Fighting talk: complex song elicits more aggressive responses in a vocally complex songbird

Song complexity in many songbirds is a trait subject to sexual selection. It is often associated with male territorial defence. Empirical studies testing differential male responses to rival song in vocally complex songbirds have, however, been scarce. We conducted playback experiments of the endemic New Zealand Tui Prosthemadera novaeseelandiae to test the aggressive response of territorial male Tui to rival songs with differing complexity levels. Overall, complex songs evoked significantly stronger responses from territorial males than did simple songs. Following playback of complex songs, focal males approached the playback more closely and rapidly, and responded with songs of higher complexity than they did to playback of simple songs. This suggests males could both distinguish between different levels of complexity within the Tui repertoire, and perceive a more complex song as a greater territorial threat. Our study is one of the first to demonstrate strong aggressive responses to increased levels of song complexity in a songbird species with highly complex vocalizations.     

Male territorial aggression does not drive conformity to local vocal culture in a passerine bird

In many songbird species, young individuals learn songs from neighbors and then settle nearby, thus creating neighborhoods of conformity to local vocal culture. In some species, individuals appear to postpone song learning until after dispersal, possibly to facilitate conformity to local dialects. Despite decades of study, we still lack a consensus regarding the selective pressures driving this delayed song learning. Two common hypothetical benefits to conformity, and thus delayed song learning, are rooted in territorial interactions; individuals preferentially produce local song either to avoid detection as new arrivals (deceptive mimicry) or to be more effectively recognized as conspecific territory holders. The dickcissel (Spiza americana) is an ideal species in which to study these hypotheses. Males of this species appear to delay song learning until they arrive at their first adult territory, each individual sings a single song type, and conformity to the local song culture is high. Using playback, we contradicted both of the territorial hypotheses described above; male dickcissels did not respond differentially to local vs foreign song playback treatment. We are confident in this lack of difference because dickcissels clearly responded less strongly to a third treatment, neighbor song, than to the other two treatments, demonstrating sufficient power in our experimental design (and providing the first evidence of the dear‐enemy effect in dickcissels). Our results raise the question of why dickcissels respond equally aggressively to both local and foreign songs when other bird species often show reduced aggression toward foreign song. If reduced aggression to foreign song is not ubiquitous in species that achieve conformity through delayed learning, then selection from aggressive territorial interaction seems unlikely to be a general explanation for such delayed learning. Reduced aggression in response to foreign songs in other species may be due to reduced exposure to the stimulus of foreign song or to different cost‐benefit trade‐offs when responding to songs that deviate from the local average.     

Sexual differentiation and seasonal variation in response to conspecific and heterospecific acoustic signals

Interspecific territoriality is frequently reported between closely related species; however, few studies have demonstrated interspecific territoriality between distantly related species living in sympatry. We conducted playback experiments to investigate territorial behaviour in male and female White‐bellied Wrens (Uropsila leucogastra) in response to simulated conspecific and heterospecific intruders during the breeding and non‐breeding seasons. We explored whether heterospecific songs of the Happy Wren (Pheugopedius felix), a distantly related species and ecological competitor, elicited antagonistic responses from focal White‐bellied Wrens, and whether such responses differed between the sexes. We also examined whether male and female responses to conspecific and heterospecific rivals varied with season. We found that male White‐bellied Wrens always responded to conspecific song, and responded significantly more to heterospecific song compared to a control stimulus (Tropical Parula, Setophaga pitiayumi). In contrast, although female White‐bellied Wrens responded strongly to conspecific song, their response to heterospecific song did not differ significantly from the control stimulus. The proportion of males that responded to heterospecific songs and the proportion of females that responded to conspecific songs varied seasonally, showing significantly lower responses during the breeding season. The intense responses of male White‐bellied Wrens to playback of heterospecific songs suggest that they recognise ecological competitors based on their vocal signals. Furthermore, the decrease in agonistic interactions during the breeding season is in line with the hypothesis that aggressive behaviour may be detrimental to reproductive and parental activity, and the hypothesis that heterospecific animals pose less of a threat during the breeding season.     

