Song type variants and aggressive context

We asked whether switching among song type variants functions as a graded signal of aggression in song sparrows (Melospiza melodia). Song type variants are different versions of a given song type and are defined by much smaller acoustic differences than those defining separate song types. The principal evidence that switching between song types functions as an aggressive signal, in song sparrows and other species, is that song type switching frequencies increase during aggressive contexts, for example during simulated territorial intrusions. We tested whether variant switching frequencies change in the same way, using playback of conspecific song to simulate intrusions. Male song sparrows showed a significant increase in variant switching frequencies during intrusion relative to before intrusion. After intrusions, variant switching frequencies returned to pre-intrusion levels. These results are consistent with the hypothesis that variant switching is a graded aggressive signal, but other interpretations are possible, for example, that variant switching is a signal of submission or represents simple production error. Received: 20 January 2000 / Revised: 15 July 2000 / Accepted: 30 July 2000     

Responses of song sparrows to song-type matching via interactive playback

The aggressive and singing responses of ten territorial male song sparrows, Melospiza melodia, to sustained song-type matching via interactive playback were examined. This auditory stimulus, which involves both synchronous switching and song-type matching, was hypothesized to be a strong aggressive threat. Responses were compared to five other trial types in which the switching pattern and song-type similarity were independently varied. The six trial types were organized into a factorial design with three switching levels (no switching, synchronized switching, and rapid unsynchronized switching) and two matching levels (shared song-types and unshared song-types). The aggressive response of the birds, a composite index of the correlated behaviors of singing rate, perch change rate, and time spent close to the speaker, showed significant main effects of both the switching and the matching factors, with no interaction effect. The response was highest for the shared/unsynchronized switching trial and lowest for the unshared/no switching trial. Since the synchronous switching trials produced aggressive responses intermediate between the no switching and rapid unsynchronized switching trials, our results corroborate earlier studies showing that switching rate, song-type diversity and song similarity are the key determinants of aggressive approach. However, the switching rate of the focal birds during playback was not correlated with the other response variables and showed a very different pattern from the aggressive response. Switching rate increased during unshared/synchronized trials and decreased during shared/synchronized (i.e., song-type matching) trials. The tendency of the birds to refrain from switching when they were song-type matched by playback led to long bouts of matched-type countersinging. These results suggest that song-type matching locks two countersinging birds into a new level of interaction in which information other than aggressive motivation exchanged.     

Song amplitude and body size in birds

Bird song is a sexually selected multidimensional signal. A fundamental question regarding the evolution of sexually selected signals is what information they convey and how their honesty is maintained. Song amplitude is a performance-related signal trait that varies considerably between individuals, but this signal dimension has been neglected in past studies. I found that median song amplitude in male nightingales (Luscinia megarhynchos) and zebra finches (Taeniopygia guttata) did not vary significantly with body size or residual body mass. In contrast, I found a significant negative correlation between body size (and also residual mass) and the maximum song amplitude during interactive singing in nightingales. However, the function of these more subtle differences in song amplitude remains to be investigated. By and large, the results of this study suggest that mean song amplitude is unlikely to indicate a bird’s body size or current condition (measured as residual mass).     

Do male barn swallows (<Emphasis Type="Italic">Hirundo rustica</Emphasis>) experience a trade-off between the expression of multiple sexual signals?

Some sexual selection models envisage exaggerated male secondary sexual characters to be costly and therefore reliable indicators of the quality of potential mates to choosy females. If male secondary sexual characters have a natural selection cost, they may be linked to each other by reciprocally constraining relationships that would prevent individual males from increasing their level of multiple signaling. Barn swallow (Hirundo rustica) males have at least two costly signals relevant to socio-sexual interactions: tail length and song. Under the hypothesis that a trade-off exists between male signals, we manipulated the maintenance cost of tail ornaments to test whether this reduced the quantity and quality of song, a condition-dependent, phenotypically plastic signal. Contrary to our expectation, tail elongation had no effect on singing activity and song complexity. However, tail-elongated males produced songs with longer terminal parts ('rattles'). Long rattles are associated with highly competitive social contexts and high circulating levels of testosterone, suggesting that tail-elongated males were more frequently involved in either male-male aggressive or inter-sexual interactions. Therefore, this study shows that males are not displaying multiple signals at the maximum possible level, implying that this system is open to unreliable communication. However, long-term trade-offs between signal expression and viability may prevent males from displaying both signals at higher rates.Communicated by: M. Webster     

