Behavioral Ecology Advance Access originally published online on November 10, 2006
Behavioral Ecology 2007 18(1):182-188; doi:10.1093/beheco/arl070
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Function of pair duets in the eastern whipbird: cooperative defense or sexual conflict?
a Department of Zoology, University of Melbourne, Melbourne, Victoria, 3010, Australia b School of Botany and Zoology, Australian National University, Canberra, 0200, Australia
Address correspondence to R.A. Mulder. E-mail: r.mulder{at}unimelb.edu.au.
Received 16 January 2006; revised 17 August 2006; accepted 21 September 2006.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Paired male and female eastern whipbirds, Psophodes olivaceus, sing precisely coordinated, male-led duets. Four broad explanations have been proposed for the function of duets: 1) cooperative resource defense, 2) prevention of partner usurpation, 3) defense of an individual's own position within the partnership, or 4) mate identification and localization. These 4 hypotheses make different predictions about how male and female residents should respond to simulated intrusion by other pairs or individuals. We compared the behavioral and vocal responses of 20 pairs of eastern whipbirds to simulated territorial intrusions by: 1) a solitary singing male, 2) a solitary singing female, and 3) a duetting pair. Males and females did not coordinate their approach to the playback speaker and showed sex-specific responses to playback. Males did not respond differently to duetting versus solo singing intruders. By contrast, females approached more closely during solo female song than during solo male song or duet playback. Females also produced specific vocalizations only in response to duet and solo female playback. Both sexes approached the speaker more closely and quickly during playback of same-sex solo songs than opposite-sex solo songs. Finally, females answered more of their mate's songs during simulated intrusion by a lone female than during simulated intrusion by a lone male. Our results suggest that duets in this species primarily function to allow females to defend their exclusive position in a partnership. Mate defense by females is unusual in birds but may be promoted in eastern whipbirds by a female-biased sex ratio and the need for exclusive access to male care. Thus, duets result from independent and conflicting strategies of mate and territory defense in males and females.
Key words: antiphonal duet, eastern whipbird, sexual conflict.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Most studies investigating avian song function have focused on solo male song and its dual role in intrasexual competition and mate attraction (Catchpole and Slater 1995
; Langmore 1998a). Why males and females of a mated pair should precisely coordinate their vocalizations to form duets is much less well understood (for reviews, see Farabaugh 1982; Hall 2004; Langmore 1998b). Duets have arisen independently in a range of phylogenetically distinct groups (Hall 2004; Von Helversen et al. 2004), suggesting that selection for these joint displays is intense. However, our understanding of the selective pressures driving their evolution has been limited by a lack of studies on color-banded populations on the one hand and poorly defined hypotheses that fail to evaluate duetting from an individual perspective on the other (Hall 2004; Langmore 2002).
In a recent comprehensive review of avian duets, Hall (2004) reevaluated traditional duet hypotheses and identified 4 promising avenues of future research (Table 1). These areas of research can be interpreted as 3 primary hypotheses that make exclusive predictions for duet function (Table 1): 1) to jointly defend a shared resource, 2) to prevent a partner from being usurped, or 3) to defend the individual's own position within the partnership (either by preventing usurpation or by signaling commitment to a partner). A more recent hypothesis, the "identity hypothesis," proposes that individuals use duet song to identify their mates during both agonistic and nonagonistic activities and, thereby, avoid misdirected aggression from their mates (Logue and Gammon 2004). There is a growing consensus that duets may have different functions in different species (Hall 2004) or multiple functions within a single species (Grafe and Bitz 2004).
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These hypotheses make different predictions about the degree of cooperation and conflict that exist between pair members (Hall 2004
). If duets function primarily in the joint defense of a shared resource, or to prevent a partner from being usurped by opposite-sex intruders, pair members have a common interest, and thus, duets should serve a primarily cooperative function. Likewise, both members of a pair benefit from avoiding misdirected intrapair aggression. By contrast, the idea that duets are required to prevent the individual from being usurped from a partnership involves selfish interest. Pair members are unlikely to have to defend their position in the partnership unless the interests of pair members diverge (Hall 2004; Seddon et al. 2002). Thus, such a function for duets is likely to involve sexual conflict (Parker 1979).
