Acoustic Monitoring Reveals Congruent Patterns of Territorial Singing Behaviour in Male and Female Tropical Wrens

Many animals defend territories against conspecific individuals using acoustic signals. In birds, male vocalizations are known to play a critical role in territory defence. Territorial acoustic signals in females have been poorly studied, perhaps because female song is uncommon in north‐temperate ecosystems. In this study, we compare male vs. female territorial singing behaviour in Neotropical rufous‐and‐white wrens Thryothorus rufalbus, a species where both sexes produce solo songs and often coordinate their songs in vocal duets. We recorded free‐living birds in Costa Rica using an eight‐microphone Acoustic Location System capable of passively triangulating the position of animals based on their vocalizations. We recorded 17 pairs of birds for 2–4 consecutive mornings and calculated the territory of each individual as a 95% fixed kernel estimate around their song posts. We compared territories calculated around male vs. female song posts, including separate analyses of solo vs. duet song posts. These spatial analyses of singing behaviour reveal that males and females use similarly sized territories with more than 60% overlap between breeding partners. Territories calculated based on solo vs. duet song posts were of similar size and similar degrees of overlap. Solos and duets were performed at similar distances from the nest for both sexes. Overall, male and female rufous‐and‐white wrens exhibit very similar spatial territorial singing behaviour, demonstrating congruent patterns of male and female territoriality.     

Differential effects of moving versus stationary territorial intruders on territory defence in a songbird

1.  In territorial contests, not only acoustic or other signals, but also the movements of a territorial intruder are likely to influence the response of a resident.
2.  We tested this movement hypothesis by simulating moving vs. stationary intruders into the territories of winter wrens Troglodytes troglodytes , using the same non-interactive song playbacks in both treatments.
3.  Male winter wrens showed a different long-term singing reaction in response to a moving than to a stationary intruder.
4.  One day after experiencing an intruder that was switching between three locations, residents started to sing earlier before sunrise, and they sang more and longer songs at dawn than before the intrusion.
5.  Residents receiving the same playback from one location only reacted by starting to sing later relative to sunrise, and by singing fewer and shorter songs than before the intrusion.
6.  We could not discriminate between the treatments when examining the short-term singing reactions during and immediately after the playbacks. However, our results clearly demonstrate an effect of the spatial behaviour of territorial intruders on the long-term territory defence of residents at dawn, about 24 h after an intrusion.
7.  We argue that spatial behaviour of territorial intruders should be an integral part of the study of animal territory defence behaviour. Investigating long-term changes in territory defence at dawn is a sensitive tool for discriminating between different types of intruders.     

SONG POPULATIONS AND DISPERSAL IN STEELBLUE AND PURPLE WIDOWFINCHES

Payne, R. B. 1985. Song populations and dispersal in Steelblue and Purple Widowfinches. Ostrich 56:135-146.

Songs of Steelblue Widowfinches Vidua chalybeata were recorded over eight years at Lochinvar National Park, Zambia. Neighbouring males share their song-type repertoires. Individual birds changed the structural details of all song types in their repertoire from year to year, and over 4–5 years the songs accumulated changes so that a song type could scarcely be recognized as the same. A few males in addition switched their song repertoires when they dispersed from one song population to another, or when their old neighbours disappeared and new neighbouring males sang a different song repertoire. Individual marked adult birds moved from one song neighbourhood to another. Purple Widowfinches had a similar song behaviour but only one song dialect was found in the study area.

An estimate of the proportion of immigrants was made from the proportion of birds with songs unlike their neighbours, and from birds that had a mixed song repertoire or that switched their song repertoires. Song populations were sampled at several localities in the Transvaal, Botswana, Zambia, Kenya, Tanzania, Cameroon, and Nigeria. An overall minimal estimate of dispersal between song neighbourhoods was 18 %. The incidence of observed dispersal and the movements indicated by the song differences among local widowfinches were considerable and more than sufficient to prevent genetic isoloation and differentiation of local song populations.     

