs when the mate was removed, nor did they change the degree of frequency shifting in the fee-bee song. We conclude that dawn singing in the black-capped chickadee acts, in part, as an intersexual signal, and that the behavior of frequency shifting in the song may be directed more toward rival males than females. Correspondence to: K. Otter     

Song sharing correlates with lifetime social pairing success but not territory tenure in the Puget Sound white-crowned sparrow

Imitative song learning in birds often results in the formation of vocal dialects or local song neighborhoods in which males in close proximity share very similar songs. If song sharing confers a selective advantage due to intra-sexual competition or inter-sexual mate choice, song sharing should enhance the singer’s territory tenure or lifetime pairing success. We tested this in a migratory population of the Puget Sound white-crowned sparrow, Zonotrichia leucophrys pugetensis, in which some territory neighbors share whole songs. Song sharing with territory neighbors in the first year on territory did not correlate with territory tenure but correlated positively with social pairing success throughout a male’s lifetime when length of territory tenure was controlled. Thus, song sharing in this migratory population does not appear to confer a benefit in male–male interactions, at least as measured by the length of territory tenure. However, males that share songs do benefit from higher lifetime pairing success. Combined with the data available for other species, it appears that intra- and inter-sexual selection may have stronger effects on song sharing in sedentary and migratory populations, respectively.     

Choice of singing sites by male bushcrickets (Tettigonia viridissima) in relation to signal propagation

Summary The spatial dispersion of singing male bushcrickets (Tettigonia viridissima) in a marshland habitat was found to be significantly clumped. Males clustered in patches of taller vegetation, buth within such clusters, males were regularly spaced with a mean distance of 6 m between nearest-neighbours. Males occupied perches on plants that were, on average, 0.3 m above the top of surrounding vegetation. Physical and acoustical interactions between males were observed more frequently when males were singing from higher sites. Excess attenuation of the male song was found to increase with frequency but decreased markedly with increasing elevation of the singing male above the ground. The maximum detection range of the song, realized when the insect was singing > 1 m above the surrounding vegetation, was estimated as 60 m for the fundamental frequency (10 kHz), 38 m for the 1st harmonic (20 kHz) and 14 m for the second harmonic (30 kHz). By contrast, when males sang from the middle of dense reed beds, the estimated detection distance was only 8 m, 6 m and 4 m for each frequency band, respectively. Males could have increased the detection range of their songs almost three fold by singing from higher positions than those usually observed in the field. This suggests that there may be a cost of singing at higher elevations such as an increased risk of predation and/or increased aggression from neighbours. We suggest the spacing strategy adopted by males reflects a compromise between maximizing the range over which their songs can be detected and accurately localized by females and minimizing interference from competing males. Offprint requests to: T. Eiriksson     

Ranging of songs with the song type on use of different cues in Carolina wrens: effects of familiarity

Male territorial song birds are usually spaced far apart and most often hear conspecific song after it has been degraded by propagation through the environment. Their ability to use the degradation of songs to assess the distance of a singing rival without approaching (called ranging) presumably increases the efficiency of defending a territory. In order to assess degradation in a song the receiver needs to compare the characteristics of the received song to its characteristics at the source or at different distances. Earlier experiments on ranging in species with song repertoires have suggested that prior familiarity with the particular song type is necessary for ranging. Here I show that male Carolina wrens (Thryothorus ludovicianus) can use either temporal or spectral characteristics for ranging song types which they were unlikely to have heard previously. Playbacks consisting of only one song prevented subjects' close-range experience with the loudspeaker, and flights beyond the loudspeaker provided direct evidence for over-assessment of distance when songs were degraded. Because ranging of songs was not affected by the degree of familiarity with the song type, this experiment provides no evidence that song repertoires hinder ranging in Carolina wrens, as suggested by Morton's ranging hypothesis. Instead, at least approximate ranging of songs is evidently possible by assessment of degradation in general features of a species' songs. Received: 9 August 1996 / Accepted after revision: 15 February 1997     

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.     

