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.