THE STRUCTURE AND FUNCTION OF FEMALE SONG IN EASTERN BLUEBIRDS (SIALIA SIALIS): INTRA-PAIR COMMUNICATION IN A TEMPERATE BREEDING SONGBIRD

Abstract

Historically, birdsong was generally considered as a male trait used in the breeding season for sexually selected functions, such as male/male contests and mate attraction. However, researchers have now identified female song across a wide range of oscine passerines and have challenged this sex-biased definition. Additionally, recent work shows that female song is likely the ancestral trait for songbirds, supporting a single phylogenetic origin for extant female song across songbird genera. More work is needed on the presence, function, and evolution of female song to complete our understanding of complex vocal communication and advance our understanding beyond the historic biases. Specifically, female song has seldom been studied in north temperate species. This dissertations addresses three fundamental aspects of female song in a temperate breeding species, the Eastern Bluebird (Sialia sialis). First, I asked whether male and female Eastern Bluebirds sing statistically equivalent songs. I developed a repeated-measures two one-sided test (rmTOST) to examine statistical equivalence in natural systems. I found that male and female songs were structurally equivalent across five standard measures of acoustic complexity. Second, I tested five hypotheses for the function of female song. My results, based on linear modelling and behavioral assays, suggest that female song in Eastern Bluebirds functions in intra-pair communication. I posit that this is likely a mechanism to help maintain long-term pair bonds. Third, I also found that female song is functionally plastic and that females modify the acoustic structure of their songs to communicate with their social mate. This modification may be a mechanism to convey individual identity or distinguish the intra-pair communication songs from those of neighboring females. This combined work has created a strong model system to further explore the evolution, ontogeny, and neurobiology of female song in a temperate species well suited to behavioral field studies. Additionally, my work further highlights the prevalence of female song throughout the oscine passerines and makes a powerful step towards discerning the selective pressures that maintain elaborate female signals in a system previously thought to be driven by sexual selection on male traits.