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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).

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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.

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