One of the difficulties of distinguishing between duet hypotheses is that they share several predictions. For example, all 4 hypotheses predict that duets will be loud, locatable signals that will be used frequently in the context of interactions with intruders (Hall 2004). However, a comparison of behavioral responses to simulated intrusions by lone and paired intruders permits discrimination of other exclusive predictions (Hall 2004). For example, if duets function in joint territory defense, males and females should approach intruders of either sex together (Hall 2000; Rogers et al. 2004). Duetting intruders should also represent a greater territorial threat and provoke a more aggressive response from subjects than solo singing intruders (Hall 2000). Males and females should therefore be more responsive (i.e., quicker, closer, approach; higher rate of vocalizations) when subjected to playback of duetting intruders than solo singing intruders, and females should answer a higher proportion of their partner's songs in response to simulated intrusion by pairs. Alternatively, if females use duets to defend their partners against male intruders, they should answer more of their partner's songs during simulated intrusion by a lone male than during simulated intrusion by a lone female (Hall 2004). If females duet in order to defend their own position within the partnership, they should respond most aggressively and answer a larger proportion of their partner's songs during simulated intrusion by a rival female, when the threat of being usurped from the partnership is greatest (Hall 2004). Finally, if duets function for mate identification, initiation rates should be high during opposite-sex playback, whereas answer rates should be high during same-sex playback. Further, a bird that fails to duet may come into physical conflict with their mate during a territorial intrusion (Logue and Gammon 2004).
Eastern whipbirds (Psophodes olivaceus) form antiphonal duets that are initiated exclusively by the male. They are socially monogamous and form extremely stable, long-term pair bonds with low rates of pair divorce (Rogers and Mulder 2004). Males and females show high levels of convergence in morphology and behavior. The sexes are of similar size and cannot easily be distinguished by differences in plumage. Both males and females feed nestlings and on fledging, the brood of 2 is divided, with each parent caring exclusively for one dependent offspring during an extended period of postfledging care (Rogers and Mulder 2004). Both sexes actively engage in interactions with intruders, together defending an exclusive territory year round (Rogers and Mulder 2004; Rogers et al. 2006). Although the similarity in roles suggests comparable selective pressures on the 2 sexes, recent studies suggest there may be sex differences in the extent of these pressures. Molecular sexing has revealed a female-biased sex ratio among both nestlings and unpaired adult "floaters" (Rogers and Mulder 2004). This could result in a particularly high level of competition between females for territories and mates. Here, we use experimental playback, simulating lone and duetting intruders, to investigate whether males and females use duets cooperatively to defend a territory or partner or whether females are forced to answer their partner to defend their own position within the partnership against rival females.
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METHODS |
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Study species and site
Eastern whipbirds are medium-sized (
60 g) insectivores that are members of the Australian endemic Psophodes genus, placed in the Corvida family Cinclostomidae (Sibley and Ahlquist 1990). They are weak fliers and are commonly found on the ground and in the understory of densely vegetated habitats through much of eastern Australia. We carried out playback experiments on pairs of individually color-banded eastern whipbirds that formed part of a resident population at Mimosa Rocks National Park near Bega, New South Wales (36°41'S 149°59'E: 60 ha). Detailed descriptions of the habitat and ecology of eastern whipbirds in the population are given in Rogers and Mulder (2004).
Male and female eastern whipbirds sing sex-specific solo songs, which are frequently combined to form a precise, male-initiated antiphonal duet (Watson 1969; Rogers 2005). Females sing 2 distinct types of song: "response songs," which can be sung as solo songs but are more commonly combined with a male song to form a duet and "type II" songs that are used only as solo songs and never in a duet context (Rogers 2005). The male commences the duet with a pure-frequency introductory whistle followed by an explosive "bullwhip-like" crack, to which the female typically replies with a simple, 2-element, "chew chew" response. Male solo song is more common than female solo song, with previous study at the site suggesting 31.5% of male songs and 71.8% of female songs are coordinated as duets (N = 4780 vocalizations, Rogers 2005). Males and females sing a repertoire of different song types and display extreme levels of repertoire sharing with neighbors, with females at the site sharing 100% and males, on average, 94% of their songs with same-sex neighbors. This high level of repertoire sharing appears to play an important role in allowing both sexes to engage in song type matching with rivals during vocal territorial encounters (Rogers et al. 2006). During duet formation, male's and female's songs are nonrandomly associated to form specific duet types (Rogers 2005).
