Species-specific song responses emerge as a by-product of tuning to the local dialect

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Evolution and plasticity: Divergence of song discrimination is faster in birds with innate song than in song learners in Neotropical passerine birds

Plasticity is often thought to accelerate trait evolution and speciation. For example, plasticity in birdsong may partially explain why clades of song learners are more diverse than related clades with innate song. This “song learning” hypothesis predicts that (1) differences in song traits evolve faster in song learners, and (2) behavioral discrimination against allopatric song (a proxy for premating reproductive isolation) evolves faster in song learners. We tested these predictions by analyzing acoustic traits and conducting playback experiments in allopatric Central American sister pairs of song learning oscines (N = 42) and nonlearning suboscines (N = 27). We found that nonlearners evolved mean acoustic differences slightly faster than did leaners, and that the mean evolutionary rate of song discrimination was 4.3 times faster in nonlearners than in learners. These unexpected results may be a consequence of significantly greater variability in song traits in song learners (by 54–79%) that requires song‐learning oscines to evolve greater absolute differences in song before achieving the same level of behavioral song discrimination as nonlearning suboscines. This points to “a downside of learning” for the evolution of species discrimination, and represents an important example of plasticity reducing the rate of evolution and diversification by increasing variability.     

Territory establishment,song learning strategies and survival in song sparrows

In most songbirds, the processes of song learning and territory establishment overlap in the early life and a young bird usually winds up with songs matching those of his territorial neighbors in his first breeding season. In the present study, we examined the relationships among the timing of territory establishment, the pattern of song learning and territorial success in a sedentary population of song sparrows (Melospiza melodia). Males in this population tend to learn their songs from their neighbors and consequently they show high song sharing with neighbors and use these shared songs preferentially in interactions with them. Males also show significant variation in the timing of territory establishment, ranging from their natal summer to the next spring. Using a three-year dataset, we found that the timing of territory establishment did not systematically affect the composition of the song repertoire of the tutee: early establishers and late establishers learned equally as much from their primary tutors and had a similar number of tutors and similar repertoire sizes, nor did timing of territory establishment affect subsequent survival on territory. Therefore, the song-learning program of song sparrows seems versatile enough to lead to high song sharing even for birds that establish territories relatively late.     

Hybridization and postzygotic isolation patterns in pigeons and doves

Abstract.— The study of the patterns of reproductive isolation in relation to species divergence is critical for the understanding of the process of speciation. Comparative analyses of this kind were previously conducted in Drosophila , Lepidoptera, frogs, ducks, and birds in general. In the present study, we used information from the literature to analyze hybrid inviability in relation to species divergence in pigeons and doves. Four main patterns arose from this analysis: (1) as in the other groups studied, F₁ hybrid inviability gradually increases as species diverge, the time needed to reach total inviability being higher in birds than in the other groups; (2) as expected, the presence of geographic overlap does not influence the evolution of postzygotic isolation; (3) the percentage of unhatched eggs does not differ between hybrids of the first generation and the backcrosses, but it increases in the second hybrid generation; and (4) pigeons and doves follow Haldane's rule, as found in the other groups studied so far. The similarity between the results of this and previous studies contributes to the growing evidence suggesting that the patterns of the evolution of postzygotic isolation, and the process of speciation in general, are shared among animal groups.     

Nightingales in space: song and extra-territorial forays of radio tagged song birds

Summary Most temperate zone passerines defend territories during the breeding season. Commonly the size of these territories is estimated by plotting the singing locations of the males. However, an individual's activities need not be restricted to the area used for singing. So far, only little quantitative information has been available to determine how the singing territory relates to the activity range of male songbirds. Here, we present a study in which we used radio-tracking techniques to collect quantitative data on the spatial behaviour of 11 male territorial Nightingales (Luscinia megarhynchos). The results show that the singing territories made up only about 50% of the activity ranges. Nevertheless, males spent over 90% of the time within the singing territory. Singing territories almost never overlapped but activity ranges overlapped in all cases with the activity range of at least one neighbour. Males intruded into neighbouring territories throughout the season but the longest excursions were made before and during the female fertile period. The time spent for forays correlated significantly with song rate and territories of males with higher song rates in turn were less frequently the object of forays by other males. Song rate can be indicative of male quality, so that our findings strongly suggest that foray behaviour is related to male quality in nightingales.