Song sharing and singing strategies in the winter wren Troglodytes troglodytes

The winter wren is a common forest bird living in groups of few adjacent neighbours during the breeding season. Inside each group, males vocally interact in the context of both territorial holding and sexual competition, forming a complex communication network. To study this network, we first analysed song type and syllable repertoires within and between distinct groups. We found a limited number of song types highly stereotyped in length, syntax and syllable composition, frequently shared among neighbours. Between groups, song type and syllable repertoires sharing decreased with increasing distance at a higher rate for song types than for syllables. Then, with continuous recordings, we focused on the dynamics of acoustic interactions between neighbours. We showed that male winter wrens can differentially use their song type repertoire (non-matching strategy), overlap their neighbours and modulate their singing rhythm producing longer inter-song intervals with no change in song length during acoustic interactions.     

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.     

Sedentary life style of Neotropical sedge wrens promotes song imitation

To what extent has the style of song development among songbirds coevolved with other life history strategies? Among Cistothorus wrens in North America, it seems that sedentary or site-faithful habits of marsh wrens, C. palustris, favour song imitation, but seminomadic habits of sedge wrens, C. platensis, favour song improvisation, whereby each male generates a large but unique song repertoire. In this study, we tested whether more sedentary populations of sedge wrens in the Neotropics would imitate songs. At our primary study site near Cartago, Costa Rica, breeding birds were colour-banded during 1995 and 1996, and follow-up surveys revealed that the birds remained at this site the year round. Extensive tape recording and analysis of songs showed that males had large song repertoires (200-300+ songs), and that many songs were shared among neighbouring males. In addition, males only 27 km distant, at La Pastora, used different songs. Furthermore, matched countersinging, in which two males answer each other with identical song types, was recorded near Brasilia, in Brazil. The sharing of songs among permanent neighbours, microgeographical variation in song, and matched countersinging can be achieved only through song imitation, thus revealing a striking difference in the style of song development among different populations of the sedge wren. In the Neotropics, having predictable neighbours throughout life appears to have favoured song imitation, so that individuals can interact using a common, learned code typical of the local population; among more mobile populations in North America, however, individuals improvise large repertoires of species-typical songs, thereby enabling singing males to communicate with any individual, no matter what the population of origin. Strategies of song development must correlate with life history features, and further surveys are needed to make sense of the great diversity of singing behaviours among songbirds. Copyright 1999 The Association for the Study of Animal Behaviour.     

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.     

Song Sharing in Neighboring and Non-Neighboring Thrush Nightingales (Luscinia luscinia) and its Implications for Communication

In many passerines, males have repertoires of different songs of which some songs are often shared with other males. Sharing of song repertoires among males can provide insights into the context in which songs were acquired and on the role of song repertoires in inter- and intrasexual communication. Here we studied repertoire sharing in male territorial thrush nightingales ( Luscinia luscinia ). We compared male vocal repertoires of the basic song components, full songs, and the sequencing of songs in a bout. The results show that males differed significantly in the size of their song repertoires but not in the size of the repertoire of basic song components. Moreover, they shared almost all (80%) the repertoire of song components but only 30% of their song types. Neighboring males shared significantly more song types than did non-neighboring males but interestingly they did not share more basic song components than non-neighboring males. These results show that the repertoire of basic song components is under much less sexual selection than the size of song repertoires. Sharing of song repertoires among neighbors presumably results from repertoire conversion over time and from males returning to their territories in the following season. Repertoire sharing then can be an indicator of territory tenure and thus it can be important in repelling rivals and in female choice.     

Countersinging as a signal of aggression in a territorial songbird

Neighbouring territorial songbirds often interact through countersinging, where birds sing in response to the singing of neighbours such that their song bouts are temporally related. Complex forms of countersinging such as song type matching or song overlapping appear to be correlated with aggressiveness and readiness to escalate confrontations. Less attention has been paid to the importance of simpler forms of countersinging, where matched song types are not used and where individual songs do not temporally overlap. I examined countersinging behaviour in male Carolina wrens, Thryothorus ludovicianus, which countersing regularly. Why they countersing and how countersinging is perceived by neighbours is unknown. By comparing singing behaviour before and after simulated intrusions, I determined that subjects countersing with their neighbours more readily when highly aroused. Comparing responses to countersinging and noncountersinging playbacks showed that countersinging elicited more aggressive responses than did noncountersinging. Carolina wrens appear to exchange aggressive signals regularly through countersinging. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

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.     

