This post was contributed by Rachael DiSciullo, a PhD candidate at Illinois State University and a 2020 WOS Research Grant recipient.
Among the most ubiquitous sounds of summer in North America, the vocal acrobatics of the Northern House Wren (Troglodytes aedon) bring joy to many. Males in this species return to my study area from migration in early spring, settle on a territory, and begin to build a crude, cup-shaped nest of twigs in any suitable cavity-like hole they can find. And they sing. Upon territory establishment and throughout the entire nesting cycle, male House Wrens produce varied, complex, and abundant song (Johnson 2014). Like many species in which members of only one sex produce such vocalizations, there is solid evidence that song in House Wrens is sexually selected. Male song is more elaborate than female song (Johnson 2014), male song incites aggressiveness when played in the territory of other males (DiSciullo, Thompson, and Sakaluk 2019), and females are attracted to areas without males when a recorded male song is played (Johnson and Searcy 1996). All of these features of House Wren song strongly suggest that it functions in both male-male competition and female mate choice, the two mechanisms of sexual selection.
What remains unknown, however, is which components of song are most attractive to females or incite the most aggressiveness in males (Cramer 2013; dos Santos, Llambías, and Rendall 2018). This mystery is what drives my dissertation work. The aim of my research is to explore, in a statistically novel and experimentally rigorous manner, how male-male competition and female mate choice have shaped different components of male Northern House Wren song. To address these unknowns, the first step I took was to record, define, and categorize male song produced on my study area in Central Illinois. I have scored representative song samples from 168 15-min recordings of songs produced by 68 different males.
From these recordings, I have identified a collection of 61 different notes that, combined in a variety of ways, produce 1,204 unique song types. Work by other groups has found no upper limit to the variety of House Wren song types, either between or within individuals (Rendall and Kaluthota 2013; dos Santos, Llambías, and Rendall 2016), and the data I have gathered reflect this as well. This revelation adds further intrigue to the question of how sexual selection has shaped male song, and why the complexity and diversity of song types appears unbounded.
When capturing song, I also obtained blood samples from all social parents and nestlings to determine how many offspring a social male actually sired, given that about 25-35% of nests in our study population host at least one nestling fathered by a different male than the one attending the nest (Soukup and Thompson 1997; Forsman et al. 2008). We generated single nucleotide polymorphism (SNP) genetic markerswith the assistance of collaborators Dr. Anna Forsman and Dr. Robert Fitak at the University of Central Florida, the first time this had been done for House Wrens. SNPs are abundant genetic markers that can be used with great accuracy in resolving parent-offspring relationships (Marth et al. 2001, Glaubitz et al. 2003). This will allow me to determine genetic paternity by genotyping the parents and offspring. This measure of genetic paternity is considered the gold standard for proxies of reproductive success, and is a critical measure for this study. Funding generously awarded to me by the Wilson Ornithological Society Research Grants has helped offset the cost of this essential genotyping process.
With these paternity data, I can conduct a multivariate statistical analysis (MSA), which is a powerful type of analysis that will allow me to identify whether the males who sire the most offspring also share specific song components (Lande and Arnold 1983). With this information, I can then use bits of previously recorded songs to synthesize new songs on the computer that reflect high and low reproductive success. These songs will be broadcast in the field to test wild male and female responses to songs that presumably advertise high- and low-quality males. This experiment is ongoing this 2022 breeding season. Stay tuned for the exciting results that I expect will provide greater insight into the evolution of such an elaborate and beautiful trait!