Changes in Song Complexity Correspond to Periods of Female Fertility in Blue Grosbeaks (Guiraca caerulea)

Song complexity is thought to be a sexually selected trait in passerine birds; however, quantifying relevant parameters of song complexity is the first step in testing the theory that song complexity is a sexually selected trait. We show here that blue grosbeak (Guiraca caerulea) males sing a single song type but the properties of that song type vary between renditions. This pattern of song delivery potentially provides females with an opportunity to assess dimensions of song complexity other then repertoire size. Here we characterize song complexity using four measures: (i) element repertoire size, (ii) proportion of distinct song variant, (iii) song versatility, and (iv) syntax consistency. We studied the functional significance of song complexity by comparing measures of song complexity before and after periods of female fertility. We found that male blue grosbeaks sing more song variants, use more versatile arrangements of elements, and maintain more syntax consistency during the fertile period of their social mate than during their social mate's non‐fertile period. These results point to a functional relationship between changes in song complexity and periods of female fertility in this species.     

Song repertoires in the red-winged blackbird (Agelaius phoeniceus): A test of the Beau Geste hypothesis

Song repertoires in the red-winged blackbird (Agelaius phoeniceus) were studied to test the hypothesis that song repertoires have evolved to mimic high density. Results were consistent with predictions of this hypothesis. (1) Song types within a repertoire were as variable as those sung by different males. (2) Males often switched song type when they switched perch. (3) Males with large repertoires sang at faster rates than those with small repertoires, but repertoire size did not affect bout length or bout rate. (4) Rate of habituation was a function of repertoire size. (5) Empty territories occupied by speakers broadcasting song repertoires were more successfully defended than those occupied by speakers broadcasting single song types. These results are also consistent with the hypothesis that song repertoires function to signal the status of the singer.     

Testosterone increases repertoire size in an open-ended learner: An experimental study using adult male European starlings (Sturnus vulgaris)

Song in songbirds is a learned secondary sexual behavior, first acquired during a sensitive phase of juvenile development, which is affected by hormones such as testosterone (T). While the latter has received much attention, the potential involvement of T in the adult repertoire changes observed in a number of species is much less understood. Yet, this may prove essential to understand the role of song as a sexually selected trait. We therefore performed a T-implantation experiment during the non-breeding season (when T is basal), using adult male European starlings (Sturnus vulgaris), a songbird species in which song repertoire size (and composition) changes seasonally and increases with age. Repertoire size increased rapidly in T-males, but not in control males, indicating a role for T in repertoire size changes. This increase resulted from a lower proportion of dropped song types in T-males than in control males, while the proportion of added song types did not differ between both groups. Interestingly, the observed repertoire turnover (adding and removing song types from the repertoire) in both groups, suggests that elevated plasma T levels were not essential for changes in repertoire composition (contrary to repertoire size). Finally, T-males (but not control males) significantly increased their song rate, while neither group showed a significant change in their song bout length and phrase repetition rate. Taken together, our results suggest a role for T in adult song learning and provide new insights into the information content of repertoire size and song bout length as sexually selected traits.     

Testosterone increases repertoire size in an open-ended learner: An experimental study using adult male European starlings (Sturnus vulgaris)

Song in songbirds is a learned secondary sexual behavior, first acquired during a sensitive phase of juvenile development, which is affected by hormones such as testosterone (T). While the latter has received much attention, the potential involvement of T in the adult repertoire changes observed in a number of species is much less understood. Yet, this may prove essential to understand the role of song as a sexually selected trait. We therefore performed a T-implantation experiment during the non-breeding season (when T is basal), using adult male European starlings (Sturnus vulgaris), a songbird species in which song repertoire size (and composition) changes seasonally and increases with age. Repertoire size increased rapidly in T-males, but not in control males, indicating a role for T in repertoire size changes. This increase resulted from a lower proportion of dropped song types in T-males than in control males, while the proportion of added song types did not differ between both groups. Interestingly, the observed repertoire turnover (adding and removing song types from the repertoire) in both groups, suggests that elevated plasma T levels were not essential for changes in repertoire composition (contrary to repertoire size). Finally, T-males (but not control males) significantly increased their song rate, while neither group showed a significant change in their song bout length and phrase repetition rate. Taken together, our results suggest a role for T in adult song learning and provide new insights into the information content of repertoire size and song bout length as sexually selected traits.     

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.     

