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1.  The deterrent effect of bird song in territory defense 
Behavioral Ecology  2008;20(1):200-206.
Using the responses of territory owners to playback to infer the territorial function of acoustic signals is common practice. However, difficulties with interpreting the results of such experiments have obscured our understanding of territorial signalling. For instance, a stronger response to playback is often interpreted as more aggressive, but there is no consensus as to whether this should be in response to the least or most threatening simulated intruder. Rather than following a gradual increase or decrease, the relationship between signal intensity and response strength may instead describe a peaked curve. We manipulated banded wren (Thryophilus pleurostictus) songs to simulate low-, median-, and high-performance singers and used these songs as stimuli in playback experiments. Banded wrens were less likely to approach the high-performance stimulus compared with the low- and median-performance stimuli. However, the birds that did approach the high-performance stimulus sang more than those that approached the low-performance stimulus. In addition, birds were more likely to match the songs when exposed to the median- and high-performance stimuli compared with the low-performance stimuli, and song matching predicted approach behavior. These results are in accordance with theoretical models of aggressive encounters in which low-performance opponents are challenged without further assessment. Median- and high-performance opponents, however, may require further assessment, and the latter may be perceived as too intimidating for approach.
doi:10.1093/beheco/arn135
PMCID: PMC2662740  PMID: 19337589
assessment; playback; sexual selection; song; territory defense
2.  Song matching, overlapping, and switching in the banded wren: the sender’s perspective 
Interpreting receiver responses to on-territory playback of aggressive signals is problematic. One solution is to combine such receiver-perspective experiments with a sender-perspective experiment that allows subjects to demonstrate how their choice of singing strategies is associated with their approach behavior. Here we report the results of a sender-perspective study on the banded wren (Thryothorus pleurostictus), and combine information on context and results of previous receiver-perspective experiments to clarify function. Territorial males were presented with a 5-min playback consisting of song types present in their repertoire. We assessed the degree to which the subjects’ song matching rate, overlapping rate, and song-type versatility were correlated with their approach latency, closeness of approach, latency to first retreat, and time spent close to the speaker. Male age, breeding stage, and features of the playback stimuli were also considered. Song matching was associated with rapid and close approach, consistent with the receiver-perspective interpretation of type matching as a conventional signal of aggressive motivation. Overlapping was associated with earlier retreat, and together with the aversive receiver response to our previous overlapping playback experiment suggests that overlapping is a defensive withdrawal signal. High versatility was associated with slower first retreat from the speaker and high levels of reciprocal matching between subject and playback. Males with fledglings sang with particularly low versatility and approached the speaker aggressively, whereas males with nestlings overlapped more and retreated quickly. Finally, older males matched more but overlapped less.
PMCID: PMC2288572  PMID: 18392112
Aggressive motivation; playback design; path analysis; signal function; singing versatility; song-type repertoires
3.  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.
doi:10.1098/rspb.2001.1757
PMCID: PMC1088843  PMID: 11571048
4.  Singing in the face of death: male banded wrens sing more to playback in their last breeding season 
Journal of avian biology  2009;40(2):10.1111/j.1600-048X.2009.04540.x.
Investment in signalling is subject to multiple trade-offs that vary with life-stage, leading to a complex relationship between survival and trait expression. We show a negative relationship between survival and song rate in response to simulated territorial intrusion in male banded wrens (Thryophilus pleurostictus), and test several explanations for this association. (1) Male age failed to explain the association: though age affected song rate in a cross-sectional analysis, longitudinal analysis showed that individuals did not increase their song rate as they got older. Reconciling these results suggests differential selection against young males that respond to intrusion with low song rates. (2) Mortality costs of high song rates did not appear to explain the negative relationship between song rate and survival because, though song rate in response to playback was condition-dependent, high song rates in a different context did not appear to impose mortality costs. (3) High levels of territorial pressure may have increased mortality, but were not associated with high song rates in response to playback. (4) Since song rates did not increase with age, but tended to increase only in the last year of life, we tentatively suggest that the negative relationship between song rate and survival could represent a terminal investment in territorial defence by males in their final breeding season, though further work is needed to confirm this conclusion.
