Female song in eastern bluebirds varies in acoustic structure according to social context

Most of our knowledge about the function, evolution, neurobiology and ontogeny of complex vocalizations in birds comes from previous work on male song. However, the recent increase in focus on female bird song over the last few decades has helped bridge that gap in our knowledge and has vastly changed our understanding of the function, phylogeny and evolution of bird song. Here we present evidence that female song may be modifiable based on signal function. Eastern bluebirds primarily use female song for within-pair communication with their social mate. We examined the female songs of eastern bluebirds sung during within-pair communication and compared the acoustic structure to female songs used in other contexts. Within-pair communication songs differed significantly from other female songs that we measured by six of the eight acoustic variables we measured. Additionally, we examined the song structure of within-pair communication songs at variable distances between mates and between neighbour females. Song structure did not vary over distance. However, we found significant differences between neighbouring females and the number of syllables and frequency modulations each female included in her within-pair communication songs. We propose that female songs have the potential to indicate individual identity, through acoustic structure modification. Our findings further highlight the need for additional study of female song and functional acoustic plasticity in bird song. Animal signals may be stereotyped to convey honest information, they may be functionally plastic to convey additional information, or elements of a signal may fall in both categories. Historically, bird song has been primarily studied as a stereotyped male trait. Whereas male bird song may have variable acoustic structure in some instances, such as extreme aggression, the functional plasticity of female song is largely unknown. We examined female song plasticity for within-pair communication songs of female eastern bluebirds. We present some of the first evidence that female song structure may be variable between functional contexts and that females may alter their song structure to encode additional information, such as individual identity. Our results inform future work on the function and ontogeny of female bird song. Additionally, our findings support the need for further study on female bird song and the functional plasticity of complex acoustic signals.