Data from: Sex Ratio and Plumage Characteristics of Juvenile American Robins in Suburban Areas of the Arkansas River Valley

Included are plumage (pdata) and sex ratio (bdata) datasets analyzed in "Sex Ratio and Plumage Characteristics of Juvenile American Robins in Suburban Areas of the Arkansas River Valley". A different submission includes the .Rmd (rmarkdown) file that has the code that was used with these datasets.<b>Abstract:</b> Bird species that exhibit male-biased sexual size dimorphism (SSD) often demonstrate a female-biased fledging sex ratio (FSR), possibly because male nestlings are more costly to raise and therefore at greater risk of mortality. However, not all male-biased size dimorphic species have been found to have a female-biased FSR, and many species have not been studied to better establish the impact of SSD on FSR. We sought to determine the FSR of the American Robin (<i>Turdus migratorius</i>) and evaluate the relevance of sexual size dimorphism (SSD) to the fledging sex ratio of the species. We also used qualitative and quantitative assessments of juvenile plumage to gauge the feasibility of sex determination through plumage. The FSR was skewed towards females; 63% of 59 nestlings were female. Male and female nestlings had substantial overlap in plumage characteristics, and we could not reliably distinguish between the sexes. Our findings support the hypothesis that SSD skews sex ratios towards the smaller sex, but more research is necessary to determine if that result is consistent with larger datasets and if other factors may affect avian FSRs. It appears that plumage differences between the sexes of juvenile robins may be insufficient to accurately determine sex in the field.