Territoriality as a paternity guard in the European robin, Erithacus rubecula

To investigate the relative importance of paternity defences in the European robin we used behavioural observations, simulated intrusions and temporary male removal experiments. Given that paired males did not increase their mate attendance, copulation rate or territory size during the female's fertile period, the most frequently quoted paternity assurance strategies in birds were absent. However, males with fertile females sang and patrolled their territories more regularly, suggesting that territorial motivation and vigilance were elevated when the risk of cuckoldry was greatest. In addition, there was a significant effect of breeding period on response to simulated intrusions: residents approached and attacked freeze-dried mounts more readily in the fertile period. During 90-min removals of the pair male in the fertile period, neighbours trespassed more frequently relative to prefertile and fertile period controls and appeared to seek copulations with unattended females. When replaced on their territories, males immediately increased both song rate and patrolling rate in comparison with controls. We propose that male robins sing to signal their presence, and increase their territorial vigilance and aggression in the fertile period to protect paternity. Copyright 2000 The Association for the Study of Animal Behaviour.     

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

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

The deterrent effect of bird song in territory defense

Using the responses of territory owners to playback to inferthe territorial function of acoustic signals is common practice.However, difficulties with interpreting the results of suchexperiments have obscured our understanding of territorial signalling.For instance, a stronger response to playback is often interpretedas more aggressive, but there is no consensus as to whetherthis should be in response to the least or most threateningsimulated intruder. Rather than following a gradual increaseor decrease, the relationship between signal intensity and responsestrength may instead describe a peaked curve. We manipulatedbanded wren (Thryophilus pleurostictus) songs to simulate low-,median-, and high-performance singers and used these songs asstimuli in playback experiments. Banded wrens were less likelyto approach the high-performance stimulus compared with thelow- and median-performance stimuli. However, the birds thatdid approach the high-performance stimulus sang more than thosethat approached the low-performance stimulus. In addition, birdswere more likely to match the songs when exposed to the median-and high-performance stimuli compared with the low-performancestimuli, and song matching predicted approach behavior. Theseresults are in accordance with theoretical models of aggressiveencounters in which low-performance opponents are challengedwithout further assessment. Median- and high-performance opponents,however, may require further assessment, and the latter maybe perceived as too intimidating for approach.     

SONG DEVELOPMENT IN CHFFINCHES: WHAT IS LEARNT AND WHEN?

Male Chaffinches share no more songs with territorial neighbours than would be expected by chance, suggesting that some song learning occurs before young birds set up their territories. Hand-reared birds are capable of learning song both in their first spring and in the previous summer one young bird produced a good copy of a song which he had only heard as a nestling and fledgling eight months previously. While song learning involves accurate copying of individual elements, the variation in number in elements in a phrase and the temporal pattern of singing shown by hand-reared birds were similar to that of wild birds rather than being influenced by the tutor tapes.     

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

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

Song Perch Height in Rufous‐and‐White Wrens: Does Behaviour Enhance Effective Communication in a Tropical Forest?

Animal signals are distorted as they transmit through the environment, but many species have evolved strategies to minimize distortion of vocal signals. Song structure can change over evolutionary time, or signallers can modify their behaviour, to reduce attenuation and degradation in a specific habitat. We investigated signalling behaviour in rufous-and-white wrens, Thyothorus rufalbus , a Neotropical songbird in which both sexes sing, with a specific focus on perch height. Based on previous findings, including a detailed sound transmission study, we predicted that rufous-and-white wrens would choose elevated song perches in order to maximize the distance their songs travel, and that males and females would show different perch height choices. We observed 30 birds in 18 territories in a tropical forest in Costa Rica to determine the perch heights of birds when singing, producing non-song vocalizations, collecting nesting material, foraging, and engaging in other behaviours. We found that both activity and sex influenced perch height. Birds sang from a variety of heights, but the highest perches were used as song posts by males. Males minimized singing from within 1 m of the ground, and they generally perched higher than females, which may reflect sex differences in communication strategies. Both sexes foraged on or near the ground. The finding that rufous-and-white wrens vary their perch height with specific activities suggests that they modify their singing behaviour to enhance long-distance communication.     

