A dawn and dusk chorus will emerge if males sing in the absence of their mate

The spring dawn and dusk chorus of birds is a widespread phenomenon, yet its origin remains puzzling. We propose that a dawn and dusk chorus will inevitably arise if two criteria are met: (1) females leave their roost later in the morning and go to roost earlier in the evening than their mate, and (2) males sing more when separated from their mate. Previous studies on blue tits (Cyanistes caeruleus) support the first criterion. We here report that males sing at a higher rate whenever they are separated from their mate and that song rate increases with the duration of female absence. These findings can explain the existence of the dawn and dusk chorus in blue tits, and they can explain why the dawn chorus is more pronounced than the dusk chorus, as is typically observed. An exhaustive literature search provides support for both criteria of the 'absent mate' hypothesis in several passerine birds. We found no evidence contradicting the hypothesis. The new hypothesis is not inconsistent with many of the existing hypotheses about dawn singing, but may be a more general explanation for the occurrence of a dawn and dusk chorus. We describe how the 'absent mate' hypothesis leads to testable predictions about daily and seasonal variation in song output.

Females occasionally create duets with males but they never sing solo-year-round singing behaviour in an Afrotropical songbird

Our knowledge of birdsong mainly comes from studies focused on male songs produced in a short breeding period, even though we know that sedentary species sing year-round, female song is quite widespread and many species sing collectively creating duets and choruses. In this study we focused on daily and seasonal changes in singing activity of an endemic, sedentary, duetting, Afrotropical songbird-the Bangwa forest warbler. We collected soundscape recordings in six recording locations and used singing activity index to examine how vocal activity of males and females varies daily and seasonally and how it correlates with the rainfall. We found that Bangwa forest warblers sing year-round, yet they do it more in wet than in dry season. The rapid increase of singing activity occurs after first rain, at the beginning of the rainy season. Males sing significantly more than females. Females never sing solo, however, in 13% of songs they create duets by joining male solos. The pattern of daily singing activity is sex-specific and seasonally variable, with two peaks (dawn and dusk) observed in males and only one in females (dawn). In Bangwa forest warbler male singing behaviour is similar to that of many songbirds, suggesting that territory defence and female attraction as main functions of singing. Females, which create duets and never sing solo may use songs in mate guarding, signalling commitment, resource defence or intersex territory defence. Duets observed year-round may suggest cooperative resource defence. Results of the study show that examining year-round singing behaviour is crucial to fully understand the evolution and functions of male and female songs.

Temporal Acoustic Patterns of the Oriental Turtle Dove in a Subtropical Forest in China

There is an abundance of bird species in subtropical areas, but studies on the vocal behavior of non-passerines in subtropical regions are limited. In this study, passive acoustic monitoring was used to investigate the temporal acoustic patterns of the vocal activities of the Oriental Turtle Dove (Streptopelia orientalis) in Yaoluoping National Nature Reserve (YNNR) in eastern China. The results show that the vocal production of the Oriental Turtle Dove exhibited a seasonal variation, peaking in the period April–August. Additionally, its diurnal vocal activity displayed a bimodal pattern in late spring and summer, with the first peak in the morning and a secondary peak at dusk. Among weather factors, temperature significantly affected the temporal sound pattern of the Oriental Turtle Dove, instead of humidity and precipitation. This study, which was focused on sound monitoring technology, provides knowledge for further research on bird behavior and ecology. In the future, long-term sound monitoring could be used for managing and conserving bird biodiversity.

The singing question: re-conceptualizing birdsong

Birdsong has been the subject of broad research from a variety of sub-disciplines and has taught us much about the evolution, function, and mechanisms driving animal communication and cognition. Typically, birdsong refers to the specialized vocalizations produced by oscines. Historically, much of the research on birdsong was conducted in north temperate regions (specifically in Europe and North America) leading to multiple biases. Due to these historic biases these vocalizations are generally considered to be highly sexually dimorphic, heavily shaped by sexual selection and essential for courtship and territoriality. Song is also typically defined as a learned trait shaped by cultural evolution. Together, this framework focuses research specifically on males, particularly during the north temperate breeding season - reflecting and thereby reinforcing this framework. The physiological underpinnings of song often emphasize the role of the hypothalamic-pituitary-gonadal axis (associated with breeding changes) and the song control system (underlying vocal learning). Over the years there has been great debate over which features of song are essential to the definition of birdsong, which features apply broadly to contexts outside males in the north temperate region, and over the importance of having a definition at all. Importantly, the definitions we use can both guide and limit the progress of research. Here, we describe the history of these definitions, and how these definitions have directed and restricted research to focus on male song in sexually selected contexts. Additionally, we highlight the gaps in our scientific knowledge, especially with respect to the function and physiological mechanisms underlying song in females and in winter, as well as in non-seasonally breeding species. Furthermore, we highlight the problems with using complexity and learning as dichotomous variables to categorize songs and calls. Across species, no one characteristic of song - sexual dimorphism, seasonality, complexity, sexual selection, learning - consistently delineates song from other songbird vocal communication. We provide recommendations for next steps to build an inclusive information framework that will allow researchers to explore nuances in animal communication and promote comparative research. Specifically, we recommend that researchers should operationalize the axis of variation most relevant to their study/species by identifying their specific question and the variable(s) of focus (e.g. seasonality). Researchers should also identify the axis (axes) of variation (e.g. degree of control by testosterone) most relevant to their study and use language consistent with the question and axis (axes) of variation (e.g. control by testosterone in the seasonal vocal production of birds).