Delayed maturation of multiple signals in a migratory songbird

Many animals use multiple signals in sexual communication, but our understanding of the interactions between multiple signals, particularly in inexperienced breeders, is limited. In birds, delayed plumage maturation (DPM) is well documented; young birds appear duller than adults, despite reaching sexual maturity. Age-related changes in song structure are also common in songbirds, though the extent to which songs of yearling males differ from those of adults (delayed song maturation, DSM) and its prevalence in species with DPM is unknown. We tested for DSM in the mate-attraction (repeat) song of a species with dramatic DPM, the American redstart (Setophaga ruticilla). Repeat song structure of territorial yearling and adult males differed significantly, based on discriminant analysis of nine non-collinear song features. Combined with previous evidence of delayed maturation in territorial (serial) song, we provide the first evidence for DSM in different song types used in different behavioral contexts during the breeding season of a migratory songbird. Within adults, variation in repeat song was associated with pairing, earlier onset of breeding, and number of offspring sired, suggesting a potential benefit for expressing more adult-like song. We found no relationship between the expression of adult-like repeat song and plumage in either age class. These results indicate that delayed maturation of repeat song in yearling redstarts is not due to a generalized delay in signal development, and suggest further work is needed to determine whether mate attraction (for both yearling and adult males) provides greater pressure to sound more “adult-like” than to look more “adult-like”.     

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.     

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.     

Function of male courtship vocalizations in red-winged blackbirds

Summary During courtship, male red-winged blackbirds (Agelaius phoeniceus) produce long sequences of simple, single-note precopulatory calls, punctuated with occasional songs. Male song has previously been shown to stimulate copulation solicitation in captive, estradiol-implanted females of this species (Searcy and Brenowitz 1988; Searcy, in press). Here I show that sequences of precopulatory calls also stimulate copulation solicitation in female redwings, but that they are not as stimulatory as song. In tests contrasting mixed bouts of song and precopulatory calls with bouts of song alone, the mixed bouts elicited significantly more female solicitation in one instance, more but not significantly more in a second, and an exactly equal amount as song alone in a third. Neither song nor precopulatory calls could be shown to affect proximity of females to the speaker.     

Effects of Population Size on Singing Behavior of a Rare Duetting Songbird

Although the genetic and ecological effects of population declines in endangered species have been well studied, little is known of the social consequences. Changes in signaling behavior may result in disrupted communication and affect both reproductive and conflict‐resolution activities. The North Island Kōkako (Callaeas wilsoni) is an endangered, duetting (i.e., alternating, coordinated singing by mated pairs) songbird endemic to New Zealand temperate rain forests. Scattered populations (approximately 1500 individuals in 13 surviving and 11 translocated populations) in isolated conservation areas of different sizes have been rescued from extirpation and are currently recovering. We examined key song attributes of the Kōkako to assess whether population size or growth rate are related to song complexity, the reduction of which may compromise effective communication. We analyzed song repertoire size and phrase‐type sharing (i.e., Jaccard index of similarity), vocal performance (singing rates, song switching rates, and diversity of phrase types), and song syntactical characteristics (i.e., unpredictability in sequences of phrase types) in surviving and translocated populations (populations of approximately 19–250 territorial individuals). Population size was positively correlated with a population's song repertoire, song diversity, and switching of song phrase types and negatively correlated with shared phrase types and variation in syntactical structure of songs. Population growth rate correlated positively with pair repertoire size, population repertoire size, and singing rates during song bouts. As for solo‐singing species in fragmented landscapes, songs in the fragmented populations of Kōkako appear to be undergoing microevolution as occurs in island colonization events. Our results suggest that vocal changes in small populations could affect population establishment and growth, particularly in multiple‐source translocations. We believe measurement of vocal behavior could be used as a supplement to periodic population censuses to allow more frequent monitoring of population size. Efectos de la Conducta de Canto sobre el Tamaño Poblacional de una Ave Canora Rara     

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.     

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.     

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.     

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.     

Sound degradation as a distance cue in great tit (Parus major) song

Summary We report an experiment designed to test the ideas that: 1. male songbirds can use cues from the distortion of song by environmental factors (degradation) to estimate the distance of another singing male; 2. song degradation is assessed by reference to an internal standard. Great tits respond more strongly to undegraded than to degraded songs when both are played at the same amplitude and from the same position in the territory. This difference in response is shown only if the playback song is familiar to the test bird; familiar songs being those sung either by the test bird or neighbours of the test bird. We interpret these results as evidence that cues from song degradation can be used to estimate the distance of a singing conspecific and that degradation assessment is only possible if the bird has an internal representation of the song (because either it and/or a neighbour sings the song). We discuss the implications of these results for Morton's (1982) ranging hypothesis, and for the distinction between learning and performance in bird song. Our results partially support the ranging hypothesis, but question the nature of unrangeable songs sensu Morton. The finding that birds can assess the degree of degradation of songs that they do not sing, supports the idea that birds learn more songs than they sing.     