doi:10.1111/j.1600-048X.2009.04540.x
PMCID: PMC3858851  PMID: 24347819
birdsong; life-history; sexual selection; terminal investment
5.  Overlapping signals in banded wrens: long-term effects of prior experience on males and females 
We tested the signal value of song overlapping in banded wrens (Thryothorus pleurostictus), using interactive playback to either overlap or alternate with their songs. Males shortened song duration and decreased variability in song length when their songs were overlapped by playback, suggesting that they were attempting to avoid being overlapped and perhaps being less aggressive. A novel finding was an effect of long-term prior experience: song lengths remained relatively short in alternating trials that followed two or more days after overlapping trials. Approach responses to the two treatments did not differ overall, but there was a parallel effect of prior experience: males tended to stay further from the speaker during alternating treatments if they had previously been overlapped by playback. Some females paired to the male subjects sang in response to playback and were also influenced by prior experience, singing more during alternating trials that had not been preceded by an overlapping trial. Male overlappers may signal dominance over a rival to other male or female receivers in a communication network, but it is currently unclear whether overlapping indicates motivation to escalate an aggressive interaction, or whether this singing strategy is related to male quality. Banded wrens are long-lived and maintain year-round territories, so modifying responses to rivals based on prior experience is likely to be important for success.
doi:10.1093/beheco/arj022
PMCID: PMC1391916  PMID: 16523216
song overlap; interactive signals; communication networks; male quality; female song
6.  Female happy wrens select songs to cooperate with their mates rather than confront intruders 
Biology Letters  2013;9(1):20120863.
Vocal duetting occurs in many taxa, but its function remains much-debated. Like species in which only one sex sings, duetting birds can use their song repertoires to signal aggression by singing song types that match those of territorial intruders. However, when pairs do not share specific combinations of songs (duet codes), individuals must choose to signal aggression by matching the same-sex rival, or commitment by replying appropriately to their mate. Here, we examined the song types used by female happy wrens (Pheugopedius felix) forced to make this decision in a playback experiment. We temporarily removed the male from the territory and then played songs from two loudspeakers to simulate an intruding female and the removed mate's response, using song types that the pair possessed but did not naturally combine into duets. Females were aggressive towards the female playback speaker, approaching it and overlapping the female playback songs, but nevertheless replied appropriately to their mate's songs instead of type matching the intruding female. This study indicates that females use song overlapping to signal aggression but use their vocal repertoires to create pair-specific duet codes with their mates, suggesting that duetting functions primarily to demonstrate pair commitment.
doi:10.1098/rsbl.2012.0863
PMCID: PMC3565499  PMID: 23097462
duetting; song matching; happy wren; territory defence; cooperation
7.  Trill performance components vary with age, season, and motivation in the banded wren 
Acoustic displays with difficult-to-execute sounds are often subject to strong sexual selection, because performance levels are related to the sender’s condition or genetic quality. Performance may also vary with age, breeding stage, and motivation related to social context. We focused on within-male variation in four components of trill performance in banded wren (Thryophilus pleurostictus) songs: note consistency, frequency bandwidth, note rate and vocal deviation. The latter is a composite measure reflecting deviation from the performance limit on simultaneously maximizing both frequency bandwidth and note rate. We compared the changes in these song parameters at three time scales: over the course of years, across the breeding season, and at different times of the day with contrasting agonistic contexts. Vocal deviation decreased and note consistency increased over years, suggesting that experience may improve individual proficiency at singing trills. Consistency also increased across the season, confirming that practice is important for this parameter. Although there was no significant seasonal change in vocal deviation, one of its components, note rate, increased during the season. Neither vocal deviation nor consistency varied with agonistic context. However, note rate increased during playback experiments simulating territorial intrusions compared to dawn chorus singing. The magnitude of a male’s increase in note rate was positively correlated with his aggressive behavior during the playback experiment. Thus consistency, bandwidth, and vocal deviation indicate age, whereas trill rate flexibly indicates the singer’s aggressive motivation. We also found evidence of a within-male trade-off between vocal deviation and consistency.
doi:10.1007/s00265-012-1461-x
PMCID: PMC3608479  PMID: 23543812
motor performance; vocal deviation; trill note consistency; frequency bandwidth; trade-off; song quality; Thryophilus pleurostictus
8.  Host response to cuckoo song is predicted by the future risk of brood parasitism 
Frontiers in Zoology  2013;10:30.
Introduction
Risk assessment occurs over different temporal and spatial scales and is selected for when individuals show an adaptive response to a threat. Here, we test if birds respond to the threat of brood parasitism using the acoustical cues of brood parasites in the absence of visual stimuli. We broadcast the playback of song of three brood parasites (Chalcites cuckoo species) and a sympatric non-parasite (striated thornbill, Acanthiza lineata) in the territories of superb fairy-wrens (Malurus cyaneus) during the peak breeding period and opportunistic breeding period. The three cuckoo species differ in brood parasite prevalence and the probability of detection by the host, which we used to rank the risk of parasitism (high risk, moderate risk, low risk).
