Adult male-female social bond advertising: The primary function of coordinated singing intensity in a small ape

Adult male and female coordinated singing occur in diverse animal taxa. Adult male-female social bond advertising and strengthening have been proposed as two important functional hypotheses of coordinated singing. Here we studied these two functions in four groups of cao vit gibbon (Nomascus nasutus), a small ape that lives in polygynous family groups and adult members coordinated their sex-specific songs to produce complex duets or trios (three members sing together), using 6-year field behavioral data. In this study, we used the number of successful great call sequences per bout or per minute, and latency period from start of the adult male call to the first successful great call sequence to represent singing intensity. We used the proportion of proximity, behavioral synchronization, and grooming between adult male and female to represent bond strength. We used linear mixed-effects model to investigate the correlation between singing intensity and adult male-female social bond strength. We found a negative correlation between all three bond strength indicators and female latency period (N = 209), and a positive correlation between the number of successful great call sequences per bout (N = 253) and per minute (N = 254) and proximity. We used paired Wilcoxon signed-rank test to investigate if adult gibbons increase bond strength after singing. We found proximity (hourly level: N = 45; daily level: N = 54), behavioral synchronization (hourly level: N = 57; daily level: N = 49), and grooming (daily level: N = 34) in most of the groups did not increase significantly after singing in an hourly or daily level. Together, these results indicate that cao vit gibbon coordinated singing serves primarily in adult male-female social bond advertising and distinct singing intensity indicators advertise different information on adult male-female social bond strength.

Structure and functions of Yellow-breasted Boubou (Laniarius atroflavus) solos and duets

Background

Birds have extremely well-developed acoustic communication and have become popular in bioacoustics research. The majority of studies on bird song have been conducted in the temperate zones where usually males of birds sing to attract females and defend territories. In over 360 bird species mostly inhabiting the tropics both males and females sing together in duets. Avian duets are usually formed when a male and female coordinate their songs. We focused on a species with relatively weakly coordinated duets, with male solo as the prevailing vocalisation type.

Methods

Instead of analysing a set of recordings spread over a long time, we analysed whole day microphone-array recordings of the Yellow-breasted Boubou (Laniarius atroflavus), a species endemic to West African montane rainforests. We described the structure of the solo and duet vocalisations and temporal characteristics of daily activity based on 5,934 vocal bouts of 18 focal pairs and their neighbours.

Results

Birds had small, sex specific repertoires. All males shared three types of loud whistles functioning as song type repertoires in both solos and duets. Females vocalised with five types of harsh, atonal notes with a more variable and usually lower amplitude. Three of them were produced both as solos and in duets, while two seem to function as alarm and excitement calls given almost exclusively as a solo. Solos were the most common vocalisation mode (75.4%), with males being more vocally active than females. Duets accounted for 24.6% of all vocalisations and in most cases were initiated by males (81%). The majority of duets were simple (85.1%) consisting of a single male and female song type, but altogether 38 unique duet combinations were described. Males usually initiated singing at dawn and for this used one particular song type more often than expected by chance. Male solo and duet activities peaked around dawn, while female solos were produced evenly throughout the day.

Discussion

Yellow-breasted Boubou is a duetting species in which males are much more vocal than females and duetting is not a dominating type of vocal activity. Duet structure, context and timing of daily production support the joint resource defence hypothesis and mate guarding/prevention hypotheses, however maintaining pair contact also seems to be important. This study provides for the first time the basic quantitative data describing calls, solos and duet songs in the Yellow-breasted Boubou.

Why do females sing?—pair communication and other song functions in eastern bluebirds

Female bird song has been underappreciated and understudied, especially in temperate species. Birdsong was originally thought to be a trait used primarily by male songbirds for mate attraction and male/male contest. However, ornithologists have long known that females sing in many tropical songbirds, often for similar functions to male song. Yet, studies of female song in temperate regions remain scarce. Increasing our understanding of the function of female song in temperate species is a powerful step towards discerning the selective pressures that maintain elaborate female signals. In the last few decades, studies of temperate species have highlighted five major functional categories of female song. Using a modeling framework, based on all known functions of song in other species, we tested the function of female song in eastern bluebirds. The modeling framework allowed us to test the effect of multiple complex behaviors simultaneously to predict female song function. Additionally, modeling mitigated issues of multiple testing across the five different functional categories. We found that female song in eastern bluebirds is primarily used in pair communication. Specifically, females sing to strengthen and maintain long-term pair bonds. Strengthening pair-bonds may be advantageous for eastern bluebirds as pairs that remain together between nesting attempts and between years have higher reproductive success. We demonstrate a clear link between the function of female song in pair communication and the likely selective force of long-term pair bonds acting on eastern bluebird reproductive success. Additionally, our study highlights a major function of female song in a temperate species.

Dynamic sex-specific responses to synthetic songs in a duetting suboscine passerine

Many bird species produce temporally coordinated duets and choruses, requiring the rapid integration of auditory perception and motor production. While males and females of some species are known to participate in these displays for sex-specific purposes, few studies have identified perceptual features that trigger sex-specific contributions of coordinated song. Especially little is known about perception and production in duetting suboscine passerines, which are thought to have innate songs and largely static, rather than dynamic, vocal behavior. Here, we used synthetic stimuli in a playback experiment on chestnut-backed antbirds (Myrmeciza exsul) to (1) test whether differences in song frequency (Hz) can trigger sex-specific vocal behavior in a suboscine passerine (2) test for the functions of duetting in males and females of this species, and (3) determine whether these suboscines can dynamically adjust the temporal and spectral features of their songs. We found sex-specific responses to synthetic playback manipulated in song frequency (Hz), providing evidence that in this context males sing in duets for general territory defense and females join in for mate guarding purposes. In addition, we found that the birds altered the frequency, duration, and timing of their songs depending on the frequency of the playback songs. Thus, we show that these birds integrate spectral and temporal information about conspecific songs and actively modulate their responses in sex-specific ways.

Females and males respond more strongly to duets than to female solos: comparing the function of duet and solo singing in a tropical songbird (Icterus icterus)

Both females and males of many animals possess elaborate displays, such as solo songs and duets of songbirds. We know little about the function of female song or what selects for duets. To examine their possible functions, we played female solos, duets and a heterospecific control to pairs of troupials (Icterus icterus). Both sexes responded strongly to duets by approaching the playback speaker significantly closer, faster, and more often compared to female solos or the control. Neither sex responded strongly to female solos; troupials approached closer, sang sooner, and duetted more following female solos than the control, but this difference was not significant. Our results indicate that troupial duets pose a greater threat than female solos and female troupial solos are not particularly threatening. Troupial duets may be especially threatening because they indicate the presence of a mated pair, which likely poses a greater territorial threat than lone, unmated birds.