Analysis of female song provides insight into the evolution of dimorphism in a widely studied songbird

Understanding the patterns and processes related to sexual dimorphism in diverse animal taxa is a foundational research topic in ecology and evolution. Within the realm of animal communication, studies have traditionally focused on male signals, assuming that female choice and male-male competition have promoted dimorphism via elaboration of male traits, but selection on females also has the potential to create sex differences. Here, we describe female song in barn swallows for the first time, report rates of female song production, and couple song data with plumage data to explore the relative degree to which dimorphism in signaling traits is consistent with contemporary selection on males versus females. During previous intensive study of male song over two years, we recorded songs for 15 females, with matched phenotypic and fitness data. We randomly selected 15 samples from our larger male dataset to test whether sexual dimorphism in song and plumage is more strongly associated with fledgling success for females or genetic paternity for males. Analyses included 35 potential sexual signals including 22 song parameters and 13 plumage traits. Outcomes indicate that: female songs are used in multiple contexts, restricted primarily to the beginning of the breeding season; song traits were more dimorphic than visual plumage traits; and trait correlations with reproductive success in females, rather than males, predicted sex-based differences in song and plumage. These results are consistent with phylogenetic studies showing that sex-based phenotypic differences are driven by changes in females, highlighting the potential role of female trait evolution in explaining patterns of sexual dimorphism. To achieve a better understanding of dimorphism, we require comprehensive studies that measure the same traits in males and females and their fitness consequences.