Playback protocol
We recorded the behavioral and vocal responses of 20 pairs of uniquely ringed eastern whipbirds to 3 different acoustic stimuli: 1) a solo male song, 2) a solo female response song, and 3) a duet. Treatments were presented to 12 pairs during November 2001 and January 2002 and a further 8 pairs between September and December 2002. Trials were conducted between 06:00 AM and 11:45 AM, with a maximum of 3 trials conducted per day. This was within the typical breeding season of the eastern whipbird as pairs in this population were found to breed between August and January (Rogers and Mulder 2004). None of the subjects was involved in a current breeding effort, although during the first season 5 of the pairs had one or more independent offspring still present on their territory. During the first experimental season, duets were presented through a single playback speaker, and during the second experimental season, we employed a "stereo duet" playback design, a new technique that provides a more realistic stimulus (Rogers et al. 2004), which was developed after collection of the first season's data. This meant the male duet contribution was played through one speaker and the female duet contribution was broadcast separately though a different speaker connected in stereo. The original temporal arrangement of the duet components was kept intact (Rogers et al. 2006).
Playback stimuli were created from high-quality recordings of banded individuals, recorded at the field site during December 2000 to February 2001 using a Sennheiser ME67 microphone and a Sony TC-D5 cassette recorder. Vocalizations selected for use in playback were duets, recorded in the context of interactions between neighbors, when the duet participants were 10–15 m from the microphone and less than 5 m apart. Song recordings were digitized, at a sampling rate of 22 050 Hz, using a 16-bit sound card, and spectrograms were generated using either Canary 1.2 (Charif et al. 1995), or Syrinx (John Burt, http://syrinxpc.com). Stimuli made with the 2 programs were indistinguishable to the human eye and ear. When choosing recordings for stimuli, we attempted to minimize reverberation and maximize signal–noise ratios. We used elements of a single recording in all 3 treatments presented to the same pair, creating "solo" male and female playbacks by digital deletion of the partner's duet contribution. To create stereo duet stimuli, we used Canary software to copy a duet into 2 song tracks. The male contribution was filtered from one (randomly assigned) track and the female contribution removed from the other track. This enabled the male song to be played through one channel and the female song through a separate channel while maintaining the integral structure and temporal precision of the original duet (Logue and Gammon 2004; Rogers et al. 2004; Rogers et al. 2006). Low-frequency noise (below 0.5 kHz) was filtered using cursor-defined filters. Songs were amplified to a standardized maximum level before being transferred to CD for playback.
Song stimuli were presented to pairs through a Realistic 30W mid-range tweeter speaker placed 1–1.3 m above the ground and attached to a battery-powered Sony CDX-L460X portable car CD-stereo via a 15-m speaker cable. During stereo duet playback, 2 identical Realistic speakers were used, placed at a distance of 2–3 m apart. This interspeaker distance was chosen as recordings used for duet stimuli were made when the male and female subjects were less than 5 m apart, and during escalated duet interactions, pairs frequently duet at this distance apart (Rogers AC, personal observation). However, speakers were too close together to allow accurate measures of which speaker each individual was more likely to approach. The speakers were placed within the territory of the subjects and within 20 m of the territory boundary. Each trial consisted of a 5-min preplayback period and a 5-min playback period, during which 15 regularly spaced song stimuli were broadcast at a volume approximating natural eastern whipbird song (70 dB at 15 m, using a Realistic digital sound level meter, model 33-2050, on slow setting). Experiments were commenced when the birds were located between 15 and 25 m from the speakers and partners were within 10 m of each other (mean initial distance: females = 20.4 ± 0.58 m, males = 19.9 ± 0.50 m). Stimuli presented to pairs were recorded from birds at least 1 km away from the subject pair to reduce the potential for effects of familiarity. Each pair was presented with recordings made from different individuals to avoid pseudoreplication. To reduce possible habituation to the playback, there was an interval of at least 1 day between trials on the same pair, and neighboring pairs were not presented with playback on the same day. The order in which treatments were presented was randomized with respect to pair and day to control for order effects.