Results
Host birds showed the strongest response to the threat of cuckoo parasitism in accordance with the risk of parasitism. Resident wrens had many alarm calls and close and rapid approach to the playback speaker that was broadcasting song of the high risk brood parasite (Horsfield’s bronze-cuckoo, C. basalis) across the year (peak and opportunistic breeding period), some response to the moderate risk brood parasite (shining bronze-cuckoo, C. lucidus) during the peak breeding period, and the weakest response to the low risk brood parasite (little bronze-cuckoo, C. minutillus). Playback of the familiar control stimulus in wren territories evoked the least response.
Conclusion
Host response to the threat of cuckoo parasitism was assessed using vocal cues of the cuckoo and was predicted by the risk of future parasitism.
doi:10.1186/1742-9994-10-30
PMCID: PMC3666891  PMID: 23692969
Cuckoo recognition; Cuckoo threat; Risk perception; Experience; Song discrimination; Deterrent behaviour
9.  Vocal performance influences male receiver response in the banded wren 
In a variety of songbirds the production of trilled song elements is constrained by a performance tradeoff between how fast a bird can repeat trill units (trill rate) and the range of frequencies each unit can span (frequency bandwidth). High-performance trills serve as an assessment signal for females, but little is known about the signal value of vocal performance for male receivers. We investigated the relationship between trill rate and frequency bandwidth in banded wren (Thryothorus pleurostictus) songs. Trilled song elements showed the same performance tradeoff found in other passerines and individuals differed in performance of some trill types. We tested the hypothesis that males of this species assess each other based on trill performance with a two-speaker experiment, in which territory owners were presented with alternating renditions of the same song type manipulated to differ in trill rate. Subjects were significantly more likely to approach the faster trill stimulus first. However, subjects that received trill types closer to the performance limit spent less time close to the fast speaker. Our results show that male banded wrens discriminate and respond differently to songs based on their vocal performance. Thus, performance of physically challenging songs may thus be important in intra- as well as intersexual assessment.
doi:10.1098/rspb.2006.3535
PMCID: PMC1634778  PMID: 16822751
bird song; sexual selection; trill rate; Thryothorus pleurostictus; performance limit; acoustic playback
10.  Vocal performance influences male receiver response in the banded wren 
In a variety of songbirds the production of trilled song elements is constrained by a performance tradeoff between how fast a bird can repeat trill units (trill rate) and the range of frequencies each unit can span (frequency bandwidth). High-performance trills serve as an assessment signal for females, but little is known about the signal value of vocal performance for male receivers. We investigated the relationship between trill rate and frequency bandwidth in banded wren (Thryothorus pleurostictus) songs. Trilled song elements showed the same performance tradeoff found in other passerines and individuals differed in performance of some trill types. We tested the hypothesis that males of this species assess each other based on trill performance with a two-speaker experiment, in which territory owners were presented with alternating renditions of the same song type manipulated to differ in trill rate. Subjects were significantly more likely to approach the faster trill stimulus first. However, subjects that received trill types closer to the performance limit spent less time close to the fast speaker. Our results show that male banded wrens discriminate and respond differently to songs based on their vocal performance. Thus, performance of physically challenging songs may be important in intra- as well as inter-sexual assessment.
doi:10.1098/rspb.2006.3535
PMCID: PMC1634778  PMID: 16822751
bird song; sexual selection; trill rate; Thryothorus pleurostictus; performance limit; acoustic playback
11.  Is song-type matching a conventional signal of aggressive intentions? 
Song-type matching is a singing strategy found in some oscine songbirds with repertoires of song types and at least partial sharing of song types between males. Males reply to the song of a rival male by subsequently singing the same song type. For type matching to serve as an effective long-distance threat signal, it must be backed up by some probability of aggressive approach and impose some type of cost on senders that minimizes the temptation to bluff. Western subspecies of the song sparrow exhibit moderate levels of song-type sharing between adjacent males and sometimes type match in response to playback of song types they possess in their repertoires. Interactive playback experiments were used in order to examine the subsequent behaviour of type-matching birds and to quantify the responses of focal birds to type-matching versus non-matching stimuli. Birds that chose to type match the playback of a shared song type subsequently approached the speaker much more aggressively than birds that did not type match. Moreover, birds approached a type-matching stimulus much more aggressively than a non-matching stimulus. These results and consideration of alternatives suggest that type matching in song sparrows is a conventional signal in which honesty is maintained by a receiver retaliation cost against bluffers.
doi:10.1098/rspb.2001.1714
PMCID: PMC1088788  PMID: 11487412
12.  Song types and their structural features are associated with specific contexts in the banded wren 
Animal behaviour  2005;70(4):921-935.
