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.     

Physically Challenging Song Traits,Male Quality,and Reproductive Success in House Wrens

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.     

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.     

Cautious response of inexperienced birds to conventional signal of stronger threat

Several studies demonstrated that bird song functions as a first line of territorial defence. The efficiency of deterring rivals depends strongly on the strategy of singing used (e.g. alternating/overlapping singing, singing with low/high rate, matching song type of a rival or singing different type). Causes of between males variation during countersinging are still not fully understood, especially when different signals have similar production costs and their meaning is assigned by arbitrary convention (conventional signalling). We tested whether an oscine bird with small repertoire size, the ortolan bunting Emberiza hortulana , differentiate strategy of responding to song of an intruder in relation to its age and threat value of signals. We performed playback experiments to measure response of second year (SY) and after second year (ASY) males to a song of low (eventual variety singing) and high (immediate variety singing) threat value. We found substantial differences in response to playback, which were related both to the type of stimuli used and age of responding males. Both SY and ASY males gave more calls than songs in response to immediate variety playback, which suggest stronger vocal response to the signal of higher threat value. Approaching loudspeaker was similar for both age classes when lower threat value signal was played back, while simultaneously SY males clearly avoided approaching loudspeaker when stronger threat values signal was played back. We conclude that ortolan bunting differentiate response to signal of different threat value and that the strength of response depends on the age of a male. This study provides experimental evidence that age of receiver affects its response to a territorial intruder. It also demonstrates that observed in many studies variation in response to playback may be an effect of age differences between males, which rarely is controlled.     

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.     

A Test of the Effect of Male Song on Female Nesting Behaviour in the Serin (Serinus serinus): a Field Playback Experiment

It is well established, through laboratory experiments, that male song in birds can stimulate female reproductive activity, affecting their behaviour and physiology, such as follicular growth, nest building and egg‐laying. However no clear demonstration has yet been provided that this effect works under natural conditions. Previous work in natural populations of serins showed that female nest‐building behaviour correlated with male singing time. Furthermore male serin song peaked exactly in the day that rapid follicular growth was estimated to start in females, suggesting that in this species song may also serve to stimulate the female's reproductive development. Direct causal evidence, however, was lacking. We conducted field playback experiments to investigate how song can influence female nesting activity during nest building. Our results show that females who listened daily to playbacks of serin songs, during the nest‐building stage, spent more time nest building than females that were not exposed to additional songs. Moreover, the singing behaviour of the mated males was not affected by the playbacks, suggesting that the song playback treatment had a direct positive effect on female nesting behaviour.     

Song duration mediates responses of territory owner in a songbird species with a small song repertoire

Song is a sexually selected trait that is involved in mate attraction and territory defence in birds. Songs may convey information about different male quality components. They are flexible in terms of frequency, amplitude, and duration. Although changes in song duration are common, the function of this behaviour has been studied less strongly. It is known that song duration changes within a singing bout and may provide information about aggressive motivation. We tested whether the elongation and shortening of songs affects the responses of territorial ortolan bunting males to neighbour song playback. If changing song duration signals level of aggressiveness, then songs differing solely in duration may elicit behavioural responses of different strength. We performed two tests with different males assigned to two experimental groups and measured approaching and vocal response. In Experiment 1, we tested 18 males, which responded to the playbacks of elongated and normal (control) neighbour songs. In Experiment 2, we tested 17 males, which responded to the playbacks of shortened and normal (control) neighbour songs. Males responded significantly stronger to longer songs in both experiments as measured by the approach variable. Vocal response was not affected by treatment, but it was affected by the order of playback presentation. Our results indicate that song duration might be used for signalling current aggressive motivation during close interactions with rivals.     

