Lack of sex-specific syllables and high female song variability support relaxed selection on female song in Orchard Orioles (Icterus spurius)

Female song in songbirds has historically been understudied compared to male song. Previous work by our lab found that female Orchard Oriole (Icterus spurius) song is acoustically reduced and more variable compared to male song. To investigate further, we categorically compared syllable usage between male and female Orchard Orioles. We created a syllable library by scoring three songs from each of 23 males and 16 females. We divided each song into syllables and visually classified syllables into syllable types. We found that individual males sang more syllable types (mean ± SD) (42.7 ± 8.2) than individual females (22.6 ± 5.0). We also found that females had a higher Song Versatility Index (SVI) (59%) in syllable usage than males (41%). A greater proportion of female syllables were shared with males than with other females, whereas a similar proportion of male syllables were shared with other males as with females. Syllable sharing between the sexes suggests that there is no sex-specific syllable usage in Orchard Orioles. High SVI in female Orchard Orioles as well as the high quantitative acoustic variability, found in the previous work, suggests that female elaborate song may be subject to relaxed selection in this species. Comparisons of song at the syllable level can provide valuable insight into sex-specific song learning, geographic patterns, and social interactions.