We studied the use of song types and their acoustic features in different social contexts in the banded wren (Thryothorus pleurostictus), a resident tropical songbird in which males possess about 20 distinctive song types varying in duration, bandwidth, note composition, and trill structure. We recorded six focal males intensively for four days each while we observed context information such as during versus after dawn chorus, presence of the female, counter-versus solo-singing, location at the edge versus centre of the territory, and proximity to the nest. All males used at least some song types differentially during each of these pairs of alternative contexts. Males also preferentially used the song types they shared with a given neighbour when interacting with that bird. Songs delivered during dawn chorus were significantly longer, wider in bandwidth, often compound (double songs), and more likely to contain a rattle or buzz and an up-sweeping trill, compared to songs delivered after dawn chorus. Similar features were also more commonly observed when birds were engaged in intense male-male interactions and boundary disputes after dawn chorus, especially when countersinging at the edge of the territory. The presence of the female caused the male to deliver song types with narrower whole-song and trill bandwidth and fewer rattles and buzzes, and song-type diversity and fraction of compound songs were higher when the female was present. Thus, in addition to using type matching and variations in song-type switching and diversity to signal different levels of aggressive intention, male banded wrens also select song types based on their acoustic structure in different social contexts.
doi:10.1016/j.anbehav.2005.02.004
PMCID: PMC1702368  PMID: 17173097
13.  Physically Challenging Song Traits, Male Quality, and Reproductive Success in House Wrens 
PLoS ONE  2013;8(3):e59208.
Physically challenging signals are likely to honestly indicate signaler quality. In trilled bird song two physically challenging parameters are vocal deviation (the speed of sound frequency modulation) and trill consistency (how precisely syllables are repeated). As predicted, in several species, they correlate with male quality, are preferred by females, and/or function in male-male signaling. Species may experience different selective pressures on their songs, however; for instance, there may be opposing selection between song complexity and song performance difficulty, such that in species where song complexity is strongly selected, there may not be strong selection on performance-based traits. I tested whether vocal deviation and trill consistency are signals of male quality in house wrens (Troglodytes aedon), a species with complex song structure. Males’ singing ability did not correlate with male quality, except that older males sang with higher trill consistency, and males with more consistent trills responded more aggressively to playback (although a previous study found no effect of stimulus trill consistency on males’ responses to playback). Males singing more challenging songs did not gain in polygyny, extra-pair paternity, or annual reproductive success. Moreover, none of the standard male quality measures I investigated correlated with mating or reproductive success. I conclude that vocal deviation and trill consistency do not signal male quality in this species.
doi:10.1371/journal.pone.0059208
PMCID: PMC3602011  PMID: 23527137
14.  Simulated Birdwatchers’ Playback Affects the Behavior of Two Tropical Birds 
PLoS ONE  2013;8(10):e77902.
Although recreational birdwatchers may benefit conservation by generating interest in birds, they may also have negative effects. One such potentially negative impact is the widespread use of recorded vocalizations, or “playback,” to attract birds of interest, including range-restricted and threatened species. Although playback has been widely used to test hypotheses about the evolution of behavior, no peer-reviewed study has examined the impacts of playback in a birdwatching context on avian behavior. We studied the effects of simulated birdwatchers’ playback on the vocal behavior of Plain-tailed Wrens Thryothorus euophrys and Rufous Antpittas Grallaria rufula in Ecuador. Study species’ vocal behavior was monitored for an hour after playing either a single bout of five minutes of song or a control treatment of background noise. We also studied the effects of daily five minute playback on five groups of wrens over 20 days. In single bout experiments, antpittas made more vocalizations of all types, except for trills, after playback compared to controls. Wrens sang more duets after playback, but did not produce more contact calls. In repeated playback experiments, wren responses were strong at first, but hardly detectable by day 12. During the study, one study group built a nest, apparently unperturbed, near a playback site. The playback-induced habituation and changes in vocal behavior we observed suggest that scientists should consider birdwatching activity when selecting research sites so that results are not biased by birdwatchers’ playback. Increased vocalizations after playback could be interpreted as a negative effect of playback if birds expend energy, become stressed, or divert time from other activities. In contrast, the habituation we documented suggests that frequent, regular birdwatchers’ playback may have minor effects on wren behavior.