The development of stimulus‐specific auditory responses requires song exposure in male but not female zebra finches

Juvenile male zebra finches develop their song by imitation. Females do not sing but are attracted to males' songs. With functional magnetic resonance imaging and event‐related potentials we tested how early auditory experience shapes responses in the auditory forebrain of the adult bird. Adult male birds kept in isolation over the sensitive period for song learning showed no consistency in auditory responses to conspecific songs, calls, and syllables. Thirty seconds of song playback each day over development, which is sufficient to induce song imitation, was also sufficient to shape stimulus‐specific responses. Strikingly, adult females kept in isolation over development showed responses similar to those of males that were exposed to songs. We suggest that early auditory experience with songs may be required to tune perception toward conspecific songs in males, whereas in females song selectivity develops even without prior exposure to song. © 2009 Wiley Periodicals, Inc. Develop Neurobiol 2010     

Male and female rufous‐and‐white wrens do not match song types with same‐sex rivals during simulated territorial intrusions

In birds with song repertoires, song‐type matching occurs when an individual responds to another individual's song by producing the same song type. Song‐type matching has been described in multiple bird species and a growing body of evidence suggests that song‐type matching may serve as a conventional signal of aggression, particularly in male birds in the temperate zone. Few studies have investigated song‐type matching in tropical birds or female birds, in spite of the fact that avian biodiversity is highest in the tropics, that female song is widespread in the tropics, and that female song is the ancestral state among songbirds. In this study of rufous‐and‐white wrens Thryophilus rufalbus, a resident neotropical songbird where both sexes sing, we presented territorial males and females with playback that simulated a territorial rival producing shared and unshared songs. In response, both males and females sang matched song types at levels statistically equal to levels expected by chance. Furthermore, males and females exhibited similar levels of aggression and similar vocal behaviours in response to playback of both shared and unshared songs. These results indicate that rufous‐and‐white wrens do not use song‐type matching in territorial conflicts as a conventional signal of aggression. We discuss alternative hypotheses for the function of song‐type sharing in tropical birds. In particular, we point out that shared songs may play an important role in intra‐pair communication, especially for birds where males and females combine their songs in vocal duets, and this may supersede the function of song‐type matching in some tropical birds.     

Differences in Frequency of Shared Song Types Enables Neighbour‐Stranger Discrimination in a Songbird Species with Small Song Repertoire

Acoustic Neighbour‐Stranger (N‐S) discrimination is widespread in birds and has evolved to settle territorial disputes with low costs. N‐S discrimination was found both in song‐learning oscines and non‐song‐learning bird taxa, irrespective of the repertoire sizes they have. Therefore, it seems that more than just a single mechanism enable N‐S discrimination. Species with larger repertoires, where males have unique phrases or syllables may rely on such interindividual differences. The majority of birds have rather small repertoires, which often are shared among neighbours. In this case, males are facing the problem of individual recognition when rivals produce songs, at least superficially, identical. To better understand the acoustic basis of N‐S discrimination in species with small and shared repertoires, I studied the ortolan bunting (Emberiza hortulana). Males of this small oscine species are able to N‐S discrimination based on a single song rendition when presented in a playback experiment, regardless of song‐type diversity and song‐sharing level within a particular population. It was also found that songs of the same type sung by different males differ in the frequency of the initial song phrases and these differences persist over years. Here, I tested whether males are able to discriminate among the natural songs and the artificially modified songs of their neighbours in which the frequency was experimentally changed by relatively small value in comparison with the variation range found in this population. Subjects responded significantly more aggressively to the songs with an artificially modified frequency, suggesting that males treat such songs as having come from the repertoire of a non‐neighbour. These results confirm an earlier prediction that differences in the frequency of shared song types enable N‐S discrimination. The study presents one of the possible mechanisms enabling N‐S discrimination in songbirds with small repertoires and stress the role of within‐song‐type variation, which is still understudied song characteristic.     

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.     

Responses to Variation in Song Length by Male White‐Crowned Sparrows

Bird song has the potential to convey a variety of information about the singer, including the singer’s current motivational state. A recent review by Searcy & Beecher (2009) (Anim. Behav. 78, 2009; 1281) emphasized how little is currently known about whether song functions as an aggressive signal in territorial interactions. A recent observational study of song length variation in the Puget Sound white‐crowned sparrow reported that males shorten the terminal trill in their single song type while singing close by another male, and shorten the trill even further immediately before chasing the opponent. Here, we report the results of two song playback experiments designed to test whether males distinguish the differences in song length that occur in agonistic and non‐agonistic contexts. We found that males gave stronger responses, as measured by close approach distances, to either a series of songs increasing in duration (compared to a series decreasing in duration), or to long loud songs (compared to short soft songs). In both experiments, males gave weaker responses to the stimuli hypothesized to be more aggressive. Males shortened their songs in response to both simulated intrusions. Combined with the observational data on song variation in different contexts, these experiments support the conclusion that song length variation contains information about aggressive motivation in the white‐crowned sparrow.     