In the 5 min before and during playback, we collected the same behavioral measures of response for both sexes. These measures were limited by low visibility in the dense understory vegetation. Birds often moved out of sight within the undergrowth. However, we could reliably locate them by their song. The first movement toward the speakers was generally the most pronounced and was observed as a low flight, often accompanied by a rustle of undergrowth. Therefore, where possible, we recorded the time taken to make the first approach toward the speaker. When we did not observe the birds move but heard them vocalizing from 2 different locations, we estimated that the bird had moved at a time midway between the 2 vocalizations and calculated the approach time accordingly. We also measured the distance from the speaker before and during playback and the closest distance approached to the speakers (the nearest speaker during stereo duet playback). Distance measures were updated with each move greater than 1 m by continuously referencing the subject's location relative to flags with unique coordinates that were positioned every 15 m in an x–y grid across the site. This enabled us to calculate the mean distance from the speaker for each individual during each minute of each experimental stage for all treatments. Although some moves by each individual could have been missed using this method, it allowed us to obtain a reasonably accurate measure of location relative to the speaker in the 5 min before and during playback. After the playback treatment, distance measurements from the speaker to the closest approach and the starting distance from the speaker were made to the nearest 1 m using a tape measure.
All subject vocalizations produced during the preplayback and playback stages were recorded using a Sennheiser ME67 microphone and a Sony TC-D5 cassette recorder to allow later analysis of the number and type of vocalizations produced. Duet production was primarily a consequence of female behavior as females answered male songs to form a duet. We were therefore particularly interested in comparing a female's responsiveness to her partner during playback, rather than simply noting duet rate, which was a more relevant measure of how often males initiated songs. Female responsiveness was calculated as the proportion of songs initiated by the male that were answered by the female (total duets/(total duets + total male solos), defined as the "answer rate" by Levin 1996a).
Statistical methods
To assess the effect of simulated intrusion, we used Wilcoxon matched-pair tests to compare: 1) the mean distance from the speaker and 2) the total number of songs produced by individuals before and during playback, for the 60 experimental trials. Within any given analysis, each of these response measures provided an independent estimate of the strength of response between the different playback stages. To investigate response to duetting and solo singing intruders, we used ANOVA models to compare 4 response measures across the different treatments: 1) closest approach to the nearest speaker (calculated as a ratio of the initial instance from the speaker to control for differences in start distance from the speaker [closest distance/initial distance]), 2) time taken to first approach the speaker, 3) total number of vocalizations by each sex, and 4) the proportion of male songs answered. ANOVA models incorporated "season" and "playback type" as treatments and were blocked for "pair identity." The effect of season and the interaction between season and playback type were found to be nonsignificant for all analyses. Therefore, we pooled data from both seasons when conducting post hoc tests to determine significant effects. We did not conduct multivariate comparisons, as we were interested in different aspects of the approach and vocal response to playback (Hall 2004; McGregor 1992). For analysis of latency data, we assigned a maximum value of 300 s to birds that did not approach the playback speaker during the 5-min playback period. Male and female behavioral responses were analyzed separately as we were interested in differences between the sexes to simulated intrusion. Where responses seemed to differ, we used ANOVA to investigate the statistical significance of the effect of sex on response. Most response variables required transforming prior to analysis with parametric statistical tests. We used arcsine transformations for proportion data and log (x + 1) transformations for variables with large extreme values. All tests are 2-tailed, all means are given ±SE, and a P value of 0.05 was taken to indicate significance. GenStat release 8 (VSN International Ltd, Hemel Hempstead, UK) was used for all statistical analyses.
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RESULTS |
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Response during preplayback versus during playback
Male and female approach and vocal responses during playback are summarized in Figure 1. Males vocalized more frequently and approached the speaker more closely during all playback treatments than they did during preplayback periods (Figure 1a,c; total vocalizations: all P
0.005; distance from speaker: all P < 0.001). Females showed a significant increase in their vocal rate during playback of a solo female and duetting intruders (Figure 1b; Wilcoxon matched-pair tests: P 0.001). They also approached the speaker more closely during solo female and duet song compared with the preplayback period (Figure 1d; Wilcoxon matched-pair tests: P 0.001). However, females did not significantly increase their song rate or approach the speaker more closely during simulated intrusion by a lone male in the playback period compared with the preplayback period (Wilcoxon matched-pair tests: total vocalizations: T = 21.0, P = 0.11; mean distance speaker: T = 97.5, P = 0.79).