doi:10.1371/journal.pone.0077902
PMCID: PMC3797570  PMID: 24147094
15.  Song type matching is an honest early threat signal in a hierarchical animal communication system 
Aggressive encounters between animals often involve significant amounts of signalling before or in lieu of physical fights. When, as is often the case, these apparent threat signals are neither inherently costly nor inherently indicative of fighting ability, we should ask whether they are in fact honest signals, i.e. do they predict that escalation is imminent? While signalling theories have indicated that such ‘conventional’ threat signals can honestly predict escalation, attempts to gather supporting empirical evidence have mostly failed. For example, recent studies in songbirds of song type matching (replying to an opponent's song with the same song type he has just sung) have failed to confirm that it predicts an eventual attack by the signaller. In the present study of song sparrows (Melospiza melodia), we tested the hypothesis that song type matching is an early threat signal in a hierarchical signalling system. We used an improved model-playback design that simulated an escalating intrusion onto the subject's territory: the simulated opponent first sang in hiding from the boundary before moving to the centre of the territory, where he revealed himself and continued to sing. We found that type matching beginning in the boundary phase and continuing into the escalation phase, or beginning immediately after the escalation, reliably predicted both subsequent escalated signalling (soft songs and wing waves) and subsequent attack on the model, supporting the hypothesis that type matching is a reliable early threat signal.
doi:10.1098/rspb.2012.2517
PMCID: PMC3574360  PMID: 23378665
animal communication; honest signals; bird song; aggression; song sparrow
16.  Testosterone Affects Song Modulation during Simulated Territorial Intrusions in Male Black Redstarts (Phoenicurus ochruros) 
PLoS ONE  2012;7(12):e52009.
Although it has been suggested that testosterone plays an important role in resource allocation for competitive behavior, details of the interplay between testosterone, territorial aggression and signal plasticity are largely unknown. Therefore, we investigated if testosterone acts specifically on signals that communicate the motivation or ability of individuals to engage in competitive situations in a natural context. We studied the black redstart, a territorial songbird species, during two different life-cycle stages, the early breeding phase in spring and the non-breeding phase in fall. Male territory holders were implanted with the androgen receptor blocker flutamide (Flut) and the aromatase inhibitor letrozole (Let) to inhibit the action of testosterone and its estrogenic metabolites. Controls received a placebo treatment. Three days after implantation birds were challenged with a simulated territorial intrusion (STI). Song was recorded before, during and after the challenge. In spring, both treatment groups increased the number of elements sung in parts of their song in response to the STI. However, Flut/Let-implanted males reacted to the STI with a decreased maximum acoustic frequency of one song part, while placebo-implanted males did not. Instead, placebo-implanted males sang the atonal part of their song with a broader frequency range. Furthermore, placebo-, but not Flut/Let-implanted males, sang shorter songs with shorter pauses between parts in the STIs. During simulated intrusions in fall, when testosterone levels are naturally low in this species, males of both treatment groups sang similar to Flut/Let-implanted males during breeding. The results suggest that song sung during a territorial encounter is of higher competitive value than song sung in an undisturbed situation and may, therefore, convey information about the motivation or quality of the territory holder. We conclude that testosterone facilitates context-dependent changes in song structures that may be honest signals of male quality in black redstarts.
doi:10.1371/journal.pone.0052009
PMCID: PMC3524101  PMID: 23284852
17.  Responses to song playback vary with the vocal performance of both signal senders and receivers 
Mating signals of many animal species are difficult to produce and thus should indicate signaler quality. Growing evidence suggests that receivers modulate their behaviour in response to signals with varying performance levels, although little is known about if and how responses are affected by receiver attributes. To explore this topic we conducted two experiments with swamp sparrows, Melospiza georgiana, in which we challenged territorial males with playback of songs with trill rates that were natural, digitally reduced, or digitally elevated (control-, low- and high-performance stimuli, respectively). In our first experiment, we found that males responded more aggressively to control songs than to low-performance stimuli, that low-performance stimuli with the most severe trill-rate reductions elicited the weakest aggressive responses, and that the subjects' own trill rates predicted aggressive responses. In our second experiment, we found that male responses to high-performance stimuli varied significantly, in ways predicted by two factors: the degree to which we had elevated stimulus performance levels of high-performance stimuli, and subjects' own vocal performance levels. Specifically, males were less aggressive towards stimuli for which we had elevated performance levels to higher degrees, and subject males with higher vocal performances themselves responded more aggressively. These findings together offer a novel illustration of how responses to aggressive signals may rely not just on signal attributes, but also on attributes of responding animals themselves.