First-Year Common Nightingales (Luscinia megarhynchos) Have Smaller Song-Type Repertoire Sizes Than Older Males

Based on the assumptions that birdsong indicates male quality and that quality is related to age, one might expect older birds to signal their age. That is, in addition to actual body condition, at least some song features should vary with age, presumably towards more complexity. We investigated this issue by comparing repertoire sizes of free‐ranging common nightingale males in their first breeding season with those of older males. Nightingales are a good model species as they are open‐ended learners, where song acquisition is not confined to an early sensitive period of learning. Moreover, nightingales develop an extraordinarily large song‐type repertoire (approx. 180 different song types per male), and differences in repertoire size among males are pronounced. We analysed repertoire characteristics of the nocturnal song of nine nightingales in their first breeding season and compared them with the songs of nine older males. The repertoire size of older males was on average 53% larger than that of yearlings. When analysing two song categories of nightingales, whistle and non‐whistle songs separately, we found similar results. Our findings show marked differences in repertoire size between age categories, suggesting that this song feature may reflect a male's age. We discuss those mechanisms that may constrain the development of larger repertoires in first‐year males. Whether repertoire sizes are crucial for female mate choice or in vocal interactions among conspecific males remains open to further investigations.     

Difficulties when assessing birdsong learning programmes under field conditions: a re-evaluation of song repertoire flexibility in the great tit

There is a remarkable diversity of song-learning strategies in songbirds. Establishing whether a species is closed- or open-ended is important to be able to interpret functional and evolutionary consequences of variation in repertoire size. Most of our knowledge regarding the timing of vocal learning is based on laboratory studies, despite the fact that these may not always replicate the complex ecological and social interactions experienced by birds in the wild. Given that field studies cannot provide the experimental control of laboratory studies, it may not be surprising that species such as the great tit that were initially assumed to be closed-ended learners have later been suggested to be open-ended learners. By using an established colour-ringed population, by following a standardized recording protocol, and by taking into account the species' song ecology (using only recordings obtained during peak of singing at dawn), we replicated two previous studies to assess song repertoire learning and flexibility in adult wild great tits elicited by social interactions. First, we performed a playback experiment to test repertoire plasticity elicited by novel versus own songs. Additionally, in a longitudinal study, we followed 30 males in two consecutive years and analysed whether new neighbours influenced any change in the repertoire. Contrary to the previous studies, song repertoire size and composition were found to be highly repeatable both between years and after confrontation with a novel song. Our results suggest that great tits are closed-ended learners and that their song repertoire probably does not change during adulthood. Methodological differences that may have led to an underestimation of the repertoires or population differences may explain the discrepancy in results with previous studies. We argue that a rigorous and standardized assessment of the repertoire is essential when studying age- or playback-induced changes in repertoire size and composition under field conditions.     

Song, sexual selection, and a song control nucleus (HVc) in the brains of European sedge warblers

Female sedge warblers select males that have more complex songs as mates. This study tests two predictions concerning HVc, a telencephalic nucleus that is essential for song learning and production: first, that males with more complex songs will have a larger HVc, and second that males who pair successfully will have a larger HVc than unpaired males. Data on song composition and pairing status were collected from wild sedge warblers breeding in Hungary. We found significant positive correlations between three song attributes (repertoire size, song complexity, and song length) and the size of HVc. Males that paired successfully also had more complex songs (repertoire size and song complexity, though not song length) than males that did not. However, we find no direct evidence that males who paired successfully had a larger HVc than unpaired males. These findings are discussed in relation to the possible functions of HVc and also to current views on sexual selection and the evolution of the song control pathway.     

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.     

Song and brain development in canaries raised under different conditions of acoustic and social isolation over two years

Early isolation experiments indicate that male songbirds learn their songs during an early sensitive period, although later work has shown that some open-ended learners modify songs in later years. Recent isolation experiments suggest that in some species song has a stronger genetic basis than previously thought. This study raised domestic canaries under different combinations of acoustic and social isolation and followed song development into the second year. Males raised alone in acoustic isolation developed songs with normal syllables, but larger repertoires and also produced syllables with lower repetition rates when compared to controls. The smallest repertoire occurred in males raised in a peer group. Isolate males had a smaller song control nucleus HVC than controls, but there was no effect on nucleus RA or on brain weight in general. In the second year, after introduction into a large normal colony, isolate and peer group males adjusted their syllable repertoire to normal size. In particular, the isolates reduced their repertoire even though the size of HVC showed a significant increase in volume. However, songs of isolate and peer group males still differ in repetition rate and number of single syllables in the common aviary. In contrast, control males showed low syllable turnover and no significant change in repertoire size. Nor did they show any significant change in the volumes of song control nuclei. It seems that complete isolation affects only some aspects of song and brain development, and later socialization corrects some but not all of these in the second year.     