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Response to duets versus solo song
Males did not show a more aggressive vocal or approach response to duetting intruders than to solo singing intruders (Figure 1a,c). There was no difference in the total number of songs they produced (Figure 1a: ANOVA: F2,59 = 0.91, P = 0.41) or in the distance they approached the speaker during playback of the 3 treatments presented (Figure 1c: ANOVA: F2,59 = 0.16, P = 0.85).
Males showed a significant difference in the speed with which they approached the speaker in response to the different treatments (ANOVA: F2,59 = 4.94, P = 0.01), but they were not significantly faster to approach the speaker during broadcast of duetting intruders than solo male intruders (Figure 2: Tukey tests: solo male vs. solo female P < 0.05; remaining pairwise comparisons all P > 0.05).
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The number of vocalizations produced by females varied across the 3 treatments. Although they vocalized significantly less frequently when presented with lone male intruders, there was no significant difference in the number of songs they sang in response to duetting intruders and lone female intruders (Figure 1b; ANOVA: F2,59 = 4.06, P = 0.03; Tukey tests: solo male vs. duet P < 0.05, remaining pairwise comparisons all P > 0.05).
Females also produced different types of vocalizations in response to the 3 treatments. Solo female type II songs (Rogers 2005) were produced only in response to duet and solo female playback and never in response to playback of solo male song (number of females giving a song type II during: solo female playback = 7, solo male playback = 0, duet playback = 4, N = 20 females). Females showed a more aggressive response to lone female intruders than to lone male or duetting intruders, approaching the speaker significantly more closely (Figure 1d: ANOVA: F2,59 = 20.3, P < 0.001) and quickly (Figure 2: ANOVA: F2,59 = 26.5, P < 0.001) during playback of solo female song (Tukey tests all comparisons P < 0.05).
Male versus female responses
Males and females did not tend to coordinate their approach of the playback speaker during simulated intrusion. In 19 out of 60 trials, only one member of the pair approached the speaker. When both partners did approach during playback, they generally did so separately. Consequently, there was only a weak negative correlation between the time taken for male and female partners to first approach the speaker (Spearman correlation: rs = –0.24, P = 0.15, N = 41 trials). Instead, males and females showed a sex-specific response to playback (Figures 1 and 2). Although males approached the speaker more closely during playback of a lone male than a lone female, females approached more closely during playback of a lone female (Figure 1c,d: interaction between sex of responder and playback type: ANOVA: F2,59 = 3.97, P = 0.05). Males were fastest to approach the speaker during lone male playback and females during lone female playback (Figure 2: interaction between sex of responder and playback type: ANOVA: F2,59 = 9.35, P = 0.003).
Female answer rate
Females showed a significant difference in the proportion of their partner's songs they answered during the 3 playback treatments (Figure 3: ANOVA: F2,58 = 5.95, P = 0.006). They answered a significantly higher proportion of their mate's songs during simulated intrusion by an unpaired female than during simulated intrusion by a lone male (Tukey test: P < 0.05) and showed a nonsignificant tendency to reply more often to their partner in the presence of an unpaired female intruder than duetting intruders (Figure 3, P > 0.05). There was no significant difference between the proportion of partner songs a female answered during simulated intrusion by duetting intruders and lone male intruders (Figure 3, P > 0.05).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Male and female eastern whipbirds responded to simulated intruders within their territories. However, they showed little evidence of coordinating their response, as might be expected if they were cooperatively defending a shared resource or preventing their partner from being usurped. We found no evidence to support any of the 3 key predictions of the joint defense hypothesis in Table 1. First, pairs did not tend to respond to intrusion as a coordinated unit. In almost one third of trials, only one member of the pair approached the speaker, and when both members approached, they did not approach at the same time. Second, neither sex showed a more aggressive vocal or approach response to duetting intruders compared with lone intruders, suggesting that duets were not perceived as more threatening territorial signals than solo songs. Finally, both sexes reacted more aggressively to same-sex playback, whereas the joint defense hypothesis predicts an equally aggressive response to intruders of either sex. Indeed, paired females did not generally approach the speaker or increase their vocal rate in response to playback of a lone male, suggesting that females did not aggressively defend against lone male intruders. During interactions between neighboring pairs, we never observed opposite-sex birds in physical conflict or chases, and aggression was invariably directed at same-sex individuals (personal observation, Rogers et al. 2006
). Our finding that females did not respond aggressively to male intruders and formed duets with their partners least often during playback of a lone male suggests that it is unlikely that female whipbirds use duets to prevent their partner from being usurped by male rivals (Table 1). Likewise, the results did not support predictions of the identity hypothesis. First, duets were always initiated by males, whereas the identity hypothesis predicts that both sexes should initiate duets. Second, during solo playback, initiation rates were higher for same-sex playback, rather than opposite-sex playback as predicted by the identity hypothesis. Lastly, females that failed to answer their mate were never subjected to aggression from their mate. Indeed, we never observed intrapair aggression in eastern whipbirds.