doi:10.1098/rspb.2013.1401
PMCID: PMC3757973  PMID: 23945685
sexual selection; competition; performance; soft song; individual differences
18.  Effects of personality on territory defence in communication networks: a playback experiment with radio-tagged great tits 
Individuals often differ consistently in behaviour across time and contexts, and such consistent behavioural differences are commonly described as personality. Personality can play a central role in social behaviour both in dyadic interactions and in social networks. We investigated whether explorative behaviour, as proxy of personality of territorial male great tits (Parus major), predicts their own and their neighbours' territorial responses towards simulated intruders. Several weeks prior to playback, subjects were taken from the wild to test their exploratory behaviour in a standard context in the laboratory. Exploratory behaviour provides a proxy of personality along a slow–fast explorer continuum. Upon release, males were radio-tracked and subsequently exposed to interactive playback simulating a more or a less aggressive territorial intruder (by either overlapping or alternating broadcast songs with the subjects' songs). At the same time, we radio-tracked a neighbour of the playback subject. Male vocal responses during playback and spatial movements after playback varied according to male explorative behaviour and playback treatment. Males with lower exploration scores approached the loudspeaker less, and sang more songs, shorter songs and songs with slower element rates than did males with higher exploration scores. Moreover, neighbour responses were related to the explorative behaviour of the subject receiving the playback but not to their own explorative behaviour. Our overall findings reveal for the first time how personality traits affect resource defence within a communication network providing new insights on the cause of variation in resource defence behaviour.
doi:10.1098/rspb.2010.0598
PMCID: PMC2982238  PMID: 20591859
personality; territorial signalling; radio-tracking; interactive playback; communication networks
19.  Flight Phases in the Song of Skylarks: Impact on Acoustic Parameters and Coding Strategy 
PLoS ONE  2013;8(8):e72768.
Skylarks inhabit open fields and perform an aerial song display which serves as a territorial signal. The particularly long and elaborate structure of this song flight raises questions about the impact of physical and energetic constraints acting on a communication signal. Song produced during the three distinct phases of the flight - ascending, level and descending phase could be subject to different constraints, serve different functions and encode different types of information. We compared song parameters during the ascending and the level phases. We found that the structure of the song varied with the phase of the flight. In particular, song had a higher tempo when skylarks were ascending which might be related to higher oxygen and energetic demands. We also explored which phase of the song flight might encode individuality. Earlier studies reported that skylarks reduced their territorial response to established neighbours if the neighbour song was broadcasted from the correct adjacent boundary, but reacted aggressively if the neighbour songs were broadcasted from an incorrect boundary (mimicking a displaced neighbour). Such differential response provides some evidence for individual recognition. Here, we exposed subjects to playback stimuli of neighbour song in which we had replaced either the song produced during the level or the ascending phase by the relevant song of the neighbour from the incorrect border. Singing response was higher towards stimuli in which the ‘level phase song’ was replaced, indicating that skylarks could be able to recognise their neighbours based on song of this phase. Thus, individuality seems to be primarily coded in the level phase of the flight song.
doi:10.1371/journal.pone.0072768
PMCID: PMC3744454  PMID: 23967338
20.  Escalation of aggressive vocal signals: a sequential playback study 
Rival conspecifics often produce stereotyped sequences of signals as agonistic interactions escalate. Successive signals in sequence are thought to convey increasingly pronounced levels of aggressive motivation. Here, we propose and test a model of aggressive escalation in black-throated blue warblers, presenting subjects with two sequential and increasingly elevated levels of threat. From a speaker outside the territorial boundary, we initiated an interaction (low-threat level), and from a second speaker inside the territory, accompanied by a taxidermic mount, we subsequently simulated a territorial intrusion (escalated threat level). Our two main predictions were that signalling behaviours in response to low-threat boundary playback would predict signalling responses to the escalated within-territory threat, and that these latter signalling behaviours would in turn reliably predict attack. We find clear support for both predictions: (i) specific song types (type II songs) produced early in the simulated interaction, in response to boundary playback, predicted later use of low-amplitude ‘soft’ song, in response to within-territory playback; and (ii) soft song, in turn, predicted attack of the mount. Unexpectedly, use of the early-stage signal (type II song) itself did not predict attack, despite its apparent role in aggressive escalation. This raises the intriguing question of whether type II song can actually be considered a reliable aggressive signal. Overall, our results provide new empirical insights into how songbirds may use progressive vocal signalling to convey increasing levels of threat.
doi:10.1098/rspb.2013.1553
PMCID: PMC3757984  PMID: 23926156
aggressive signalling; birdsong; black-throated blue warbler; escalation; Setophaga caerulescens; soft song
21.  Social Context–Induced Song Variation Affects Female Behavior and Gene Expression 
PLoS Biology  2008;6(3):e62.
