Sex ratio and plumage characteristics of juvenile American Robins ( Turdus migratorius ) in suburban areas of the Arkansas River valley

Bird species that exhibit male-biased sexual size dimorphism (SSD) often demonstrate a female-biased fledging sex ratio (FSR). One hypothesis for this relationship is that 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 relationship between SSD and FSR. We sought to determine the FSR of the American Robin (Turdus migratorius) and evaluate the relationship of SSD in the nestling stage to the FSR of the species. We also used qualitative and quantitative assessments of juvenile plumage to gauge the feasibility of sex determination through plumage. Male and female nestlings had substantial overlap in plumage characteristics, and we could not reliably distinguish between the sexes. Plumage differences between the sexes of juvenile robins may be insufficient to accurately determine sex in the field. The FSR was significantly skewed toward females; 63% of 59 nestlings were female. Size differences between male and female nestlings were minimal. Our findings do not provide strong support for the hypothesis that SSD leads to differential mortality of nestlings that skews fledging sex ratios toward the smaller sex. Other factors probably have a greater influence on the FSR of the species. More research is necessary to determine the primary drivers of skewed sex ratios in American Robins and other bird species.