Our results provide the most support for the hypothesis that females use duets to defend their own position within the partnership (Table 1). Intrasexual aggression is indicative of individuals independently defending their position within a partnership (Seddon et al. 2002). In addition, paired females answered their partner more often in the presence of a lone female intruder than a lone male intruder and tended to respond to their partner more often in the presence of an unpaired female than a paired female rival. Consequently, the proportion of vocalizations that were duets was highest when the focal female was most likely to be usurped from the partnership. The fact that females duet with their partner in the apparent absence of a rival female (during playback of a lone male) could be because females are trying to ascertain whether another female is present. Eastern whipbirds inhabit areas of dense vegetation where visual cues are limiting, and male songs that receive an answer from a female do not appear to differ from those that do not (Watson 1969). Thus, on hearing a male intruder, a female may not immediately know whether he is alone. The costs of not responding to her partner in this situation may exceed any benefits of remaining silent.
Two mechanisms could explain how responding precisely to a partner might enable a female to maintain her exclusive position in the partnership (Table 1). These are mutually nonexclusive, and it is possible that both play a role in duet function in the eastern whipbird. Firstly, duets could be used to signal paired status to a same-sex rival. By responding precisely to their partners, females prevent mates singing solo songs that may otherwise attract female rivals ("acoustic mate defense": Brown and Lemon 1979; Sonnenschein and Reyer 1983; Levin 1996a,b; Seddon et al. 2002; Gill et al. 2005). The following field observations suggest that unpaired females may be attracted to solo male song if a female fails to reply to her partner. While paired females did not tend to approach the speaker during playback of lone male song, opportunistic playback of lone male song to 4 unpaired females resulted in close approaches to within less than 5 m and frequent antiphonal duets with the speaker. It also seems possible that paired male eastern whipbirds actively seek out females. We never observed males attacking female intruders and our findings that males reacted strongly to playback of a lone female could have been indicative of attraction rather than territorial aggression. It is likely that in this climate of conflict, duets may have arisen to allow paired females to protect their position within the partnership through an acoustic display of pair status that deters rival females.
A second possible mechanism of partnership defense involves the use of duets as an intrapair signal. By signaling their commitment and willingness to invest in the partnership, females may attempt to elicit reciprocal investment from their partners ("maintaining the pair bond": Wickler 1980, Smith 1994, Hall 2004). Wickler (1980) suggested that if duet coordination required effort to achieve, for example, if it required a period of learning, duets could be used as an effective signal of commitment. However, most studies since then have shown that antiphonal duets do not require a lengthy learning period (reviewed in Hall 2004). Further, in eastern whipbirds, females respond to the songs of male strangers as precisely as they respond to their own partner (Rogers, forthcoming), suggesting that signaling intrapair commitment is unlikely to be the only role of duets in the eastern whipbird.