Social cues modulate the performance of communicative behaviors in a range of species, including humans, and such changes can make the communication signal more salient. In songbirds, males use song to attract females, and song organization can differ depending on the audience to which a male sings. For example, male zebra finches (Taeniopygia guttata) change their songs in subtle ways when singing to a female (directed song) compared with when they sing in isolation (undirected song), and some of these changes depend on altered neural activity from a specialized forebrain-basal ganglia circuit, the anterior forebrain pathway (AFP). In particular, variable activity in the AFP during undirected song is thought to actively enable syllable variability, whereas the lower and less-variable AFP firing during directed singing is associated with more stereotyped song. Consequently, directed song has been suggested to reflect a “performance” state, and undirected song a form of vocal motor “exploration.” However, this hypothesis predicts that directed–undirected song differences, despite their subtlety, should matter to female zebra finches, which is a question that has not been investigated. We tested female preferences for this natural variation in song in a behavioral approach assay, and we found that both mated and socially naive females could discriminate between directed and undirected song—and strongly preferred directed song. These preferences, which appeared to reflect attention especially to aspects of song variability controlled by the AFP, were enhanced by experience, as they were strongest for mated females responding to their mate's directed songs. We then measured neural activity using expression of the immediate early gene product ZENK, and found that social context and song familiarity differentially modulated the number of ZENK-expressing cells in telencephalic auditory areas. Specifically, the number of ZENK-expressing cells in the caudomedial mesopallium (CMM) was most affected by whether a song was directed or undirected, whereas the caudomedial nidopallium (NCM) was most affected by whether a song was familiar or unfamiliar. Together these data demonstrate that females detect and prefer the features of directed song and suggest that high-level auditory areas including the CMM are involved in this social perception.
Author Summary
Vocal communication in many species, including humans, is affected by social cues. In the zebra finch, for example, males make subtle changes to the length, tempo, and variability of their courtship songs (directed songs) relative to songs performed in isolation (undirected songs). Using a behavioral approach assay, we found that female zebra finches strongly prefer the sound of directed over undirected song. Interestingly, female preferences were influenced by the variability of note pitch, showing stronger preferences for directed songs when they were less variable in pitch than the undirected songs. Pitch variability is controlled by a forebrain–basal ganglia circuit, which may represent a neural substrate on which selection acts to shape behavior. Preference for directed song was also increased when the singer was familiar to the listener, suggesting that song preferences are enhanced by experience. Based on the expression of an immediate early gene associated with memory formation and plasticity, we found that two high-level auditory areas were differentially responsive to the category of song females heard, with one area responding to whether songs were directed or undirected, and a second area to whether songs were familiar or unfamiliar. Together, these data demonstrate that females detect and prefer the male's changed performance during courtship singing and suggest that neurons in high-level auditory areas are involved in this social perception.
Female songbirds are attentive to subtle changes in male song, particularly to the variability of pitch. High-level auditory areas may generate females' behavioral preferences.
doi:10.1371/journal.pbio.0060062
PMCID: PMC2267820  PMID: 18351801
22.  Song environment affects singing effort and vasotocin immunoreactivity in the forebrain of male Lincoln’s sparrows 
Hormones and behavior  2010;58(3):544-553.
Male songbirds often establish territories and attract mates by singing, and some song features can reflect the singer’s condition or quality. The quality of the song environment can change, so male songbirds should benefit from assessing the competitiveness of the song environment and appropriately adjusting their own singing behavior and the neural substrates by which song is controlled. In a wide range of taxa social modulation of behavior is partly mediated by the arginine vasopressin or vasotocin (AVP/AVT) systems. To examine the modulation of singing behavior in response to the quality of the song environment we compared the song output of laboratory-housed male Lincoln’s sparrows (Melospiza lincolnii) exposed to one week of chronic playback of songs categorized as either high or low quality, based on song length, complexity and trill performance. To explore the neural basis of any facultative shifts in behavior, we also quantified the subjects’ AVT immunoreactivity (AVT-IR) in three forebrain regions that regulate socio-sexual behavior: the medial bed nucleus of the stria terminalis (BSTm), the lateral septum (LS) and the preoptic area. We found that high quality songs increased singing effort and reduced AVT-IR in the BSTm and LS, relative to low quality songs. The effect of the quality of the song environment on both singing effort and forebrain AVT-IR raises the hypothesis that AVT within these brain regions plays a role in the modulation of behavior in response to competition that individual males may assess from the prevailing song environment.
doi:10.1016/j.yhbeh.2010.04.002
PMCID: PMC2916047  PMID: 20399213
arginine vasotocin (AVT); bird song; competition; sexual selection; social behavior network
23.  Delayed song maturation and territorial aggression in a songbird 
Biology Letters  2012;8(3):369-371.