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Das Raum- und Gesangsverhalten von männlichen Nachtigallen in der Brutsaison: radiotelemetrische Untersuchungen

Zusammenfassung Männliche Singvögel markieren mit ihrem Gesang ein Revier. Empirische Studien zeigen allerdings, dass Männchen und Weibchen das Revier verlassen und in Reviere von Nachbarn eindringen. Solche Ausflüge sind vor allem in dichteren Habitaten schwierig zu beobachten, so dass für die meisten Vogelarten kaum Datenmaterial für die Raumnutzung außerhalb der Gesangsreviere vorliegt. In dieser Studie haben wir über den Verlauf der Brutsaison 11 männliche territoriale Nachtigallen telemetriert. Die Ergebnisse zeigen, dass die Aktionstradien im Mittel doppelt so groß waren, wie die Gesangsreviere und dass die Männchen ca. 8% der Zeit außerhalb der Gesangsreviere verbracht haben. Dabei überlappten die Aktionsräume benachbarter Männchen deutlich. Die Männchen drangen vor allem vor und während der fertilen Phasen der Weibchen in Nachbarreviere vor. Darüber hinaus drangen Männchen mit höherer Gesangsaktivität signifikant länger in Nachbarreviere ein, als Männchen mit geringerer Gesangsaktivität. Reviere von Männchen mit höherer Gesangsaktivität wurden dabei auch signifikant seltener von anderen Männchen besucht. Da die Gesangsaktivität mit der Qualität eines Männchens zusammenhängen kann, weisen die Ergebnisse darauf hin, dass qualitativ bessere Männchen mehr in Nachbarreviere eindringen als qualitativ schlechtere Männchen und die Reviere der qualitativ besseren Männchen auch weniger häufig von Rivalen besucht werden. Dass solche Ausflüge von Männchen für Versuche zu außerpaarlichen Kopulationen genutzt werden, ist anzunehmen, muss aber noch geklärt werden. Die räumlichen Beziehungen territorialer Männchen reflektieren darüber hinaus vermutlich Aspekte der sozialen Beziehungen.

     

Rapid evolution of courtship song pattern in Drosophila willistoni sibling species

Courtship behaviours may provide a more reliable means of identifying reproductively isolated taxa than traits such as morphology or many genetic markers. Here we describe the courtship songs of the Drosophila willistoni sibling species group, which consists of several species and subspecies. We find that song pattern is species-specific, despite significant differences among strains within species. D. paulistorum has the most variable song pattern, which reflects this species' traditional subdivision into semispecies. All the other species could be unambiguously identified by song. The major differences among these species was in the interpulse interval, as has been found in other studies of fly song. However, the interpulse intervals of the species studied here were often multimodal. This was partly due to the presence of multiple song types within the courtship repertoire, but it also reflected changes in interpulse interval within a song type by some males. Unusually, some species had distinctively patterned variation in interpulse interval. Song must have evolved rapidly within the species complex, probably due to sexual selection.     

To trill or not to trill? Territorial response to a heterospecific vocal trait in male collared doves, Streptopelia decaocto

Responding of individuals outside the conspecific range hasbeen largely explained by biases in sensory or cognitive systemstoward particular traits or trait values. More recently, ithas been shown that such responses might occur if individualsstill respond to signal traits that have been lost over time.However, empirical evidence remains scarce. We report a casesupporting the latter mechanism. Phylogenetic analysis suggeststhat the collared dove, Streptopelia decaocto, had lost thetrilled vocalization present in most congeneric taxa. We testedwhether males retained the ability to respond to these trillsin the context of territory defense. We synthesized trilledsongs by inserting trills from the sister species S. roseogriseainto S. decaocto songs. We show that trilled songs yielded higherresponses than did natural conspecific songs, and that the intensityof the response depended on the number of trilled elements.We also show that trilled songs elicited as strong reactionsas frequency modulated coos, which are stronger releasers ofterritorial response than are nonmodulated coos, but are notproduced by every males. Additional tests suggest that the frequencypattern is the most important feature of the trill used by males.However, it is still unknown whether reactions to trilled andfrequency modulated coos have the same perceptual basis. Toour knowledge, this study is the first report of a stronglydeviating signal that is still effective in vocal intrasexualcommunication in birds.     