Divergence in mating signals correlates with genetic distance and behavioural responses to playback

Animals use acoustic signals to defend resources against rivals and attract breeding partners. As with many biological traits, acoustic signals may reflect ancestry; closely related species often produce more similar signals than do distantly related species. Whether this similarity in acoustic signals is biologically relevant to animals is poorly understood. We conducted a playback experiment to measure the physical and vocal responses of male songbirds to the songs of both conspecific and allopatric‐congeneric animals that varied in their acoustic and genetic similarity. Our subjects were territorial males of four species of neotropical Troglodytes wrens: Brown‐throated Wrens (Troglodytes brunneicollis), Cozumel Wrens (T. beani), Clarion Wrens (T. tanneri) and Socorro Wrens (T. sissonii). Our results indicate that birds respond to playback of both conspecific and allopatric‐congeneric animals; that acoustic differences increase with genetic distance; and that genetic divergence predicts the strength of behavioural responses to playback, after removing the effects of acoustic similarity between subjects’ songs and playback stimuli. Collectively, these results demonstrate that the most distantly related species have the most divergent songs; that male wrens perceive divergence in fine structural characteristics of songs; and that perceptual differences between species reflect evolutionary history. This study offers novel insight into the importance of acoustic divergence of learned signals and receiver responses in species recognition.     

Song Amplitude of Rival Males Modulates the Territorial Behaviour of Great Tits During the Fertile Period of Their Mates

Bird song is a widely used model in the study of sexual selection. Variation in the expression of sexually selected traits is thought to reflect variation in male genetic and/or phenotypic quality. Vocal amplitude is a song parameter that has received little attention in the context of sexual selection, but there is some evidence that the intensity of bird song affects female preferences. Here, we tested whether the amplitude of broadcast song plays a role in male–male competition. We used song playback with varying song amplitude (within the natural amplitude range of the species) and a dummy bird taxidermy to simulate territorial intrusions in the great tit, Parus major, during the fertile period of the female and measured the response of the local male. The results show that playback amplitude significantly affected the subjects’ behaviour: after approaching to within 25 m around the loudspeaker, territorial males stayed longer within that perimeter after the playback of high‐amplitude songs compared with low‐amplitude songs. Our findings add to the small but growing body of evidence suggesting that vocal amplitude may be a sexually selected song trait.     

Signalling through acoustic windows: nightingales avoid interspecific competition by short-term adjustment of song timing

The function of bird song is closely linked to sexual selection through female choice and male–male competition, and thus variation in communication success is likely to have major fitness consequences for a singing male. A crucial constraint on signal transmission is imposed by background noise, which may include songs from other species. I investigated whether singing nightingales (Luscinia megarhynchos) avoid temporal overlap with the songs of other bird species in a playback experiment. I analysed the temporal song patterns of six males, each of which were exposed to songs of other species. The nightingales significantly avoided overlapping their songs with the playback songs, and started singing preferentially during the silent intervals between the heterospecific songs. This timing of song onset produced a greater variability in pause duration compared to the nightingales’ undisturbed solo singing. These findings suggest that birds adjust their song timing to avoid acoustic interference on short temporal scales, and thus are able to improve the efficiency of acoustic communication in complex sonic environments. Moreover, the results indicate that temporal song patterns can be affected by the songs of other species, and thus such influences should be taken into account when studying bird song in the field.     

Changes in Performance of Shared and Unshared Songs Within and Between Years in the White‐crowned Sparrow

Song of passerine birds is one of the few animal signals that is learned and that improves with practice. Vocal practice is crucial early in life to perfect a song imitation, but it also occurs throughout life and may continue to improve aspects of song performance. Differences in song performance among males that share song types, that is sing structurally similar songs may be particularly salient to receivers. We here test the hypothesis that aspects of song performance improve in a songbird species that deletes song types from its repertoire early in the first breeding season to share their final single song type with territorial neighbours. Over 3 yrs, we recorded songs in a population of Puget Sound white‐crowned sparrows Zonotrichia leucophrys pugetensis and measured percentage peak performance and consistency thereof in all of the song types in each male's repertoire. We found that within the first year on territory, percentage peak performance was higher in shared than unshared songs but did not change from first to second recording. Contrary to the hypothesis that song performance improves with age, song performance declined from the first to the second year. Our results support the hypothesis that high‐performance singers share songs. We did not find support for song performance improving within or between years, like it does in some other songbird species.     