Song correlates with social context,testosterone and body condition in male barn swallows

Bird song, like many other male secondary sexual characters, may have evolved as intra- or inter-sexual signals of male phenotypic quality. The hypotheses that song rate and song features reflect androgen levels and body condition, qualities useful in male–male competition, and that they are also influenced by social context, was tested for the first time in the present correlational study. The relationships between song rate and 14 variables describing song structure, respectively, and absolute plasma testosterone levels, body mass, body condition, number of neighbouring males and distances between nest sites in male barn swallows,Hirundo rustica, were analysed. Song rate was not correlated with any of the song features nor with male or social context characteristics. By contrast, a harsh song syllable, the ‘rattle’, was positively related to plasma testosterone levels, and its peak amplitude frequency varied inversely with male body mass and condition. In addition, eight features of song varied according to the social environment of each male. In particular, males sang longer and more varied songs when they had few or no neighbours, whereas males in highly competitive contexts uttered short songs, interrupted them more frequently, and emphasized the rattle. Neighbouring males also sang more similar songs than distant males, and this resulted in matched countersinging. The quality of song output therefore reflects aspects of male competitive potential, and relationships between song structure and social context suggest that some features, such as the rattle, might have originally evolved to serve in male–male interactions; a female preference may have further promoted song evolution leading to complex syllable repertoires.     

Song learning and deceptive mimicry

Payne, 1978, Payne, 1981a, Payne, 1981b has suggested that young males of some bird species copy the songs of the previous occupant or the songs of local established males when setting up a territory and thereby increase their likelihood of success. According to this view, song copying is a form of deceptive mimicry. We show that first-year great tits do not share more songs than expected with the previous occupants of their territories, nor are shared songs used more often than other songs in their repertoires. Songs are shared with neighbours but not selectively with established neighbours and there is no difference in reproductive success between great tits sharing with established neighbours and with first-year neighbours. We discuss the general applicability of the deceptive mimicry hypothesis and suggest that song sharing between neighbours may be advantageous for reasons other than deceptive mimicry.     

Evidence of repertoire sharing and stability despite a high turnover rate in a duetting neotropical wren

In songbirds, the spatial pattern of song sharing among individuals is influenced by the song learning and dispersal strategies within each species. In birds where females and males sing and create joint acoustic displays (duets), the processes defining the patterns of song sharing become more complex as there might be different selection pressures shaping the behaviour of each sex. To provide further insight into the vocal development and the dispersal strategy of duetting tropical species, we investigated the patterns of individual and pair repertoire sharing, as well as the stability of these repertoires, in a colour-marked population of riverside wrens, Cantorchilus semibadius, located in the Osa Peninsula, Costa Rica. Using data collected over a five-year period, we found considerable variation in the sharing levels of phrase and duet type repertoires among neighbouring individuals coupled with a general decline of repertoire sharing as distance increased between birds’ territories. These results are consistent with a pattern predicted in age-restricted learners that establish preferentially near their tutors. Furthermore, we found no evidence of individuals changing their phrase type repertoires over time, including after remating events. Duet type repertoires were also stable when pairs remained together. However, we observed a surprisingly high turnover rate. When individuals remated, even though the majority of the previous duet type repertoire remained, several new duet types were included. Taken together, our findings suggest that riverside wrens might create their individual repertoires by copying their same-sex parent and neighbouring individuals before dispersal. Additionally, we speculate that even though birds were able to create new duet types after changing partners, a substantial portion of their duet type repertoire might also be copied from their parents and neighbouring pairs during the initial critical period of song learning.     

Song types and their structural features are associated with specific contexts in the banded wren

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.