Comparative Analysis of Female Song Structure and Function in Two Temperate and Tropical Oriole Species

Abstract

Our understanding of the structure and function of elaborate signaling traits, such as coloration or vocalizations, has been dramatically impacted by a long-held bias towards studying these traits in male animals. Many female animals also have functional elaborate signals, but in the past, these traits were not likely to be studied and were often assumed to have only evolved as a consequence of shared genetic architecture with males. Work from our lab has demonstrated that the common ancestor of all songbirds likely had both male and female bird song, but female bird song remains significantly understudied. In tropical species, which are more likely to defend territories year-round and have sexual monomorphism, female song is more common than in temperate-breeding, migratory species. As a result, there is growing interest in the associations of tropical vs. temperate natural history characteristics with the incidence and function of female song. In this dissertation, we compared male and female song in two closely related New World Blackbirds (Icteridae): Orchard Orioles (Icterus spurius), a migratory, temperate-breeding species, and in Puerto Rican Orioles (Icterus portoricensis), a non-migratory, tropical species. We found that female song in Orchard Orioles is significantly reduced compared to male song in acoustic structure, rate, and function. We were able to reliably distinguish between female and male song in the field, and we did not observe female song produced in any consistent functional contexts. Puerto Rican Oriole female song was also acoustically reduced compared to male song, but to a much lesser degree than in Orchard Orioles. We did not find any reliable acoustic metric for distinguishing between female and male Puerto Rican Oriole song in the field, though females did not typically participate in the dawn chorus. However, females did seem to sing more than males sporadically throughout the day. These results suggest that female song in Orchard Orioles is likely under relaxed selection and may be in the process of being lost, which may parallel its loss of elaborate coloration. Female song in Puerto Rican Orioles seems to experience similar selective pressures to male song and is more likely to be functional, which would be predicted in a tropical species with year-round territory defense. To our knowledge, this is the first detailed comparative study of female song in closely related temperate and tropical species.