Duetting behavior in a Neotropical ovenbird: sexual and seasonal variation and adaptive signaling functions

Duetting is a collective behavior and might have multiple functions, including joint territory defense and mate guarding. An important step toward understanding the adaptive function of bird song is to determine if and how singing behavior varies seasonally. However, seasonal patterns for duetting species are different from the pattern described for species in which only the male sings, because song function may vary according to sex, singing role (initiator vs responder) and level of duet organization (individual vs pair). We investigated whether patterns of seasonal variation in duetting depends on these factors, which would suggest different interpretations of song function. We studied social pairs of a Neotropical bird species (rufous hornero Furnarius rufus) for seven consecutive months, recording vocal and territorial behaviors. Overall, partners coordinated 61% of their songs into duets and many song traits (song initiation rate, song output and duet rate) peaked in territorial contexts. Males engaged in territorial interactions with strangers more often, initiated more songs, and answered proportionately more of their partners’ songs than females. Male song initiation rate peaked during the pre‐ and post‐breeding stages, whereas females initiated more songs during the non‐breeding season. Both sexes answered partner songs faster and at higher rates during the pre‐breeding and female fertile stages. Partners duetted at a higher rate during the pre‐ and post‐breeding stages. Finally, song initiation rates and duet rate, but not song answering rates, correlated with frequency of territorial interactions with strangers. Although our findings indicate that song function may vary with sex, singing role and level of duet organization, our results suggest that in general duet functions to defend common territories and as a mutual mate guarding strategy in the rufous hornero.     

Blackcaps (Sylvia atricapilla) increase the whistle part of their song in response to simulated territorial intrusion

Bird song is a sexually selected signal that serves two main functions, attracting a mate and deterring rivals. Different signal parameters may be important in advertising to females compared to advertising to rival males. Species solve the problem of this dual function in a variety of ways, one of which may be to have separate parts of song directed at male and female receivers. The blackcap song has two distinct parts, a complex warble, assumed to be directed at female receivers, followed by a louder and more stereotyped whistle putatively directed at males. We simulated territorial intrusions by broadcasting blackcap song in territories. Comparing songs sung prior, with those produced in response to playback, showed that the proportion of the whistle component of songs increased, but not the warble. This study thus provides empirical evidence that the whistle component of the blackcap song plays a prominent role in male–male competition. The warble component of the blackcap song may be directed at females, but this requires further testing.     

Do Blackbirds Signal Motivation to Fight with Their Song?

Conflicts about resources can be solved with physical fights, but animals will in general try to avoid them due to the costs and risks associated with fighting. Fight outcome depends on the fighting ability as well as the motivation to fight, which means that individuals should try to assess both during a pre‐escalation phase. In the present study, we investigated whether blackbirds (Turdus merula) acoustically signal their motivation to fight with their advertisement song, which consists of a motif part with pure‐toned low frequency elements followed by a twitter part with more complex and higher‐pitched elements. We tested (1) whether the temporal and spectral characteristics of spontaneous songs broadcasted by a territorial male could predict its subsequent aggressive behaviour to a playback and (2) if the song of territorial males changed in case they responded aggressively to the playback. We found no effects in the temporal song structure, which contradicts results from a previous study involving playback experiments with blackbirds. We did however find an interesting pattern in the spectral characteristics of the twitter, since aggressively responding males significantly increased the twitter frequency after the playback. The post‐playback songs of aggressive responders were nevertheless indistinguishable from spontaneous pre‐playback songs of non‐aggressive responders. Therefore, we conclude that changes in twitter frequency, rather than absolute twitter frequencies, are advertising information about motivation to fight.     