Nestling growth and song repertoire size in great reed warblers: evidence for song learning as an indicator mechanism in mate choice

Females of many songbird species show a preference for mating with males that have larger song repertoires, but the advantages associated with this preference are uncertain. We tested the hypothesis that song complexity can serve as an indicator of male quality because the development of the brain regions underlying song learning and production occurs when young birds typically face nutritional and other stresses, so that song reflects how well a male fared during post-hatch development. A key prediction of this hypothesis is that variation in nestling condition should correspond to variation in the adult song repertoires of individuals. We used data from a long-term study of the great reed warbler (Acrocephalus arundinaceus) to test this prediction, correlating two measures of nestling development with subsequent repertoire size of males. We found that the length of the innermost primary feather, a standard measure of development, significantly predicted first-year repertoire size. The relationship between repertoire size and body mass was nearly significant, in spite of the large variance inherent in this measure. These data support the idea that song may provide females with information about a male's response to developmental stress, which in turn is expected to correlate with indirect or direct benefits she might receive.     

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.     

No overlap in song repertoire size between yearling and older Starlings Sturnus vulgaris

We recorded the song of 25 male Starlings Sturnus vulgaris to investigate differences in song characteristics between yearling and older males. Older males had significantly larger repertoire sizes and sang significantly longer average song bouts than yearling males. Since older males had almost twice as large a repertoire size as yearling males and since there was no overlap in repertoire size between yearling and older males, our data indicate that the repertoire size of Starlings increases after the first year of life. The largest difference between yearling and older males occurred in the 'variable song types' which are the song types containing most of the heterospecific imitations sung in a song bout.     

Blue Tit song repertoire size, male quality and interspecific competition

A recent hypothesis suggested that bird song repertoire size is a reliable indication of male quality because the number of songs emitted by an individual and the expression of many traits important for fitness are highly dependent on developmental conditions. To test this hypothesis, we studied the relationship between song repertoire size and a trait known to be strongly affected by developmental conditions: tarsus length of Blue Tits Parus caeruleus . We studied this relationship in three populations presenting differences in density, quantity of food available and relative and absolute densities of Great Tits Parus major . Great Tit density may be an important factor because this species is supposed to be in competition for acoustic space with the Blue Tit. We found a positive correlation between Blue Tit dawn repertoire size and tarsus length in the three study populations which is consistent with the tested hypothesis. However, this correlation was only significant in the population with the lower Great Tit density. We discuss the significance of this variation and propose that interspecific competition may limit the action of sexual selection on song repertoire size.     

Territory tenure in song sparrows is related to song sharing with neighbours, but not to repertoire size

Song repertoires may be a product of sexual selection and several studies have reported correlations of repertoire size and reproductive success in male songbirds. This hypothesis and the reported correlations, however, are not sufficient to explain the observation that most species have small song repertoire sizes (usually fewer than 10, often fewer than five song types). We examined a second important aspect of a male's song repertoire, the extent to which he shares songs with his neighbours. Song sharing has not been measured in previous studies and it may be partially confounded with repertoire size. We hypothesized that in song sparrows, Melospiza melodia, song sharing rather than repertoire size per se is crucial for male territorial success. Our longitudinal study of 45 song sparrows followed from their first year on territory showed that the number of songs a bird shares with his neighbourhood group is a better predictor of lifetime territory tenure than is his repertoire size. We also found that song sharing increases with repertoire size up to but not beyond eight to nine song types, which are the most common repertoire sizes in the population (range in our sample 5-13). This partial confound of song sharing and repertoire size may account for some earlier findings of territory tenure-repertoire size correlations in this species and other species having small- or medium-sized repertoires. Copyright 2000 The Association for the Study of Animal Behaviour.     

Are there age-related differences in the song repertoire size of Eurasian blackbirds?

Most oscine bird species possess a repertoire of different song patterns, and repertoire size is thought to signal aspects of male quality. As age is assumed to be related to male quality in terms of experience and/or viability, repertoire size may be expected to reflect male age. Here, we investigated the relationship between repertoire size and age (yearlings or older) in Eurasian blackbirds, Turdus merula, a species with a large repertoire delivered in a highly variable manner. We found that older males tended to have larger repertoires than yearlings though the two age groups overlapped considerably. Thus, compared to other species with large repertoires, age-related differences in repertoire size seem rather small in male Eurasian blackbirds. We also compared repertoires of three individuals in two successive years (as yearlings and in the year following) and found a large element turnover. Our investigation revealed that this turnover was almost complete in the quiet terminating twitter part of the song. Such turnover may allow a young bird to adjust his repertoire to his neighbours?? repertoires, which could be useful for song matching interactions.     