Incomplete song divergence between recently diverged taxa: syllable sharing by Orchard and Fuertes’ orioles

ABSTRACT Orchard Orioles (Icterus spurius) and Fuertes’ Orioles (I. fuertesi) recently diverged from each other, making them an ideal system for investigating trait evolution and mechanisms of reproductive isolation during the early stages of speciation. These taxa differ in adult male plumage coloration and in their breeding and wintering ranges, but quantitative comparisons of their song characteristics have revealed no discernible differences. We assessed evolutionary song divergence in this group by investigating patterns of syllable‐type sharing within and between populations. Of 529 distinct syllable types, 142 (26.8%) were shared among individuals, and sharing appeared to decrease with geographic distance. The total number of syllables shared between Orchard and Fuertes’ orioles (26; 4.9% of the total) was similar to levels of sharing between populations of Orchard Orioles. Furthermore, hierarchical cluster analyses showed individuals of the two taxa intermixed. Syllables also used as calls were shared more frequently within and between taxa, suggesting that they have evolved more slowly than those used exclusively in songs. Our results show that at least some aspects of song have not yet diverged between these incipient species, either due to cultural exchange or because songs have evolved relatively slowly compared to plumage colors.     

Mating patterns of Darwin's Finch hybrids determined by song and morphology

Three species of Darwin's Finches hybridize on the Galapagos island of Daphne Major. We examined mating patterns to determine if hybrids exhibit mate preferences. Geospiza fortis x G. scandens F₁ hybrids backcrossed to both of the parental species, whereas all backcrosses bred with the parental species to which they were most related, or with hybrids. Paternal song was shown to be the crucial factor determining the mating pattern of G. fortis x G. scandens F₁ hybrids and their offspring. Song is culturally inherited, transmitted faithfully from father to son (with few exceptions) as a result of an imprinting-like process. Size also contributes to the choice of mates. G. fortis x G. scandens F₁ hybrid females paired with large G. scandens -like G. fortis males. G. fortis x G. fuliginosa F₁ hybrids paired negatively assortatively with respect to the size of their G. fortis mates. Non-random mating of hybrids based on song, a non-generic trait, has interesting evolutionary consequences. Song characteristics and nuclear and mitochondrial genes flow from G. fuliginosa into the G. fortis population, whereas the direction of transfer of genetic and song information between G. fortis and G. scandens depends on which song was sung by the father of the hybrids.     

Losses of female song with changes from tropical to temperate breeding in the New World blackbirds

Birds in which both sexes produce complex songs are thought to be more common in the tropics than in temperate areas, where typically only males sing. Yet the role of phylogeny in this apparent relationship between female song and latitude has never been examined. Here, we reconstruct evolutionary changes in female song and breeding latitude in the New World blackbirds (Icteridae), a family with both temperate and tropical representatives. We provide strong evidence that members of this group have moved repeatedly from tropical to temperate breeding ranges and, furthermore, that these range shifts were associated with losses of female song more often than expected by chance. This historical perspective suggests that male-biased song production in many temperate species is the result not of sexual selection for complex song in males but of selection against such songs in females. Our results provide new insights into the differences we see today between tropical and temperate songbirds, and suggest that the role of sexual selection in the evolution of bird song might not be as simple as we think.     

Song matching in a long-lived,sedentary bird with a low song rate: The importance of song type,song duration and intrusion

Territorial songbirds often match the song features or singing patterns of rivals, commonly as an aggressive signal. Most studies of song matching have been on Northern Hemisphere species with short lifespans and high song rates, but vocal matching is predicted to be affected both by longevity and territorial stability. We studied song matching in males of the white-browed scrubwren, Sericornis frontalis, a long-lived, sedentary, territorial Australian songbird. We quantified natural song rate and diversity, and then conducted three playback experiments to test: (a) whether males match by song type; (b) how they respond physically and vocally to territorial intrusion; and (c) whether they match by song length, and use it as an agonistic signal. Males naturally had very low song rates, singing on average less than three times per hour, and moderate repertoires, with an estimated mean of 17.5 song types for individual males. Males did not engage in extended counter-singing bouts. The first experiment showed that males matched the song type of immediate neighbours almost 90% of the time, if that type was in their repertoire. The remaining experiments revealed that song-type matching was an aggressive signal; males responded more aggressively to, and were more likely to match, playback simulating a neighbour's territorial intrusion than song from their shared boundary. Males did not match songs by length, but they produced longer songs after simulated intrusion. Males also responded more aggressively to playback of longer songs that simulated intrusion, but less aggressively to longer songs from the territory boundary. Overall, we show that sedentary, long-lived songbirds with low song rates, can use song-type matching as an aggressive signal to communicate with neighbours and intruders. Song length had a different role in communication, possibly related to individual quality or territory ownership.     