ASYMMETRIC RESPONSE PATTERNS TO SUBSPECIES‐SPECIFIC SONG DIFFERENCES IN ALLOPATRY AND PARAPATRY IN THE GRAY‐BREASTED WOOD‐WREN

Song divergence between closely related taxa may play a critical role in the evolutionary processes of speciation and hybridization. We explored song variation between two Ecuadorian subspecies of the gray‐breasted wood‐wren (Henicorhina leucophrys) and tested the impact of song divergence on response behaviors. Songs were significantly different between the two subspecies, even between two parapatric populations 10 km apart. Playback experiments revealed an asymmetric response pattern to these divergent subspecies specific songs; one subspecies responded more to songs of its own subspecies than to the other subspecies’ songs, whereas the second responded equally strongly to songs of both subspecies. While song parameters revealed a mixed pattern of divergence between allopatric and parapatric populations, the majority of spectral characteristics showed increased divergence in parapatry, suggestive of character displacement. This increased song divergence in parapatry appeared to affect behavioral responses to playback as discriminating responses were most prominent in parapatry and against parapatric songs. The clear behavioral impact of subspecies‐specific song differences supports a potential role for song as an acoustic barrier to gene flow. The asymmetric nature of the responses suggests that song divergence could affect the direction of gene flow and the position of the subspecies‐specific transition.     

Imitation and Invention in Song Learning in Nightingales (Luscinia megarhynchos B., Turdidae)

Song learning in oscine birds is often defined solely as a process of song imitation; nonetheless, not all songs produced by laboratory‐tutored birds are imitations of the model songs. If song learning were strictly a process of imitation, these non‐imitated songs (inventions) would be expected to contain no learned attributes. To determine whether species‐typical song attributes can be learned in the absence of imitation, we compared the imitations and inventions of laboratory‐tutored nightingales (Luscinia megarhynchos B.) with the songs of wild nightingales and the songs of laboratory‐reared, untutored nightingales. The species‐typical song attribute measured was stereotypy. We quantified stereotypy by four variables: (1) percentage of notes shared between two renditions of the same song type (2) difference in repetition rates of the same trill in two renditions of the same song type (3) acoustic similarity of the same note in two renditions of the same song type, and (4) acoustic similarity of the same note repeated within a trill. Wild songs and imitated songs were significantly more stereotypical than the songs of untutored birds for all measures. For the percentage of notes shared (1), and the acoustic similarity of notes in two renditions of the same song type (3), invented songs did not differ from the songs of untutored birds, suggesting that imitation is necessary for the acquisition of these song characteristics. However, invented songs were significantly more stereotypical than the songs of untutored birds for measures of stereotypy related to trills (2 and 4), and neither imitated nor invented songs differed significantly from the songs of wild birds in terms of trill rate stereotypy (2). Thus, it appears that the process of learning to produce trills may differ from the process of learning non‐repetitive song components: increased stereotypy in trills occurs even when the trills themselves are not copied from song models. Strict imitation does not fully account for the acquisition of some learned song attributes.     

Song learning by prairie warblers: When,where, and from whom

Songbirds of many species acquire their songs by imitating the songs of conspecific singers. Conclusive evidence of such imitation comes from controlled laboratory studies, but such studies do not reveal when and where songbirds learn their songs under natural conditions. To determine the timing and location of song learning in a population of prairie warblers, we compared the songs of yearling prairie warblers of known hatching location to the songs of other birds in the yearlings' natal and first breeding areas. The comparisons yielded a likely model song (and model singer) for each of the song types used by the focal yearlings. We supplemented our findings from the song comparisons with inferences drawn from an analysis of local geographic variation in songs. This analysis revealed that shared song types showed no tendency to be geographically clustered within the study area. Taken together, our data suggest that prairie warblers learn their songs during the hatch year, at locations somewhat distant (mean distance 1,437 m) from their natal site, most likely as birds wander about during the post-fledging period.     

Testing alternative hypotheses for evolutionary diversification in an African songbird: rainforest refugia versus ecological gradients

Geographic isolation in rainforest refugia and local adaptation to ecological gradients may both be important drivers of evolutionary diversification. However, their relative importance and the underlying mechanisms of these processes remain poorly understood because few empirical studies address both putative processes in a single system. A key question is to what extent is divergence in signals that are important in mate and species recognition driven by isolation in rainforest refugia or by divergent selection across ecological gradients? We studied the little greenbul, Andropadus virens, an African songbird, in Cameroon and Uganda, to determine whether refugial isolation or ecological gradients better explain existing song variation. We then tested whether song variation attributable to refugial or ecological divergence was biologically meaningful using reciprocal playback experiments to territorial males. We found that much of the existing song variation can be explained by both geographic isolation and ecological gradients, but that divergence across the gradient, and not geographic isolation, affects male response levels. These data suggest that ecologically divergent traits, independent of historical isolation during glacial cycles, can promote reproductive isolation. Our study provides further support for the importance of ecology in explaining patterns of evolutionary diversification in ecologically diverse regions of the planet.