Song matching, overlapping, and switching in the banded wren: the sender's perspective

Interpreting receiver responses to on-territory playback ofaggressive signals is problematic. One solution is to combinesuch receiver-perspective experiments with a sender-perspectiveexperiment that allows subjects to demonstrate how their choiceof singing strategies is associated with their approach behavior.Here we report the results of a sender-perspective study onthe banded wren (Thryothorus pleurostictus) and combine informationon context and results of previous receiver-perspective experimentsto clarify function. Territorial males were presented with a5-min playback consisting of song types present in their repertoire.We assessed the degree to which the subjects' song-matchingrate, overlapping rate, and song-type versatility were correlatedwith their approach latency, closeness of approach, latencyto first retreat, and time spent close to the speaker. Maleage, breeding stage, and features of the playback stimuli werealso considered. Song matching was associated with rapid andclose approach, consistent with the receiver-perspective interpretationof type matching as a conventional signal of aggressive motivation.Overlapping was associated with earlier retreat and togetherwith the aversive receiver response to our previous overlappingplayback experiment suggests that overlapping is a defensivewithdrawal signal. High versatility was associated with slowerfirst retreat from the speaker and high levels of reciprocalmatching between subject and playback. Males with fledglingssang with particularly low versatility and approached the speakeraggressively, whereas males with nestlings overlapped more andretreated quickly. Finally, older males matched more but overlappedless.     

Vocal Plasticity – are Pied Flycatchers,Ficedula Hypoleuca,Open‐Ended Learners?

In some songbird species, large song repertoires are advantageous in female attraction, whereas song sharing with neighbours may give an advantage in male–male competition. Open‐ended learners, with the ability to memorize new song elements throughout their lives, may learn from territorial neighbours and thus benefit from increasing both repertoire size and song sharing. A distinction needs to be made between true adult song learning, i.e. memorization of novel song elements, and vocal plasticity resulting in changes in the use of previously memorized elements, such as the use of hidden repertoires or increased production of previously rare syllable types. We assessed the ability of adult pied flycatcher Ficedula hypoleuca males to learn previously unheard song elements and to change their song production in response to playback of unfamiliar, conspecific song, emulating a singing neighbour. After a 1‐week playback treatment, three out of 20 subjects had learned foreign song elements, providing evidence from the wild that pied flycatchers are true open‐ended learners. However, the syllable sharing with the playback stimulus repertoires had not changed, and the males’ repertoires had decreased rather than increased. Hence, we did not find support for increased syllable sharing with neighbours or increased repertoire size as functions of adult song learning in pied flycatchers. Because pied flycatcher song seems to serve mainly for mate attraction, copying of attractive syllable types is a possible alternative function of adult song learning in this species.     

Importance of Frequency and Temporal Song Matching in Black-Capped Chickadees: Evidence from Interactive Playback

Song-type matching during territorial contests may allow males of some bird species to direct their signal to a particular receiver. By matching the song-type and also responding immediately to a rival (temporal matching), a signalling male may indicate his willingness to escalate the contest. Black-capped chickadees ( Poecile atricapilla ) sing a single song-type, but are able to sing this song over a wide range of absolute frequencies. By using interactive playback to instigate and control the level of matching during trials, we investigated whether matching the frequency and the temporal patterning of song escalates contests. Males that were matched for both the frequency and the temporal pattern of their songs during trials escalated contests more than males that were not matched, while males that were only matched temporally had an intermediate response. During trials that consisted of temporal matching only, focal males often shifted frequency to match the playback. Our results confirm that frequency matching and temporal matching using a single song-type allows graded signalling during aggressive interactions in chickadees.     

Interspecific Response to Playback of Bird Song

Responses to bird song have usually only been studied at the intraspecific level. I experimentally tested whether playback of the song of the black wheatear Oenanthe leucura in an area in S Spain resulted in responses from conspecifics as well as heterospecific birds by comparing the numbers of individuals singing before and after playback. The number of singing male black wheatears increased considerably, but also the number of singing males of five other passerine species increased significantly. The heterospecific response to playback may be due (1) to interspecific territoriality, (2) to black wheatear song signalling the absence of predators, or (3) to heterospecifics confusing the species-identity of the singer. The second alternative is considered more likely, since an ecologically wide array of species increased their song rate following playback. The conspicuous dawn (and dusk) chorus of bird song may be augmented by social facilitation due to the singing of conspecifics as well as heterospecifics.     