Avian vocalisations: the female perspective

Research on avian vocalisations has traditionally focused on male song produced by oscine passerines. However, accumulating evidence indicates that complex vocalisations can readily evolve outside the traditional contexts of mate attraction and territory defence by male birds, and yet the previous bias towards male song has shaped - and continues to shape - our understanding of avian communication as a whole. Accordingly, in this review we seek to address this imbalance by synthesising studies on female vocalisations from across signalling contexts throughout the Aves, and discuss the implications of recent empirical advances for our understanding of vocalisations in both sexes. This review reveals great structural and functional diversity among female vocalisations and highlights the important roles that vocalisations can play in mediating female-specific behaviours. However, fundamental gaps remain. While there are now several case studies that identify the function of female vocalisations, few quantify the associated fitness benefits. Additionally, very little is known about the role of vocal learning in the development of female vocalisations. Thus, there remains a pressing need to examine the function and development of all forms of vocalisations in female birds. In the light of what we now know about the functions and mechanisms of female vocalisations, we suggest that conventional male-biased definitions of songs and calls are inadequate for furthering our understanding of avian vocal communication more generally. Therefore, we propose two simple alternatives, both emancipated from the sex of the singer. The first distinguishes song from calls functionally as a sexually selected vocal signal, whilst the second distinguishes them mechanistically in terms of their underlying neurological processes. It is clear that more investigations are needed into the ultimate and proximate causes of female vocalisations; however, these are essential if we are to develop a holistic epistemology of avian vocal communication in both sexes, across ecological contexts and taxonomic divides.

Breeding season length predicts duet coordination and consistency in Neotropical wrens (Troglodytidae)

Many animals produce coordinated signals, but few are more striking than the elaborate male-female vocal duets produced by some tropical songbirds. Yet, little is known about the factors driving the extreme levels of vocal coordination between mated pairs in these taxa. We examined evolutionary patterns of duet coordination and their potential evolutionary drivers in Neotropical wrens (Troglodytidae), a songbird family well known for highly coordinated duets. Across 23 wren species, we show that the degree of coordination and precision with which pairs combine their songs into duets varies by species. This includes some species that alternate their song phrases with exceptional coordination to produce rapidly alternating duets that are highly consistent across renditions. These highly coordinated, consistent duets evolved independently in multiple wren species. Duet coordination and consistency are greatest in species with especially long breeding seasons, but neither duet coordination nor consistency are correlated with clutch size, conspecific abundance or vegetation density. These results suggest that tightly coordinated duets play an important role in mediating breeding behaviour, possibly by signalling commitment or coalition of the pair to mates and other conspecifics.

New insights from female bird song: towards an integrated approach to studying male and female communication roles

Historically, bird song has been regarded as a sex-specific signalling trait; males sing to attract females and females drive the evolution of signal exaggeration by preferring males with ever more complex songs. This view provides no functional role for female song. Historic geographical research biases generalized pronounced sex differences of phylogenetically derived northern temperate zone songbirds to all songbirds. However, we now know that female song is common and that both sexes probably sang in the ancestor of modern songbirds. This calls for research on adaptive explanations and mechanisms regulating female song, and a reassessment of questions and approaches to identify selection pressures driving song elaboration in both sexes and subsequent loss of female song in some clades. In this short review and perspective we highlight newly emerging questions and propose a research framework to investigate female song and song sex differences across species. We encourage experimental tests of mechanism, ontogeny, and function integrated with comparative evolutionary analyses. Moreover, we discuss the wider implications of female bird song research for our understanding of male and female communication roles.

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.