Singing activity stimulates partner reproductive investment rather than increasing paternity success in zebra finches

Song is used as a signal in sexual selection in a wide range of taxa. In birds, males of many species continue to sing after pair formation. It has been suggested that a high song output after pair formation might serve to attract extra-pair females and to minimise their own partner’s interest in extra-pair copulations. A non-exclusive alternative function that has received only scant attention is that the amount of song might stimulate the own female’s investment into eggs in a quantitative way. We address these hypotheses in a captive population of zebra finches, Taeniopygia guttata, by relating male undirected song output (i.e. non-courtship song) to male egg siring success and female reproductive investment in two different set-ups. When allowed to breed in aviaries, males with the highest song output were no more attractive than others to females in an analysis of 4,294 extra-pair courtships involving 164 different males, and they also did not sire more offspring (both trends were against the expectation). When breeding in cages with two different partners subsequently, females produced larger eggs with more orange yolks when paired to a male with a high song output. These findings suggest that singing activity in paired zebra finch males might primarily function to stimulate the partner and not to attract extra-pair females.     

Reproductive strategy and singing activity: blue tit and great tit compared

The costs and benefits of bird song are likely to vary among species, and different singing patterns may reflect differences in reproductive strategies. We compared temporal patterns of singing activity in two songbird species, the blue tit (Cyanistes caeruleus) and the great tit (Parus major). The two species live side by side year round, and they have similar breeding ecology and similar rates of extra-pair paternity. However, they differ in two aspects of reproductive strategy that may have an influence on song output: blue tits are facultatively polygynous and have a fairly short breeding season with almost no second broods, whereas great tits are socially monogamous but more commonly raise second broods. We found that great tit males continued singing at high levels during the egg-laying and incubation periods, while monogamously paired blue tit males strongly reduced singing activity after the first days of egg-laying by their female. Since males of both species sang much more intensely shortly before sunrise than after sunrise, at midday or in the evening, this difference was most conspicuous at dawn. No differences in singing activity were found within species when testing for male age. We suggest that in contrast to blue tits, great tit males continued singing after egg-laying to defend the territory and to encourage the female for a possible second brood.     

Energetic state and the performance of dawn chorus in silvereyes (Zosterops lateralis)

Stochastic dynamic programming (SDP) models predict that males singing to attract a mate should concentrate singing in what has been termed the dawn chorus. This is because male birds should have a variable surplus of fat in the morning that can be used to fuel singing, with the amount of fat available dependent upon such factors as his quality, foraging success and risk of predation. In this manner, the dawn chorus can act as an indicator of male quality in the context of female mate choice. We test a key prediction of SDP models of singing behaviour that males with greater fat levels should sing more. We conducted an experiment where we recorded the dawn chorus of male silvereyes (Zosterops lateralis) on three consecutive days. Each male received supplementary food on the second day, which enabled us to sample his dawn chorus before, during and after food supplementation. We also collected data on the effect of supplementary food on the body mass of silvereyes. As predicted by SDP models, we found that silvereyes sang for a greater proportion of the time after receiving supplementary food. Supplementary food also had a significant effect on the complexity of a male song, indicating that males not only increased the quantity of their song but also the quality of their song when they received extra food. As the provision of supplementary food significantly increased the mass of fed birds, our results support a causal link between male energy reserves and his ability to perform the dawn chorus.     

No evidence for acoustic mate-guarding in duetting buff-breasted wrens (Thryothorus leucotis)

There are few empirical tests of the acoustic mate-guarding hypothesis for the function of duetting in birds. This hypothesis states that when females are fertile, males initiate many songs or answer most of their mates solo songs to form duets and repel rival males seeking extra-pair copulations. We tested the hypothesis by comparing song initiation and answer rates of males and females in socially monogamous buff-breasted wrens (Throthorus leucotis) during pre-fertile and fertile periods. During pre-fertile periods, males often sang for short periods before being answered by their mates, yet first duets were formed earlier relative to dawn and more duets were given during the dawn chorus on pre-fertile than fertile mornings. Males initiated more songs during pre-fertile than fertile periods, whereas there was no difference between stages in female song initiation rates. The proportion of songs answered by individuals of both sexes did not differ between breeding stages. Other mate-guarding behaviours, such as frequent copulation and maintaining close proximity to mates when fertile, did not appear to be important in this species, as no copulations were observed and there was no difference in the time pairs spent in close proximity when females were fertile or not. Parentage analysis revealed that only 3% of 31 broods had young that were likely the result of extra-pair paternity. These findings do not support the acoustic mate-guarding hypothesis, and suggest that the low rate of extra-pair paternity in buff-breasted wrens was maintained without the use of acoustic or traditional paternity guards.Communicated by R. Gibson     

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.