Relationships of song structure to phylogenetic history,habitat, and morphology in the vireos,greenlets, and allies (Passeriformes: Vireonidae)

Acoustic signals show immense variation among passerines, and several hypotheses have been proposed to explain this diversity. In this study, we tested, for the first time, the relationships of song structure to phylogeny, habitat type, and morphology in the vireos and allies (Vireonidae). Every measure of song structure considered in this study had moderate and significant phylogenetic signal. Furthermore, two song-constraining morphological traits, bill shape and body mass, also exhibited significant phylogenetic signal. Song length showed the largest within-clade similarity; longer songs were highly conserved in part of the greenlet (Hylophilus) clade, whereas shorter songs characterized the remaining seven genera. We found no differences in song structure among vireonids living in different habitat types. However, vireonids with shorter, stouter bills and larger bodies sang songs with lower minimum and maximum peak frequency, compared with species with longer, thinner bills and smaller bodies. We conclude that Vireonidae song evolution is driven partially by phylogenetically conserved morphological traits. Our findings support the phylogenetic signal and morphological constraints hypotheses explaining structural diversity in avian acoustic signals.     

Aromatase inhibition affects testosterone‐induced masculinization of song and the neural song system in female canaries

Songbirds have a specialized steroid‐sensitive network of brain nuclei, the song system, for controlling song. Most nuclei of the song system express androgen receptors, and the sensory‐motor integration nucleus High Vocal Center (HVC) alone also expresses estrogen receptors. Apart from expressing estrogen receptors in the vocal control system, songbirds are unique among birds because they have high concentrations of the estrogen‐synthesizing enzyme aromatase in the neostriatum surrounding HVC. However, the role of estrogen in controlling the development of the song structure has been scarcely investigated. In this work, we show that blocking the production of estrogen during testosterone‐induced song motor development in adult female canaries alters the song pattern compared to control females treated with testosterone only. These effects were correlated with inhibition of the expression of estrogen‐sensitive genes, such as brain‐derived nerve growth factor, in HVC. The expression of the ATP‐synthase gene, an indicator of cell activity, in HVC, and the size of HVC, were not affected by the treatment. Our results provide the first example of estrogen‐sensitive mechanisms controlling the structural features of adult birdsong. © 2002 Wiley Periodicals, Inc. J Neurobiol 54: 370–379, 2003     

Seasonal changes in song nuclei and song behavior in Gambel's white-crowned sparrows

In males of several songbird species, the morphology of forebrain nuclei that control song changes seasonally. The only seasonally breeding songbird in which seasonal changes in the structure of song control nuclei have been reported not to occur is the nonmigratory Nuttall's subspecies of white-crowned sparrow. In the present study, we manipulated photoperiod and plasma testosterone concentrations in captive male white-crowned sparrows of the migratory Gambel's subspeices. Males exposed to photoperiods and plasma testosterone concentrations typical of those experienced by wild breeding males had larger song control nuclei than males held on a winter photoperiod. We also found seasonal change in stereotypy of spectral and temporal parameters of song in wild Gambel's white-crowned sparrows. We hypothesize that seasonal changes in song control nuclei may correlate with seasonal changes in song stereotypy. © 1995 John Wiley & Sons, Inc.     