Rock wrens preferentially use song types that improve long distance signal transmission during natural singing bouts

When animals are capable of producing variable signals they may preferentially use some signal types over others. Among songbirds, individuals are known to alter song type form and usage patterns in contest and mating situations, but studies have not examined how song choice improves signal efficacy during broadcast song. For this study we investigated rock wren Salpinctes obsoletus song type use rates during natural singing bouts. We tested three hypotheses for adaptive song use during broadcast song: 1) birds improve signal content by increasing the use of high quality songs, 2) birds optimize for signal propagation by preferentially using songs that transmit well, and 3) birds maintain energy by reducing the use of costly songs. The study included 19 058 songs sung by 12 individuals, each of which had a measured song repertoire of between 52 and 117 song types which were produced at highly variable rates. Results indicated that rock wrens did not preferentially sing song types with shorter durations or fewer frequency switches, as would be expected if they selected song types to minimize delivery costs. They also did not favor songs with more rapid trills or more frequency switches, as would be expected if they adjusted song use primarily to indicate quality. Focal birds did preferentially sing significantly longer songs with lower bandwidths, lower frequencies, and slower trill rates. Results suggest that natural broadcast singing patterns are shaped more by the benefits of long distance transmission than by the benefits of advertising performance ability or the costs of song production.     

Song as an honest signal of past developmental stress in the European starling (Sturnus vulgaris)

Bird song is a sexually selected male trait where females select males on the basis of song quality. It has recently been suggested that the quality of the adult male song may be determined by nutritional stress during early development. Here, we test the 'nutritional-stress hypothesis' using the complex song of the European starling. Fledgling starlings were kept under experimental treatment (unpredictable short-term food deprivations) or control conditions (ad libitum food supply), for three months immediately after independence. We measured their physiological and immune responses during the treatment and recorded song production during the following spring. Birds in the experimental group showed increased mass during the treatment and also a significantly suppressed humoral response compared with birds in the control group. There was no difference between the groups in the cell-mediated response. Next spring, males in the experimental group spent less time singing, sang fewer song bouts, took longer to start singing and also sang significantly shorter song bouts. These data support the hypothesis that both the quality and quantity of song produced by individual birds reflect past developmental stress. The results also suggest the 'nutritional-stress hypothesis' is best considered as a more general 'developmental-stress hypothesis'.     

Songbird cheaters pay a retaliation cost: evidence for auditory conventional signals

Conventional signals impose costs on senders through receiver retaliation rather than through investment in signal production. While several visual conventional signals have been described (mainly 'badges of status'), acoustic examples are rare; however, several aspects of repertoire use in songbirds are potential candidates. We performed interactive playback experiments to determine whether song-type matches (responding to a song with the same song type), repertoire matches (responding to a song with a different song type, but one in the repertoires of both singers) and unshared song types serve as conventional signals during male-male territorial interactions in banded wrens, Thryothorus pleurostictus. Our results demonstrate that these three signals incite varying levels of receiver aggression: song-type matches induce faster approach than do repertoire matches, and repertoire matches induce faster approach than do unshared song types. Production costs do not differ, while the receiver response does. Because territorial banded wrens approach opponents who signal aggressively, such opponents risk attack. This system will punish and prevent cheaters, as weak males signalling aggression will be subject to escalation by stronger or more-motivated opponents.     

Complex modulation of singing behavior by testosterone in an open-ended learner, the European Starling

In many temperate zone songbird species males only produce song during the breeding season, when plasma testosterone (T) levels are high. Males of some species sing throughout the year, even when T levels are low, indicating a dissociation between high T levels and song rate. Given that few studies have taken advantage of these species, we compare here song traits expressed under high versus low T concentrations and we study the role of testosterone in adult song learning in the European Starling, an open-ended learner in which repertoire size dramatically increases with age. We performed a detailed comparison of song complexity and song rate between fall and spring in 6-year-old intact male European starlings. In parallel, we investigated whether potential seasonal changes were regulated by the gonadally induced increase in plasma T, by comparing seasonal changes in intact and castrated males of the same age (castrated as juveniles during their first fall) and by subsequently experimentally elevating T in half of the castrated males. While song rate and stereotypy did not differ between intacts and castrates or between fall and spring, both groups increased their average song bout length from fall to spring, but only intact males increased their repertoire size, indicating that effects of seasonal T changes differ between song traits. Intact males overall displayed a larger song repertoire and a longer bout length than the castrates, and implantation with T caused a turnover in repertoire composition in castrates. However, as the castrates had never experienced high T levels and yet displayed a markedly higher repertoire size than that of typical yearling males, this suggests that the progressive increase of song repertoire with age in male starlings is not dependent on gonadal T, although it may be T-enhanced.