Why should female eastern whipbirds need to defend their position in the partnership? Two features of eastern whipbird life history may explain the need for acoustic mate defense in this species. First, it is unlikely that females could successfully reproduce without access to a territory and paternal care. Male eastern whipbirds play an important role in caring for offspring (Rogers and Mulder 2004), and the high level of paternal care displayed suggests that polygyny and division of male care between 2 nests would involve significant reproductive costs to his original partner (Slagsvold and Lifjeld 1994). By defending their position in the partnership, females simultaneously maintain their position on a breeding territory and their exclusive access to male parental care. Using an acoustic method to defend their position may be particularly effective in the densely vegetated habitat preferred by eastern whipbirds. It also allows for the possibility of long-distance defense of a partner from variable distances. Second, female defense of her exclusive position within the partnership may be particularly necessary as competition for partners and territories appears intense. During our 3-year study, eastern whipbirds displayed extremely high rates of adult survival, limited dispersal, and low rates of territory turnaround, despite the presence of numerous unpaired adult floaters (Rogers and Mulder 2004). The population sex ratio was female biased both among nestlings and unpaired adults (Rogers and Mulder 2004) and several lines of evidence suggest that this may have resulted in high levels of intraspecific competition between females. Elsewhere, we have shown that female eastern whipbirds type match the songs of female intruders during solo singing bouts (Rogers et al. 2006) and may also use a novel vocal strategy (forming extrapair duets with male neighbors) in order to type match and temporally overlap the song of female rivals in escalating song duels (Rogers, forthcoming). The development of a distinctive female solo song type II which does not feature in duets is also indicative of a high incidence of rivalry between female eastern whipbirds (Rogers 2005). A high level of female–female competition has been identified as a principal factor promoting the development of female solo song (Langmore 1998b, 2000). In this study, females only produced a song type II in response to playback involving female intruders and never in response to solo male playback. A male-biased sex ratio has been used to explain why males may use duets to defend their female partners from rival males in a species forming female-initiated duets (subdesert mesites (Monias benschi): Seddon et al. 2002). However, to the best of our knowledge, this is the first time it has been suggested that a female-biased sex ratio, and high female–female competition, might explain the development of male-initiated duets as a method for females to defend their position in the partnership. Our results show a striking correspondence to those of a recent study on duetting in a suboscine passerine, the warbling antbird Hypocnemis cantator (Seddon and Tobias 2006). Like eastern whipbirds, warbling antbirds produced male-initiated duets, females responded to same-sex solo songs more strongly than to duets, and females answered their partner's songs more often in response to female solos than male solos or duets. It would be interesting to know whether acoustic mate guarding in antbirds is likewise underpinned by high levels of female competition and a female-biased sex ratio.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Our results do not support hypotheses that female eastern whipbirds reply to their partner to identify themselves to jointly defend a shared resource or to prevent their partner being usurped. Rather, duets in this species appear to play a primary role in allowing females to defend their exclusive position in a partnership. Female whipbirds may reply precisely to their partner to signal commitment to their partner or, more likely, as a method of acoustic mate defense. Defence of their position in the partnership may be necessary because a female-biased sex ratio is likely to create conditions of intense intrasexual competition between females for mate, and exclusive access to male care plays an important role in female reproductive success. This study suggests that duets are a consequence of the sexes following independent strategies of mate and territory defense and highlights the existence of possible conflicts between partners. Further studies are required to determine whether duets function in a similar way in other species that form duets initiated exclusively by males.
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ACKNOWLEDGEMENTS |
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We are grateful to the New South Wales National Parks and Wildlife Service, Australian Bird and Bat Banding Scheme, and the Animal Experimentation and Ethics Committee of the University of Melbourne for permission to conduct this research. We also thank Murray Littlejohn for lending recording equipment and Sophie Allebone-Webb, Zoe Birtle, Chris Boland, Elisa Bone, Eric Liebgold, Joah Madden, Golo Maurer, James Nicholls, Dean Portelli, Tami Ramson, Jennifer Reimer, Carl Rothfelds, Stacie Schoppman, Reagan Szabo, and Wouter van Dongen for providing help in the field. ACR was supported by an Australian Geographic research grant, a Cayley Memorial Scholarship from the Gould League of New South Wales (NSW), an Ethyl Mary Read Grant from the Linnean Society of NSW, a Frank M. Chapman Memorial Fund research award from the American Museum of Natural History, the Holsworth Wildlife Research Fund, a Johnstone and Florence Stoney Studentship from the British Federation of Women Graduates, an IPRS award from the University of Melbourne, the Joyce W. Vickery Scientific Research Fund of the Royal Zoological Society of NSW, the Norman Wettenhall Foundation, and a Stuart Leslie Research Award from Birds Australia.
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