Use of trace element analysis of feathers as a tool to track fine-scale dispersal in birds

Tracking dispersal and migratory movements of animals over small and large spatial scales is a challenge. In birds, a promising yet underutilized tool is the trace elemental composition of feathers. The elemental profile of a feather may reveal information about the geographic origin of a bird provided that molting occurs on the breeding grounds and that elemental differences exist between breeding areas. Here, we explore the use of trace elemental composition in body feathers of the Puget Sound white-crowned sparrow Zonotrichia leucophrys pugetensis as a tool (1) to discriminate among birds collected in four different song dialect populations along a 400-km stretch of the Pacific Northwest coast and (2) to assign males singing nonlocal dialects in one population to potential natal populations. Inductively coupled plasma mass spectrometry detected 34 trace elements in sampled feathers and in a discriminant function analysis seven of these elements differed among the four source populations. Half of the six nonlocal dialect singers, who were likely to have immigrated into the focal population, were assigned to a population that matched their song dialect. Our study suggests that feather microchemistry is a promising tool for identifying geographic origins of dispersing birds over small geographic scales and in combination with other markers, such as song, may give insight into ecological and evolutionary processes.     

Structure and evolution of song form in the wrens Thryothorus sinaloa and T. felix

Summary The songs of two species of wrens, Thryothorus sinaloa and T. felix, are less similar than previously believed. We have found that song structure, bout patterning, and other aspects of singing behavior are widely divergent between the two species, which are sympatric over much of their ranges. The nature of these differences supports the hypothesis of contrast reinforcement in bird song. Further evidence is provided by essentially parallel differences in another pair of sympatric species, T. pleurostictus and T. maculipectus. A cluster analysis of song form of the above four species plus one other, T. ludovicianus, suggests that those species which are allopatric are the most similar in song form, while sympatric species are the most dissimilar. We relate various features of singing behavior in the two species to some aspects to their ecology, and examine relationships among various song parameters within the genus Thryothorus. The parallel differences in pairs of sympatric Thryothorus in structure of song units, temporal organization of song, and duetting, along with the close correlation of groups of song parameters in the five species of Thryothorus leads us to propose that strategies or adaptive complexes of singing characteristics may exist in the genus.     

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.     

Does song reflect age and viability? A comparison between two populations of the great reed warbler Acrocephalus arundinaceus

Song complexity is often regarded as a sexually selected trait that reflects the overall quality of a male. In many passerine species, old males possess larger song repertoires than younger males. This may be either because individual males improve their performance as they get older (longitudinal increase) or because poor singers have reduced viability and, hence, are underrepresented in old age classes (cross-sectional increase). We studied the age dependence of repertoire size and other song traits in a German and a Swedish great reed warbler (Acrocephalus arundinaceus) population. We found marked differences between longitudinal and cross-sectional approaches, as well as between the two study populations. In the German population, we found that syllable switching, a measure of immediate versatility and strophe length, increased with age in a cross-sectional analysis. This was not because birds improved with age (longitudinally) but because syllable switching was positively correlated with male longevity. However, in Sweden, syllable switching seemed to be unrelated to age and longevity. In the Swedish population, individual males increased their repertoire size as they got older (longitudinal increase), but this did not happen in the German population. Hence, two populations, even when belonging to the same subspecies, may differ in whether or not they show delayed song maturation.     

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.     

Colony nutritional status modulates worker responses to foraging recruitment pheromone in the bumblebee Bombus terrestris

Foraging activity in social insects should be regulated by colony nutritional status and food availability, such that both the emission of, and response to, recruitment signals depend on current conditions. Using fully automatic radio-frequency identification (RFID) technology to follow the foraging activity of tagged bumblebees (Bombus terrestris) during 16,000 foraging bouts, we tested whether the cue provided by stored food (the number of full honeypots) could modulate the response of workers to the recruitment pheromone signal. Artificial foraging pheromones were applied to colonies with varied levels of food reserves. The response to recruitment pheromones was stronger in colonies with low food, resulting in more workers becoming active and more foraging bouts being performed. In addition to previous reports showing that in colonies with low food successful foragers perform more excited runs during which they release recruitment pheromone and inactive workers are more prone to leave the nest following nectar influx, our results indicate that evolution has shaped a third pathway that modulates bumblebee foraging activity, thus preventing needless energy expenditure and exposure to risk when food stores are already high. This new feedback loop is intriguing since it involves context-dependent response to a signal. It highlights the integration of information from both forager-released pheromones (signal) and nutritional status (cue) that occurs within individual workers before making the decision to start foraging. Our results support the emerging view that responses to pheromones may be less hardwired than commonly acknowledged. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.