Phylogenetic comparative studies of sex differences in birdsong
Classically, birdsong is thought to be sexually selected in males. However, comparative analyses by my colleagues and I revealed that female song is ancestral and widespread in songbirds. This means that birdsong originally evolved in both males and females. Furthermore, the absence of song in females of many songbird species is because females lost song in these species; not that elaborate song was solely gained in males. Through continued phylogenetic comparative analyses, we are currently investigating the evolutionary pressures that led to maintenance, reduction, or exaggeration of male and female song in particular species, resulting in the extreme variation seen in male and female song today. Important factors promoting song in both sexes appear to be monogamy, bi-parental care, and year-round territoriality leading to similar sex roles.
Integrative field studies on seasonal changes in song
Understanding how environmental and social cues lead to variation in male and female song within an individual's lifetime can help us pin-point the adaptive mechanisms leading to these behaviors. To do this, I investigate seasonal changes in male and female singing behavior. During my Ph.D. I showed that female troupials (a tropical oriole) sing consistently between the breeding and nonbreeding season whereas males sing most at dawn during the breeding season. This suggests that both social and sexual selection pressures are shaping sex-specific behaviors in this tropical species.
I am currently examining hormone regulation of male and female song rates in house wrens throughout the breeding season. In temperate songbirds, large increases in male singing early in the breeding season is associated with increased circulating concentrations of testosterone. However, we are finding that female house wren testosterone concentrations are very low throughout the breeding season, despite a peak in female singing during egg laying. We are currently investigating alternative explanations for this peak in female house wren song, including the role of other steroid hormones and increased neural sensitivity to testosterone in females.
Neuroendocrine regulation of sex differences in song
I am beginning to explore how neuroanatomy, hormone profiles, and gene expression interact to lead to differences in male and female song in wild songbirds. To do this, we are comparing changes in neuroanatomy, steroid hormone concentrations, and gene expression between the breeding and non-breeding season in female and male house wrens. These results will help us understand how genes and hormones up and down regulate seasonal changes in male and female song. This will also help us understand how environmental variation can lead to increases or decreases in female song. Our goal is to eventually understand how these mechanisms contribute to the sex differences in song we see across species.
Using machine learning to quantify & detect female and male birdsong
My colleagues and I have been working on approaches to quantify and compare birdsong across species. Our methods include using unsupervised random forest to classify song structure and determine the overlap in acoustic structure between sexes or species. In addition to publications describing these methods, we have made them readily available in the R package PhenotypeSpace. We are also applying a deep learning approach to train algorithms to detect female and male house wren songs within hours-long passively collected recordings.