Determining the sex of monomorphic species, such as Red-tailed Hawks (Buteo jamaicensis), can be very difficult in the field as the plumage of males and females is similar and body masses overlap between sexes. Our purpose was to develop age-class specific methods that could be used in field settings to accurately determine the sex of Red-tailed Hawks within an Upper Midwestern USA landscape. Red-tailed Hawk carcasses (n = 648) were grouped by age class, weighed, measured, and necropsied to determine the sex of each individual. We compared morphological measurements between males and females and used discriminant function analysis to identify morphological characteristics that were useful in sexing Red-tailed Hawks in-hand. We developed separate equations for hatch-year, second-year, after-second-year, and after-hatch-year Red-tailed Hawks that were 80–88% accurate in correctly classifying eastern Red-tailed Hawks (B. j. borealis) to sex. The region-specific methods we developed should be useful to raptor researchers and managers.