Asymmetries in competitive ability can determine the outcome of social interactions in animals and are often expressed through differences in sexual traits. Competitive ability (resource holding potential, RHP), trait expression and ultimately reproductive success may vary with an individual's age or experience. In some species, reproductively mature males delay acquisition of some adult traits and thereby signal their young age. Theory on animal contests predicts that individuals assess the RHP of an opponent relative to their own, such that escalation is more common between evenly matched opponents. Here, we test predictions from this hypothesis that males respond to a territorial intruder based on their RHP relative to the intruder's RHP. We simulated white-crowned sparrows (Zonotrichia leucophrys) intruding into the territory of a recruit or return. Playback of a song repertoire simulating a young male (recruit) elicited a weaker response from established territory holders (return), but a stronger response from recruits. Playback of a single song type simulating an older male elicited the opposite responses. This indicates that males distinguished between simulated young and old intruders based on song, and responded differently depending on their own experience. Our study highlights the possibility that receiver as well as sender traits should be considered when interpreting animal interactions.
doi:10.1098/rsbl.2011.1119
PMCID: PMC3367755  PMID: 22319096
mutual assessment; birdsong; delayed maturation; sexual traits; Zonotrichia leucophrys
24.  RESPONSES OF MALE TROPICAL MOCKINGBIRDS TO VARIATION IN WITHIN-SONG AND BETWEEN-SONG VERSATILITY 
The Auk  2007;124(1):185-196.
Despite their large vocal repertoires and otherwise highly versatile singing style, male mockingbirds sometimes sing in a highly repetitive fashion. We conducted a playback experiment to determine the possible signal value of different syllable presentation patterns during simulated male intrusions in the Tropical Mockingbird (Mimus gilvus) testing the hypothesis that more repetitive singing represents a stronger threat and generates a stronger aggressive response. Responses were measured in terms of approach and singing behavior and were analyzed using McGregor’s (1992) multivariate method. We also introduce the use of survival analysis for analyzing response variables for which subjects do not perform the behavior in question in at least one of the replicates (known as ‘right-censored variables’ in the statistical literature). As predicted by theory, experimental subjects responded more aggressively to songs composed of a single note than to variable ones. However, versatility at the between-song level had an opposite effect as high song switching rates generated stronger responses than low ones. Given the lack of a statistical interaction between within-song versatility and switching rate, we conclude that these two parameters may serve independent purposes and possibly transmit different information. We discuss the possibility that the signal value of variation in vocal versatility lies in the mediation of territorial conflicts, the attraction of female partners and/or the mediation of conflicts over access to reproductive females.
doi:10.1642/0004-8038(2007)124[185:ROMTMM]2.0.CO;2
PMCID: PMC2396498  PMID: 18509510
mockingbird; survival analysis; song complexity; switching rate; versatility
25.  Behavioural response of a migratory songbird to geographic variation in song and morphology 
Frontiers in Zoology  2014;11(1):85.
Introduction
Sexually selected traits contribute substantially to evolutionary diversification, for example by promoting assortative mating. The contributing traits and their relevance for reproductive isolation differ between species. In birds, sexually selected acoustic and visual signals often undergo geographic divergence. Clines in these phenotypes may be used by both sexes in the context of sexual selection and territoriality. The ways conspecifics respond to geographic variation in phenotypes can give insights to possible behavioural barriers, but these may depend on migratory behaviour. We studied a migratory songbird, the Stonechat, and tested its responsiveness to geographic variation in male song and morphology. The traits are acquired differently, with possible implications for population divergence. Song can evolve quickly through cultural transmission, and thus may contribute more to the establishment of geographic variation than inherited morphological traits. We first quantified the diversity of song traits from different populations. We then tested the responses of free-living Stonechats of both sexes to male phenotype with playbacks and decoys, representing local and foreign stimuli derived from a range of distances from the local population.
Results
Both sexes discriminated consistently between stimuli from different populations, responding more strongly to acoustic and morphological traits of local than foreign stimuli. Time to approach increased, and time spent close to the stimuli and number of tail flips decreased consistently with geographic distance of the stimulus from the local population. Discriminatory response behaviour was more consistent for acoustic than for morphological traits. Song traits of the local population differed significantly from those of other populations.
Conclusions
Evaluating an individual’s perception of geographic variation in sexually selected traits is a crucial first step for understanding reproductive isolation mechanisms. We have demonstrated that in both sexes of Stonechats the responsiveness to acoustic and visual signals decreased with increasing geographic distance of stimulus origin. These findings confirm consistent, fine discrimination for both learned song and inherited morphological traits in these migratory birds. Maintenance or further divergence in phenotypic traits could lead to assortative mating, reproductive isolation, and potentially speciation.
Electronic supplementary material
The online version of this article (doi:10.1186/s12983-014-0085-6) contains supplementary material, which is available to authorized users.
doi:10.1186/s12983-014-0085-6
PMCID: PMC4256809  PMID: 25484906
Sexual selection; Population divergence; Reproductive isolation; Phenotypic traits; Geographic clines; Simulated territorial intrusion; Saxicola torquata; Songbird; Behavioural isolation barrier

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