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.     

Dawn song of male blue tits as a predictor of competitiveness in midmorning singing interactions

During the dawn chorus, territorial male songbirds vocalise intensively within signalling range of several conspecific males and can therefore be considered members of a busy communication network. The more or less continuous singing over a long period of time under standardised stimulus conditions makes the dawn song a potentially important information source both for simple receivers and for eavesdroppers. Male blue tits (Parus caeruleus) vary in features of their dawn song, e.g. older males sing longer strophes, and females choose males that sing longer strophes as extra-pair partners. However, so far, dawn song in the blue tit has been investigated separately from other singing behaviour of the same males. In this study, we investigate aspects of blue tit male quality, reflected in dawn song characteristics, and their predictive value for how males behave during singing interactions later in the morning. We acted as simple receivers by recording the singing activity of one male at a time at dawn and compared features of its dawn song, such as onset before sunrise, repertoire size, mean bout length, strophe length and percentage performance time to responses of the same male to a territory intrusion simulated by playback of synthesised songs later during the same morning. We assume that an aggressive response towards an intruder will involve a fast approach to the loudspeaker broadcasting strophes of blue tit song, searching for the intruder (flying around), and a high amount of counter singing and overlapping of the intruders songs. Aspects of vigour of response to the simulated intrusion could be predicted from all five investigated dawn song parameters as well as male age. This is, to our knowledge, the first indication that a simple receiver could extract reliable information from a males dawn singing behaviour about its competitiveness later in the day.Communicated by P.K. McGregor     

Song organisation and patterns of variation in the serin (Serinus serinus)

This article describes the organisation of song in the serin (Serinus serinus) and analyses its variation among individuals. Serins have a repertoire of about 50 complex syllables that are sung at a very fast rate and in a very stereotyped order, forming discrete songs. Songs are high pitched for the serin’s body size. Song organisation is circular, with a limited number of starting points. Songs can stop at any point in their cycling. Within songs there are trilled sections and fast, non-repeated sections that account for the greatest part of songs. These two modes of singing also differ in average inter-element intervals and probably in their respiratory kinematics. Bird repertoire size was measured and the difficulties of measuring it in this species are discussed. Repertoires are individually specific and have a variable amount of syllable sharing with other birds. We found evidence for geographical variation in the composition of repertoires. Considering our current knowledge of song in carduelines, the stereotyped and circular nature of serin song appear to be unique within this group of birds. Received: 30 May 2000 / Received in revised from: 14 August 2000 / Accepted: 12 September 2000     

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.     

Neighbour recognition by resident males in the banded wren, Thryothorus pleurostictus, a tropical songbird with high song type sharing

Several studies and reviews have suggested that the ability to discriminate between neighbours and strangers decreases as neighbour song repertoire size and song type sharing increase. We tested the recognition capabilities of territorial male banded wrens by comparing the aggressive approach responses of focal birds to three playback treatments: shared song types sung by an adjacent neighbour (neighbour song), shared song types sung by unfamiliar birds (mimic song), and unshared song types sung by unfamiliar birds (unfamiliar song). All three treatments for each male were broadcast from the same location on the territorial boundary shared with the appropriate neighbour. As expected, focal males responded nonaggressively to the neighbour treatment and aggressively to the unfamiliar song treatment. The approach response to the mimic treatment was statistically indistinguishable from the unfamiliar treatment and significantly higher than the neighbour treatment, suggesting that most males were able to recognize unfamiliar singers even when the song types played were very similar to those of their neighbours. The relative strength of responses to the mimic varied: some males treated the mimic song with low aggression levels typical of responses to neighbour song. Repertoire sizes of focal and neighbour birds, the fraction of song types shared among neighbouring males, and the similarity of neighbour and mimic song types did not explain this variation. Therefore, within the short 3-min period of our playback experiments, some birds may have used repertoire composition as a recognition cue and confused the mimic with the neighbour. Copyright 2001 The Association for the Study of Animal Behaviour.     

