Sexual Dimorphism in Plumage Coloration of Swainson's Hawks (Buteo swainsoni)

Plumage pigmentation and ornamentation can indicate male phenotypic quality in many avian species and represent traits that are often selected for by females. Melanins are some of the most widespread classes of pigments in the animal kingdom and form the many shades of black and brown feather colors across raptor taxa. Eumelanin is responsible for the darker shades of black and brown, whereas pheomelanin is responsible for the lighter brown shades and rusty red coloration that are found in many buteonine raptors, including Swainson's Hawks (Buteo swainsoni). Plumage polymorphism has been examined in various raptor species along with morphometric differences as a basis for sexual dimorphism; however, the potential for sexual dimorphism in the form of pheomelanin-pigmented plumage has rarely been discussed. We examined this phenomenon in Swainson's Hawks by digitally sampling breast feathers from adult museum specimens, and we utilized an imaging toolbox for multispectral image analysis in the open-source ImageJ software to compare red pixel values between sexes. We found a significant difference in normalized red pixel value means between male and female birds, with higher redness values strongly correlating with male individuals, regardless of morph type. This indicates a relationship between pheomelanin pigmentation and sex, illustrating a basis for sexual dimorphism within the species, as well as the potential for sexual selection of redder males.