A preliminary comparison of a songbird’s song repertoire size and other song measures between an urban and a rural site

Characteristics of birdsong, especially minimum frequency, have been shown to vary for some species between urban and rural populations and along urban–rural gradients. However, few urban–rural comparisons of song complexity—and none that we know of based on the number of distinct song types in repertoires—have occurred. Given the potential ability of song repertoire size to indicate bird condition, we primarily sought to determine if number of distinct song types displayed by Song Sparrows (Melospiza melodia) varied between an urban and a rural site. We determined song repertoire size of 24 individuals; 12 were at an urban (‘human‐dominated’) site and 12 were at a rural (‘agricultural’) site. Then, we compared song repertoire size, note rate, and peak frequency between these sites. Song repertoire size and note rate did not vary between our human‐dominated and agricultural sites. Peak frequency was greater at the agricultural site. Our finding that peak frequency was higher at the agricultural site compared to the human‐dominated site, contrary to many previous findings pertaining to frequency shifts in songbirds, warrants further investigation. Results of our pilot study suggest that song complexity may be less affected by anthropogenic factors in Song Sparrows than are frequency characteristics. Additional study, however, will be required to identify particular causal factors related to the trends that we report and to replicate, ideally via multiple urban–rural pairings, so that broader generalization is possible.     

Conveying information with one song type: changes in dawn song performance correspond to different female breeding stages

Many bird species participate in dawn singing, a behaviour categorized by intensive singing at dawn; however, many of these species deliver only one song type at dawn. While there are many proximate and ultimate hypotheses for why birds sing at dawn, little is known about whether males are able to vary one simple song to convey different information. We used autonomous acoustic recorders to record dawn songs of field sparrows and quantified three parameters of singing performance: 1) bout length, 2) song rate, and 3) song complexity. We found that males sang the longest dawn bouts during their mate's fertile period, the highest song rates during the post-fertile period, and the most complex songs during the pre-fertile period. The change in dawn singing behaviour with their mate's breeding stage suggest the purpose of dawn song may be context dependent. Our results demonstrate that male field sparrows, while only having a single song type sung at dawn, may convey information for both intra- and intersexual purposes. While it is generally assumed that dawn song has a specific function, the variability in the duration, rate, and complexity of dawn song in field sparrows suggests that they are conveying different information and that dawn song likely has multiple functions.     

Beak morphology and song features covary in a population of Darwin's finches (Geospiza fortis)

Animal mating signals evolve in part through indirect natural selection on anatomical traits that influence signal expression. In songbirds, for example, natural selection on beak form and function can influence the evolution of song features, because of the role of the beak in song production. In this study we characterize the relationship between beak morphology and song features within a bimodal population of Geospiza fortis on Santa Cruz Island, Galápagos. This is the only extant population of Darwin's finches that is known to possess a bimodal distribution in beak size. We test the hypothesis that birds with larger beaks are constrained to produce songs with lower frequencies and decreased vocal performance. We find that birds with longer, deeper, and wider beaks produce songs with significantly lower minimum frequencies, maximum frequencies and frequency bandwidths. Results from the analysis of the relationship between beak morphology and trill rate are mixed. Measures of beak morphology correlated positively with 'vocal deviation', a composite index of vocal performance. Overall these results support a resonance model of vocal tract function, and suggest that beak morphology, a primary target of ecological selection in Darwin's finches, affects the evolution of mating signals. We suggest that differences in song between the two modes of the distribution may influence mate recognition and perhaps facilitate assortative mating by beak size and population divergence.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 489–498.     

Age-related changes in song repertoire size and song type sharing in the Whinchat Saxicola rubetra

In many oscine passerines males’ songs, the repertoire size increases with age. At the same time it often remains unknown when and where males learn new songs. To infer the Whinchat Saxicola rubetra song learning strategy, we described and catalogued song-type repertoire, revealed age differences and examined song sharing strategies among neighbouring and distant males. We recorded song vocalizations of 40 males in a limited (104 ha) study plot during four years. Whinchats produce short and discrete songs with clear intersong pauses. In total 45 song types were allocated, the individual repertoire size averaged 23.5 ± 7.6 song types (range 9–34 song types). The males’ age significantly influenced the song-type repertoire size. The second calendar year (first breeding) males had a lower repertoire size than the older males. The majority of song types were shared by less than half of males in our sample. The Jaccard similarity indexes varied from 0.5 to 0.7. We could not find a relationship between males’ song sharing and geographic distances between their nests. We assume that Whinchat males learned new songs in the local population before territory establishment.