The functions of multiple singing modes: experimental tests in yellow warblers, Dendroica petechia

In several groups of songbirds, including the North American wood warblers (Parulidae), males subdivide their song repertoires into two different categories, or ‘modes’, of singing. Previous studies of wood warblers suggest that the two singing modes in this group are each specialized for interacting with one of the two sexes: Type 1 singing for interacting with females and Type 2 for interacting with other males. Here I examine the responses of male and female yellow warblers to the two singing modes produced by males. Using song playback experiments, I found that male yellow warblers responded to potential intruders with Type 1 singing, rather than Type 2 singing, as predicted. In tests using the copulation solicitation assay, females implanted with oestradiol responded equally strongly to both Type 1 and Type 2 singing. These results suggest that male wood warblers use both singing modes to interact with males and females, and that the singing modes do not have distinct, sex-specific functions as previously thought. Factors other than intra- and intersexual selection are therefore likely to have helped shape the evolution of distinct singing modes.     

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.     

Memory-dependent adjustment of vocal response latencies in a territorial songbird

Vocal interactions in songbirds can be used as a model system to investigate the interplay of intrinsic singing programmes (e.g. influences from vocal memories) and external variables (e.g. social factors). When characterizing vocal interactions between territorial rivals two aspects are important: (1) the timing of songs in relation to the conspecific’s singing and (2) the use of a song pattern that matches the rival’s song. Responses in both domains can be used to address a territorial rival. This study is the first to investigate the relation of the timing of vocal responses to (1) the vocal memory of a responding subject and (2) the selection of the song pattern that the subject uses as a response. To this end, we conducted interactive playback experiments with adult nightingales (Luscinia megarhynchos) that had been hand-reared and tutored in the laboratory. We analysed the subjects’ vocal response latencies towards broadcast playback stimuli that they either had in their own vocal repertoire (songs shared with playback) or that they had not heard before (unknown songs). Likewise, we compared vocal response latencies between responses that matched the stimulus song and those that did not. Our findings showed that the latency of singing in response to the playback was shorter for shared versus unknown song stimuli when subjects overlapped the playback stimuli with their own song. Moreover birds tended to overlap faster when vocally matching the stimulus song rather than when replying with a non-matching song type. We conclude that memory of song patterns influenced response latencies and discuss possible mechanisms.     

Phonotaxis and aggression in the coneheaded katydid Neoconocephalus affinis

ABSTRACT. Positive phonotaxis to tape recorded conspecific song by males of the katydid Neoconocephalus affinis (Beauvois) (Orthoptera: Tettigoniidae) was studied in the field, in Puerto Rico. N.affinis males stridulated loudly during evening hours, and most males were territorial. Aggression between neighbours occurred, albeit rarely, and playback experiments using tape recordings of N.affinis song showed that aggressive behaviour, in the form of jumps directed toward a loudspeaker, could be elicited from a male by song broadcast above a threshold sound intensity. Response thresholds for different males varied from 76 to 93 dB, intensities that corresponded to calculated inter-neighbour distances of 1.6-0.26 m, respectively. Insects that had lower response thresholds were on average more isolated from neighbours and also stridulated more persistently and at a higher sound intensity than insects with higher response thresholds. N.affinis males displayed no phonotaxis toward the song of Neoconocephalus maxillosus , a sympatric species whose song is similar to that of N.affinis in principal frequency but differs in wingstroke rate. Phonotaxis toward conspecific song could only be elicited from males while they were silent.